GRE Biochemistry, Cell and Molecular Biology Subject Test — Questions and Answers

Question 1: Which of the following is a key regulatory enzyme in the citric acid cycle?

Citrate synthase
Isocitrate dehydrogenase (Correct answer)
Succinate dehydrogenase
Malate dehydrogenase

Correct answer: Isocitrate dehydrogenase

Isocitrate dehydrogenase is a key regulatory enzyme in the citric acid cycle, controlling its overall rate based on the cell's energy demands. Its activity is allosterically inhibited by high levels of ATP and NADH, signaling ample energy, and activated by ADP, indicating a need for more energy. This regulation ensures efficient energy production.

Question 2: Scurvy results from Vitamin C deficiency primarily because without ascorbate:

Iron cannot be absorbed from the diet
Prolyl and lysyl hydroxylases cannot function, impairing collagen synthesis (Correct answer)
Coagulation factors cannot be gamma-carboxylated
The immune system cannot produce antibodies

Correct answer: Prolyl and lysyl hydroxylases cannot function, impairing collagen synthesis

Ascorbate is required to maintain the Fe²⁺ state of prolyl and lysyl hydroxylases; without it, hydroxylation of proline and lysine in collagen is deficient, preventing proper triple helix formation.

Question 3: Tay-Sachs disease results from accumulation of which lipid in neurons due to a missing enzyme?

Ceramide
Glucocerebroside
Sphingomyelin
GM2 ganglioside (Correct answer)

Correct answer: GM2 ganglioside

Tay-Sachs is caused by deficiency of hexosaminidase A, leading to accumulation of GM2 ganglioside in neuronal lysosomes, causing progressive neurodegeneration.

Question 4: Von Gierke disease (Type I glycogen storage disease) is caused by deficiency of:

Debranching enzyme
Glycogen synthase
Glycogen phosphorylase
Glucose-6-phosphatase (Correct answer)

Correct answer: Glucose-6-phosphatase

Glucose-6-phosphatase deficiency in Von Gierke disease prevents the liver from releasing free glucose from glycogen (or gluconeogenesis), causing severe fasting hypoglycemia.

Question 5: In glycolysis, which enzyme catalyzes the conversion of glucose to glucose-6-phosphate?

Pyruvate kinase
Aldolase
Phosphofructokinase
Hexokinase (Correct answer)

Correct answer: Hexokinase

Hexokinase is the enzyme that catalyzes the first committed step of glycolysis, converting glucose into glucose-6-phosphate. This phosphorylation traps glucose inside the cell, preventing its diffusion out and initiating its breakdown for energy. This reaction is crucial for cellular glucose metabolism.

Question 6: Alpha-helices and beta-pleated sheets are examples of which level of protein structure?

Quaternary
Tertiary
Primary
Secondary (Correct answer)

Correct answer: Secondary

Secondary structure describes local folding patterns stabilized by hydrogen bonds between backbone amide and carbonyl groups.

Question 7: Which class of proteins contains immunoglobulin-fold domains and is central to adaptive immunity?

Antibodies (immunoglobulins) (Correct answer)
Fibrous proteins
Nuclear receptors
G-protein coupled receptors

Correct answer: Antibodies (immunoglobulins)

Antibodies are glycoproteins with characteristic immunoglobulin folds consisting of beta-sandwich domains, forming variable regions that bind specific antigens.

Question 8: Thiamine (Vitamin B1) deficiency causes which neurological and cardiovascular condition?

Pellagra
Scurvy
Beriberi (Correct answer)
Rickets

Correct answer: Beriberi

Thiamine deficiency causes beriberi, characterized by peripheral neuropathy (dry beriberi) or cardiovascular dysfunction (wet beriberi), due to impaired pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase.

Question 9: Which disaccharide is composed of glucose and fructose joined by an alpha(1→2) glycosidic bond?

Maltose
Cellobiose
Sucrose (Correct answer)
Lactose

Correct answer: Sucrose

Sucrose (table sugar) consists of glucose and fructose linked by an unusual alpha(1→2) glycosidic bond involving both anomeric carbons, making it a non-reducing sugar.

Question 10: Chaperone proteins assist in which process?

Protein degradation
Post-translational glycosylation
Correct protein folding (Correct answer)
mRNA translation initiation

Correct answer: Correct protein folding

Molecular chaperones bind to unfolded or misfolded proteins and provide a protected environment that facilitates correct folding, preventing aggregation.

Question 11: Which of the following is true about the DNA replication?

It is semiconservative, where each daughter DNA molecule has one parent and one new strand. (Correct answer)
It does not require a primer to start synthesis.
It is conservative, where both parent strands remain together.
It occurs only during mitosis.

Correct answer: It is semiconservative, where each daughter DNA molecule has one parent and one new strand.

DNA replication is a semiconservative process, meaning that each new double helix consists of one original (parental) strand and one newly synthesized strand. This mechanism ensures that genetic information is accurately copied and passed on to daughter cells. The parental strand serves as a template for the synthesis of the new complementary strand.

Question 12: Heme biosynthesis requires which vitamin as a cofactor for the enzyme delta-aminolevulinate synthase?

Biotin
Pyridoxal phosphate (B6) (Correct answer)
Folate (B9)
Vitamin B12 (cobalamin)

Correct answer: Pyridoxal phosphate (B6)

Delta-aminolevulinate synthase, which catalyzes the first committed step of heme synthesis, requires PLP (pyridoxal phosphate) as a cofactor for the condensation of glycine and succinyl-CoA.

Question 13: Which amino acid can form disulfide bonds that stabilize protein structure?

Methionine
Threonine
Serine
Cysteine (Correct answer)

Correct answer: Cysteine

Cysteine residues contain thiol (-SH) groups that can be oxidized to form covalent disulfide (-S-S-) bonds between chains or within a single chain.

Question 14: The pentose phosphate pathway (PPP) primarily generates which two products needed for biosynthesis?

ATP and NADH
Pyruvate and lactate
NADPH and ribose-5-phosphate (Correct answer)
Acetyl-CoA and oxaloacetate

Correct answer: NADPH and ribose-5-phosphate

The oxidative PPP produces NADPH for reductive biosynthesis and antioxidant defense, and ribose-5-phosphate for nucleotide synthesis.

Question 15: Phosphofructokinase-1 (PFK-1) is the key regulatory enzyme of glycolysis. Which molecule allosterically activates it?

AMP (Correct answer)
Citrate
ATP
Acetyl-CoA

Correct answer: AMP

AMP (and ADP) allosterically activates PFK-1, signaling low energy status in the cell and stimulating glycolysis to increase ATP production.

Question 16: Vitamin D3 (cholecalciferol) is hydroxylated first in the liver to 25-hydroxyvitamin D, then in the kidney to its active form:

1,24-dihydroxyvitamin D
24,25-dihydroxyvitamin D
1,25-dihydroxyvitamin D (calcitriol) (Correct answer)
7-dehydrocholesterol

Correct answer: 1,25-dihydroxyvitamin D (calcitriol)

1,25-dihydroxyvitamin D₃ (calcitriol) is the biologically active hormone that binds to nuclear vitamin D receptors to increase intestinal calcium absorption and regulate bone mineralization.

Question 17: Which reaction does hexokinase catalyze in the first step of glycolysis?

Fructose-6-phosphate → fructose-1,6-bisphosphate
Glucose → glucose-6-phosphate (using ATP) (Correct answer)
Glucose → glucose-1-phosphate
Glucose-6-phosphate → fructose-6-phosphate

Correct answer: Glucose → glucose-6-phosphate (using ATP)

Hexokinase catalyzes the phosphorylation of glucose to glucose-6-phosphate using ATP, trapping glucose in the cell and committing it to metabolism.

Question 18: Carnitine plays an essential role in fatty acid oxidation by:

Activating fatty acids to acyl-CoA in the cytosol
Transporting long-chain fatty acyl groups across the inner mitochondrial membrane (Correct answer)
Serving as a coenzyme for beta-oxidation enzymes
Regenerating CoA from acetyl-CoA

Correct answer: Transporting long-chain fatty acyl groups across the inner mitochondrial membrane

Carnitine shuttles long-chain fatty acyl groups across the inner mitochondrial membrane as acylcarnitine, which is essential because the inner membrane is impermeable to acyl-CoA.

Question 19: In the pyrimidine salvage pathway, uridine is converted directly to UMP by which enzyme?

Uridine phosphorylase
CTP synthetase
Uridine kinase (Correct answer)
UMP synthase

Correct answer: Uridine kinase

Uridine kinase phosphorylates uridine (using ATP) to produce UMP in the salvage pathway, allowing cells to recycle preformed pyrimidine nucleosides without de novo synthesis.

Question 20: Niacin (Vitamin B3) is a precursor for which essential electron carrier coenzymes?

FAD and FMN
Pyridoxal phosphate and pyridoxamine
NAD⁺ and NADP⁺ (Correct answer)
Coenzyme A and acetyl-CoA

Correct answer: NAD⁺ and NADP⁺

Niacin (nicotinic acid/nicotinamide) is the precursor for NAD⁺ and NADP⁺, which are crucial electron carriers in oxidative metabolism and anabolic reactions respectively.

Question 21: The saponification number (saponification value) of a fat is a measure of:

The number of free hydroxyl groups
The degree of unsaturation
The average molecular weight of fatty acids (Correct answer)
The amount of glycerol present

Correct answer: The average molecular weight of fatty acids

The saponification value reflects the amount of alkali required to saponify a fat; higher values indicate shorter-chain fatty acids, since more moles (and thus more alkali) are needed per gram.

Question 22: Cyclic AMP (cAMP) functions as a second messenger primarily by activating which intracellular target?

Protein kinase C (PKC)
Calmodulin-dependent kinase
Protein kinase A (PKA) (Correct answer)
Phospholipase C

Correct answer: Protein kinase A (PKA)

cAMP binds to the regulatory subunits of protein kinase A, causing their dissociation and releasing active catalytic subunits that phosphorylate serine/threonine residues on downstream target proteins.

Question 23: In transcription, which of the following statements is correct?

RNA polymerase synthesizes RNA in the 5' to 3' direction. (Correct answer)
Transcription occurs in the cytoplasm of eukaryotic cells.
Both DNA strands are transcribed into RNA.
The newly synthesized RNA remains permanently bound to the DNA template.

Correct answer: RNA polymerase synthesizes RNA in the 5' to 3' direction.

During transcription, RNA polymerase synthesizes RNA in the 5' to 3' direction, reading the DNA template strand in the 3' to 5' direction. This directional synthesis is a fundamental characteristic of nucleic acid polymerization. Only one DNA strand serves as the template for RNA synthesis, and in eukaryotes, this process occurs in the nucleus.

Question 24: PRPP (5-phosphoribosyl-1-pyrophosphate) is best described as which of the following?

A substrate exclusive to pyrimidine de novo synthesis
An allosteric inhibitor of purine salvage enzymes
The final catabolic product of both purines and pyrimidines
An activated ribose donor used in both de novo synthesis and salvage pathways (Correct answer)

Correct answer: An activated ribose donor used in both de novo synthesis and salvage pathways

PRPP donates the activated ribose-5-phosphate moiety in both de novo nucleotide synthesis and salvage pathways for purines (HGPRT, APRT) and pyrimidines (OPRT).

Question 25: Familial hypercholesterolemia is caused by a defect in:

LDL receptor function (Correct answer)
Cholesterol ester transfer protein (CETP)
HMG-CoA reductase activity
Bile acid synthesis

Correct answer: LDL receptor function

Familial hypercholesterolemia is typically caused by loss-of-function mutations in the LDL receptor gene, impairing LDL uptake and causing elevated plasma LDL cholesterol.

Question 26: Which of the following best describes passive transport?

Diffusion of molecules down their concentration gradient without energy input (Correct answer)
Use of ATP to pump ions across the membrane
Movement of substances against their concentration gradient using energy
Endocytosis of large molecules into the cell

Correct answer: Diffusion of molecules down their concentration gradient without energy input

Passive transport describes the movement of substances across a cell membrane down their concentration gradient, meaning from an area of higher concentration to an area of lower concentration. This process does not require the cell to expend metabolic energy (ATP) because it is driven by the natural tendency of molecules to spread out. Examples include simple diffusion and facilitated diffusion.

Question 27: Glycogen is the storage polysaccharide in animals. Its branching involves which type of linkage?

Alpha(1→2) linkages throughout
Beta(1→3) linkages at branch points
Beta(1→4) linkages only
Alpha(1→4) in chains with alpha(1→6) at branch points (Correct answer)

Correct answer: Alpha(1→4) in chains with alpha(1→6) at branch points

Glycogen has linear alpha(1→4)-linked glucose chains with alpha(1→6) branch points approximately every 8–12 residues, allowing rapid glucose release from multiple ends simultaneously.

Question 28: Pyruvate dehydrogenase complex (PDC) converts pyruvate to acetyl-CoA with release of:

CO₂ and NADH (Correct answer)
H₂O and NADPH
CO₂ and FADH₂
CO₂ and ATP

Correct answer: CO₂ and NADH

PDC catalyzes the oxidative decarboxylation of pyruvate: pyruvate + CoA + NAD⁺ → acetyl-CoA + CO₂ + NADH, using five cofactors including thiamine pyrophosphate.

Question 29: Which bond is primarily responsible for maintaining the tertiary structure of a protein in an aqueous environment?

Ionic bonds only
Covalent disulfide bonds only
Peptide bonds
Hydrophobic interactions (Correct answer)

Correct answer: Hydrophobic interactions

Hydrophobic interactions drive the burial of nonpolar side chains in the protein core and are the dominant force in tertiary structure stabilization.

Question 30: Methotrexate inhibits nucleotide synthesis by targeting which enzyme?

Dihydrofolate reductase (DHFR) (Correct answer)
Ribonucleotide reductase
PRPP synthetase
Thymidylate synthase

Correct answer: Dihydrofolate reductase (DHFR)

Methotrexate competitively inhibits DHFR, preventing reduction of dihydrofolate to tetrahydrofolate; without THF cofactors, one-carbon transfer reactions required for both purine ring synthesis and dTMP formation are blocked.

Question 31: In a Western blot, after SDS-PAGE and transfer to membrane, what is the purpose of the primary antibody?

To generate a detectable signal directly
To transfer protein from gel to membrane
To specifically recognize the target protein (Correct answer)
To block non-specific binding sites

Correct answer: To specifically recognize the target protein

The primary antibody binds specifically to the target protein (antigen) on the membrane, providing specificity; a secondary antibody conjugated to an enzyme or fluorophore then generates the detectable signal.

Question 32: Cholesterol in animal cell membranes functions to:

Serve as the primary energy storage lipid
Increase membrane permeability to ions
Provide receptor sites for hormones only
Modulate membrane fluidity (Correct answer)

Correct answer: Modulate membrane fluidity

Cholesterol inserts between phospholipid tails, reducing fluidity at high temperatures and preventing crystallization at low temperatures, thereby buffering membrane fluidity.

Question 33: Which type of fatty acid contains one double bond?

Saturated fatty acid
Polyunsaturated fatty acid
Trans fatty acid
Monounsaturated fatty acid (Correct answer)

Correct answer: Monounsaturated fatty acid

Monounsaturated fatty acids contain exactly one carbon-carbon double bond, such as oleic acid (18:1, Δ9), and are liquid at room temperature.

Question 34: What is the final end product of purine catabolism in humans?

Allantoin
Xanthine
Hypoxanthine
Uric acid (Correct answer)

Correct answer: Uric acid

Humans lack uricase and therefore cannot convert uric acid to allantoin as most other mammals do; uric acid is thus the terminal product of purine breakdown, and its accumulation causes gout.

Question 35: Which sugar transporter (GLUT) is expressed in pancreatic beta-cells and serves as a glucose sensor?

GLUT5
GLUT4
GLUT2 (Correct answer)
GLUT1

Correct answer: GLUT2

GLUT2 in pancreatic beta-cells has a high Km for glucose, allowing glucose uptake proportional to blood glucose concentration, enabling beta-cells to sense and respond to glucose levels.

Question 36: What is the isoelectric point (pI) of a protein?

The pH at which the protein has maximum enzymatic activity
The pH at which the protein carries no net charge (Correct answer)
The pH at which the protein has maximum solubility
The pH at which the protein denatures

Correct answer: The pH at which the protein carries no net charge

The isoelectric point is the pH at which the net charge of the protein is zero, meaning positive and negative charges are balanced.

Question 37: Collagen's unique triple-helix structure is stabilized primarily by which amino acid's repeated occurrence?

Glycine
Both B and C (Correct answer)
Alanine
Proline and hydroxyproline

Correct answer: Both B and C

Collagen's Gly-X-Y repeat sequence requires glycine (smallest residue) at every third position, and proline/hydroxyproline at X and Y positions to stabilize the helix with hydrogen bonds.

Question 38: During the signal transduction process, what role does a second messenger, such as cAMP, play?

Degrades the signaling molecule
Directly activates the receptor on the cell surface
Amplifies and transmits the signal inside the cell (Correct answer)
Acts as the primary messenger outside the cell

Correct answer: Amplifies and transmits the signal inside the cell

In signal transduction, a second messenger like cAMP amplifies and transmits the signal received by a cell surface receptor to various target molecules inside the cell. It acts as an intracellular relay, propagating the initial signal throughout the cytoplasm and initiating a cascade of biochemical events. This amplification ensures a robust and coordinated cellular response to external stimuli.

Question 39: The Bradford assay measures protein concentration using which dye?

Coomassie Brilliant Blue G-250 (Correct answer)
Lowry reagent
BCA (bicinchoninic acid)
Biuret reagent (copper sulfate)

Correct answer: Coomassie Brilliant Blue G-250

The Bradford assay uses Coomassie Brilliant Blue G-250, which shifts its absorbance maximum from 465 nm to 595 nm upon binding to proteins; the absorbance at 595 nm is proportional to protein concentration.

Question 40: Beta-oxidation of fatty acids occurs primarily in which cellular compartment?

Peroxisome only
Cytosol
Smooth ER
Mitochondrial matrix (Correct answer)

Correct answer: Mitochondrial matrix

Beta-oxidation of most fatty acids occurs in the mitochondrial matrix, where fatty acyl-CoA undergoes sequential rounds of oxidation to produce acetyl-CoA, NADH, and FADH₂.

Question 41: Which metabolic pathway is activated during prolonged starvation to supply energy to the brain?

Glycolysis
Beta-oxidation
Ketogenesis (Correct answer)
Citric acid cycle

Correct answer: Ketogenesis

During prolonged starvation, the body activates ketogenesis to provide an alternative energy source for the brain when glucose reserves are severely depleted. The liver produces ketone bodies from fatty acids, which can then be utilized by the brain for energy. This metabolic adaptation helps conserve muscle protein by reducing the need for gluconeogenesis from amino acids.

Question 42: The Cori cycle involves the transfer of metabolites between muscle and liver. Muscle exports lactate; the liver converts it back to:

Glycogen stored permanently in the liver
Glucose via gluconeogenesis, which is returned to muscle (Correct answer)
Acetyl-CoA for ketogenesis
Pyruvate only, which is excreted

Correct answer: Glucose via gluconeogenesis, which is returned to muscle

In the Cori cycle, lactate from anaerobic muscle glycolysis travels to the liver, where gluconeogenesis converts it back to glucose, which is released into the blood for muscle use.

Question 43: NAD+ is synthesized in cells using which vitamin as the key precursor of the nicotinamide ring?

Pyridoxine (B6)
Niacin (B3) (Correct answer)
Riboflavin (B2)
Thiamine (B1)

Correct answer: Niacin (B3)

Niacin (vitamin B3), supplied as nicotinic acid or nicotinamide, provides the nicotinamide moiety of NAD+ and NADP+; deficiency causes pellagra and impairs all NAD+-dependent redox reactions.

Question 44: The cooperative binding of oxygen to hemoglobin is best described by which model?

Induced fit model
Lock-and-key model
Michaelis-Menten model
Concerted (MWC) model (Correct answer)

Correct answer: Concerted (MWC) model

The concerted (Monod-Wyman-Changeux) model proposes that all subunits switch simultaneously between T (tense, low affinity) and R (relaxed, high affinity) states.

Question 45: Fatty acid synthesis occurs in which cellular compartment?

Mitochondrial matrix
Cytosol (Correct answer)
Smooth ER
Peroxisome

Correct answer: Cytosol

Fatty acid synthesis by fatty acid synthase (FAS) occurs in the cytosol, using NADPH as the reducing agent and malonyl-CoA as the two-carbon donor.

Question 46: CRISPR-Cas9 gene editing creates targeted double-strand DNA breaks using which component to guide it to the correct genomic location?

A single guide RNA (sgRNA) complementary to the target sequence (Correct answer)
A restriction endonuclease with sequence specificity
A DNA-binding zinc finger protein
A TAL effector nuclease (TALEN)

Correct answer: A single guide RNA (sgRNA) complementary to the target sequence

The single guide RNA (sgRNA) base-pairs with the complementary target DNA sequence, directing the Cas9 nuclease to cut precisely at that location in the genome.

Question 47: Orotic aciduria type I is caused by a deficiency of which bifunctional enzyme?

ATCase
UMP synthase (UMPS) (Correct answer)
CPS II
Dihydroorotate dehydrogenase

Correct answer: UMP synthase (UMPS)

UMP synthase possesses both orotate phosphoribosyltransferase and OMP decarboxylase activities; its deficiency blocks the last two steps of pyrimidine synthesis, causing orotic acid accumulation and megaloblastic anemia.

Question 48: Biotin serves as a cofactor for carboxylase enzymes by acting as a carrier of:

Acyl groups
Amino groups
CO₂ (carboxyl groups) (Correct answer)
Methyl groups

Correct answer: CO₂ (carboxyl groups)

Biotin is covalently attached to carboxylase enzymes and serves as a mobile carboxyl group carrier, transferring CO₂ in ATP-dependent carboxylation reactions such as those catalyzed by pyruvate carboxylase and ACC.

Question 49: Which phospholipase cleaves the fatty acid at the sn-2 position of glycerophospholipids to release arachidonic acid?

Phospholipase A2 (Correct answer)
Phospholipase C
Phospholipase A1
Phospholipase D

Correct answer: Phospholipase A2

Phospholipase A2 cleaves the ester bond at the sn-2 position, releasing arachidonic acid (or other fatty acids) and a lysophospholipid.

Question 50: Protein denaturation by heat or chemicals disrupts all levels of structure EXCEPT which?

Secondary
Quaternary
Tertiary
Primary (Correct answer)

Correct answer: Primary

Denaturation disrupts non-covalent interactions and disulfide bonds but does not break the covalent peptide bonds that define primary structure.

Question 51: Flow cytometry can be used to:

Separate proteins by molecular weight
Measure enzyme kinetics in real time
Count and sort cells based on fluorescent markers (Correct answer)
Determine protein tertiary structure

Correct answer: Count and sort cells based on fluorescent markers

Flow cytometry passes cells single-file through a laser beam and detects fluorescent signals from labeled antibodies or dyes, allowing rapid analysis and sorting of cell populations.

Question 52: Vitamin B12 (cobalamin) is required for which two reactions in humans?

Fatty acid synthesis and glycolysis
Heme synthesis and bilirubin metabolism
DNA replication and RNA transcription
Methionine synthesis and methylmalonyl-CoA mutase reaction (Correct answer)

Correct answer: Methionine synthesis and methylmalonyl-CoA mutase reaction

Cobalamin is required for methionine synthase (converting homocysteine to methionine) and methylmalonyl-CoA mutase (converting methylmalonyl-CoA to succinyl-CoA) in the metabolism of odd-chain fatty acids and some amino acids.

Question 53: In enzyme kinetics, the Michaelis constant (Km) is defined as:

The substrate concentration at half-maximal velocity (Correct answer)
The rate constant for product release
The maximum reaction velocity
The equilibrium constant for enzyme-substrate binding

Correct answer: The substrate concentration at half-maximal velocity

Km is the substrate concentration at which reaction velocity equals Vmax/2; it is a measure of substrate affinity (lower Km = higher affinity) but is not a true equilibrium dissociation constant.

Question 54: 2,3-Bisphosphoglycerate (2,3-BPG) affects hemoglobin oxygen binding by:

Increasing oxygen affinity by stabilizing the R state
Promoting dissociation of hemoglobin subunits
Decreasing oxygen affinity by stabilizing the T state (Correct answer)
Blocking the heme iron from binding oxygen

Correct answer: Decreasing oxygen affinity by stabilizing the T state

2,3-BPG binds in the central cavity of deoxyhemoglobin and stabilizes the low-affinity T state, promoting oxygen release to tissues.

Question 55: Folate (Vitamin B9) deficiency causes megaloblastic anemia primarily because folate is required for synthesis of:

Thymidylate (dTMP) for DNA synthesis (Correct answer)
Globin protein chains
Iron-binding proteins
Heme precursors

Correct answer: Thymidylate (dTMP) for DNA synthesis

Folate as 5,10-methylene-THF is required for thymidylate synthase to convert dUMP to dTMP; without sufficient folate, DNA synthesis is impaired, causing large, immature red blood cells.

Question 56: Which vitamin deficiency is associated with Wernicke-Korsakoff syndrome, commonly seen in chronic alcoholics?

Vitamin B12
Folate (B9)
Thiamine (B1) (Correct answer)
Niacin (B3)

Correct answer: Thiamine (B1)

Thiamine deficiency due to poor diet and impaired absorption in alcoholics causes Wernicke-Korsakoff syndrome, characterized by ophthalmoplegia, ataxia, and confabulation from neuronal damage.

Question 57: What is the primary function of the phospholipid bilayer in the cell membrane?

Structural support for the cytoskeleton
Transport of proteins into the nucleus
Formation of a selective barrier for molecules (Correct answer)
Production of ATP

Correct answer: Formation of a selective barrier for molecules

The primary function of the phospholipid bilayer in the cell membrane is to form a selective barrier that controls the movement of molecules into and out of the cell. Its hydrophobic interior prevents the free passage of most polar molecules, while its overall structure maintains cellular integrity and homeostasis. This selectivity is vital for cell function.

Question 58: The anomeric carbon of glucose is carbon number:

C-2
C-1 (Correct answer)
C-6
C-3

Correct answer: C-1

C-1 of glucose is the anomeric carbon, the former carbonyl carbon that becomes the new chiral center when the open-chain form cyclizes to form alpha or beta glucopyranose.

Question 59: Ubiquitin-mediated proteasomal degradation requires that ubiquitin chains be attached to which amino acid residue of the target protein?

Lysine (Correct answer)
Aspartate
Cysteine
Serine

Correct answer: Lysine

Ubiquitin is attached via an isopeptide bond to lysine residues on target proteins; a chain of at least four ubiquitin molecules targets the protein to the 26S proteasome.

Question 60: Galactosemia results from accumulation of galactose-1-phosphate due to deficiency of which enzyme?

UDP-galactose 4-epimerase
Galactose-1-phosphate uridylyltransferase (GALT) (Correct answer)
Phosphoglucomutase
Galactokinase

Correct answer: Galactose-1-phosphate uridylyltransferase (GALT)

Classic galactosemia is caused by GALT deficiency, which prevents conversion of galactose-1-phosphate to UDP-galactose, leading to toxic accumulation of galactose-1-phosphate in the liver, brain, and kidneys.

Question 61: Reverse transcriptase PCR (RT-PCR) begins with which step?

Conversion of mRNA to complementary DNA (cDNA) (Correct answer)
Amplification of genomic DNA
Ligation of DNA fragments
Denaturation of double-stranded DNA

Correct answer: Conversion of mRNA to complementary DNA (cDNA)

RT-PCR starts by using reverse transcriptase to synthesize a complementary DNA (cDNA) copy of the mRNA template, which can then be amplified by conventional PCR.

Question 62: Eicosanoids (prostaglandins, thromboxanes, leukotrienes) are derived from which fatty acid precursor?

Arachidonic acid (20:4) (Correct answer)
Palmitic acid (16:0)
Linoleic acid (18:2)
Stearic acid (18:0)

Correct answer: Arachidonic acid (20:4)

Arachidonic acid (20:4, ω-6) is released from membrane phospholipids by phospholipase A2 and serves as the substrate for cyclooxygenases and lipoxygenases to produce eicosanoids.

Question 63: Allopurinol treats hyperuricemia and gout by inhibiting which enzyme?

HGPRT
Xanthine oxidase (Correct answer)
Adenosine deaminase
Purine nucleoside phosphorylase

Correct answer: Xanthine oxidase

Allopurinol (and its oxidized form oxipurinol) inhibits xanthine oxidase, the enzyme that converts hypoxanthine to xanthine and xanthine to uric acid, thereby lowering serum urate levels.

Question 64: Hemoglobin is an example of a protein with which level of structural organization?

Primary
Quaternary (Correct answer)
Secondary
Tertiary

Correct answer: Quaternary

Quaternary structure describes the assembly of two or more polypeptide subunits, as seen in hemoglobin's four globin chains.

Question 65: Which of the following best describes the function of tRNA in translation?

Catalyzes peptide bond formation between amino acids.
Carries amino acids to the ribosome and matches them to the mRNA codon. (Correct answer)
Acts as the template for protein synthesis.
Transports mRNA from the nucleus to the cytoplasm.

Correct answer: Carries amino acids to the ribosome and matches them to the mRNA codon.

Transfer RNA (tRNA) plays a crucial role in translation by acting as an adaptor molecule. Each tRNA carries a specific amino acid and has an anticodon that base-pairs with a complementary codon on the mRNA molecule. This ensures that the correct amino acid is delivered to the ribosome and incorporated into the growing polypeptide chain according to the mRNA sequence.

Question 66: Which prosthetic group is found in myoglobin and hemoglobin, enabling oxygen binding?

Heme group (Correct answer)
Flavin adenine dinucleotide (FAD)
Biotin
Pyridoxal phosphate (PLP)

Correct answer: Heme group

The heme group contains an iron atom (Fe²⁺) coordinated within a porphyrin ring, which reversibly binds molecular oxygen.

Question 67: Mass spectrometry in proteomics identifies proteins by measuring:

The mass-to-charge ratio (m/z) of peptide fragments (Correct answer)
Their fluorescence emission wavelength
Their migration speed in an electric field without gel
Their UV absorbance spectra

Correct answer: The mass-to-charge ratio (m/z) of peptide fragments

Mass spectrometry measures the m/z ratio of ionized peptide fragments with high precision; the resulting mass spectra are matched against protein databases for identification.

Question 68: Riboflavin (Vitamin B2) is the precursor for which two coenzymes?

NAD⁺ and NADP⁺
FAD and FMN (Correct answer)
Biotin and lipoic acid
CoA and acyl-CoA

Correct answer: FAD and FMN

Riboflavin is the precursor for flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN), which serve as prosthetic groups for flavoenzymes in oxidative metabolism.

Question 69: Which nucleotide intermediate serves as the branch point from which both AMP and GMP are synthesized in the purine de novo pathway?

IMP (inosine monophosphate) (Correct answer)
PRPP
XMP
Adenylosuccinate

Correct answer: IMP (inosine monophosphate)

IMP is the first complete purine nucleotide formed in de novo synthesis and serves as the branch-point intermediate from which AMP and GMP diverge via separate enzymatic routes.

Question 70: The Bohr effect refers to which property of hemoglobin?

Increased O₂ affinity when 2,3-BPG binds
Decreased O₂ affinity at lower pH and higher CO₂ (Correct answer)
Increased O₂ affinity at low temperature
Decreased O₂ affinity at high altitude

Correct answer: Decreased O₂ affinity at lower pH and higher CO₂

The Bohr effect describes hemoglobin's decreased oxygen affinity in acidic, high-CO₂ environments (active tissues), facilitating O₂ release where it is needed most.

Question 71: Vitamin C (ascorbate) is required for collagen synthesis because it is needed for:

Secretion of procollagen from fibroblasts
Cross-linking of collagen fibrils
Glycosylation of collagen fibers
Hydroxylation of proline and lysine residues (Correct answer)

Correct answer: Hydroxylation of proline and lysine residues

Ascorbate is a cofactor for prolyl and lysyl hydroxylases, which add hydroxyl groups to proline and lysine residues essential for helix stability and cross-linking.

Question 72: Lipoic acid (lipoate) functions as a cofactor in alpha-keto acid dehydrogenase complexes by:

Acting as a transient carrier of acyl groups and electrons via its dithiol/disulfide couple (Correct answer)
Donating CO₂ for carboxylation reactions
Carrying amino groups between reactions
Serving as an electron acceptor in the electron transport chain

Correct answer: Acting as a transient carrier of acyl groups and electrons via its dithiol/disulfide couple

Lipoic acid is covalently bound to a lysine residue and cycles between its oxidized (disulfide) and reduced (dithiol) forms to transfer acyl groups and electrons during oxidative decarboxylation.

Question 73: Which fat-soluble vitamin acts as the most important lipid-soluble antioxidant in membranes?

Vitamin A (retinol)
Vitamin D (calcitriol)
Vitamin E (alpha-tocopherol) (Correct answer)
Vitamin K (phylloquinone)

Correct answer: Vitamin E (alpha-tocopherol)

Vitamin E (alpha-tocopherol) is the primary chain-breaking antioxidant in lipid membranes, donating a hydrogen atom to lipid peroxyl radicals to terminate lipid peroxidation chain reactions.

Question 74: What distinguishes a nucleotide from a nucleoside?

Nucleosides contain deoxyribose; nucleotides contain ribose
Nucleotides are found only in DNA; nucleosides are found only in RNA
Nucleosides contain a phosphate group; nucleotides do not
Nucleotides contain one or more phosphate groups; nucleosides do not (Correct answer)

Correct answer: Nucleotides contain one or more phosphate groups; nucleosides do not

A nucleotide consists of a nitrogenous base, a pentose sugar, and one or more phosphate groups, whereas a nucleoside contains only the base and the sugar without any phosphate.

Question 75: Adenosine deaminase (ADA) deficiency causes which clinical condition, and why?

Hemolytic anemia, due to erythrocyte purine depletion
Gout, due to uric acid overproduction from adenosine catabolism
Orotic aciduria, due to blocked pyrimidine conversion
Severe combined immunodeficiency (SCID), due to toxic dATP accumulation in lymphocytes (Correct answer)

Correct answer: Severe combined immunodeficiency (SCID), due to toxic dATP accumulation in lymphocytes

ADA deficiency allows deoxyadenosine to accumulate and be phosphorylated to dATP, which is selectively toxic to lymphocytes (inhibits ribonucleotide reductase and induces apoptosis), resulting in SCID.

Question 76: Lipid rafts in cell membranes are enriched in which components?

Lysophospholipids and free fatty acids
Cholesterol and sphingolipids (Correct answer)
Cardiolipin and phosphatidylserine
Phosphatidylethanolamine and unsaturated fatty acids

Correct answer: Cholesterol and sphingolipids

Lipid rafts are dynamic microdomains enriched in cholesterol and sphingolipids that form a more ordered (liquid-ordered) phase and serve as platforms for signaling proteins.

Question 77: Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors. Which of the following is NOT a gluconeogenic precursor?

Lactate
Oxaloacetate
Acetyl-CoA (Correct answer)
Glycerol

Correct answer: Acetyl-CoA

Acetyl-CoA cannot be converted to net glucose in mammals because the two carbons entering the TCA cycle as acetyl-CoA are lost as CO₂ before the cycle can regenerate oxaloacetate.

Question 78: Which lipid class forms the structural backbone of the cell membrane bilayer?

Cholesterol
Sphingomyelin
Phospholipids (Correct answer)
Triglycerides

Correct answer: Phospholipids

Phospholipids are amphipathic molecules with a hydrophilic phosphate head group and hydrophobic fatty acid tails that spontaneously form bilayers in aqueous environments.

Question 79: Sphingolipids differ from glycerophospholipids in that their backbone is:

Sphingosine (Correct answer)
Cholesterol
Serine
Glycerol

Correct answer: Sphingosine

Sphingolipids are built on a sphingosine backbone (an amino alcohol) rather than glycerol, with a fatty acid attached via an amide bond to form ceramide.

Question 80: Which enzyme is the rate-limiting step in cholesterol biosynthesis?

HMG-CoA synthase
HMG-CoA reductase (Correct answer)
Lanosterol synthase
Squalene synthase

Correct answer: HMG-CoA reductase

HMG-CoA reductase catalyzes the conversion of HMG-CoA to mevalonate and is the primary regulated, rate-limiting step in cholesterol synthesis; statins inhibit this enzyme.

Question 81: Iron absorption from the diet is enhanced by Vitamin C because ascorbate:

Increases hepcidin levels to regulate iron stores
Binds transferrin to increase iron transport
Reduces Fe³⁺ to Fe²⁺, which is better absorbed by DMT1 (Correct answer)
Forms a chelate with iron that prevents oxidation

Correct answer: Reduces Fe³⁺ to Fe²⁺, which is better absorbed by DMT1

Ascorbate reduces dietary non-heme iron from the insoluble Fe³⁺ (ferric) form to the soluble Fe²⁺ (ferrous) form, which is the substrate for the divalent metal transporter DMT1 in intestinal cells.

Question 82: Which technique separates proteins based on their isoelectric points?

Affinity chromatography
Size-exclusion chromatography
Isoelectric focusing (Correct answer)
SDS-PAGE

Correct answer: Isoelectric focusing

Isoelectric focusing separates proteins in a pH gradient; each protein migrates until it reaches the pH equal to its pI, where it has no net charge and stops.

Question 83: During fatty acid synthesis, malonyl-CoA is formed from acetyl-CoA by which enzyme?

HMG-CoA reductase
Acetyl-CoA carboxylase (ACC) (Correct answer)
Citrate lyase
Fatty acid synthase (FAS)

Correct answer: Acetyl-CoA carboxylase (ACC)

Acetyl-CoA carboxylase (ACC) catalyzes the ATP-dependent carboxylation of acetyl-CoA to malonyl-CoA, the committed step in fatty acid synthesis.

Question 84: Vitamin C (ascorbic acid) acts as an antioxidant primarily by:

Activating superoxide dismutase
Inhibiting lipid peroxidation by binding membranes
Chelating iron to prevent Fenton reactions
Donating electrons to neutralize free radicals (Correct answer)

Correct answer: Donating electrons to neutralize free radicals

Ascorbic acid is a potent reducing agent that donates electrons to neutralize reactive oxygen species and can regenerate other antioxidants such as vitamin E.

Question 85: During oxidative phosphorylation, the final electron acceptor in the electron transport chain is:

Oxygen (O₂) (Correct answer)
NAD⁺
Water (H₂O)
FAD

Correct answer: Oxygen (O₂)

During oxidative phosphorylation, the electron transport chain passes electrons through a series of protein complexes, ultimately delivering them to oxygen. Oxygen acts as the final electron acceptor, combining with electrons and protons to form water. This crucial step drives the entire electron transport process and enables ATP synthesis.

Question 86: Southern blotting detects:

Specific RNA sequences
Specific DNA sequences (Correct answer)
Specific protein antigens
Specific lipid molecules

Correct answer: Specific DNA sequences

Southern blotting (named after Edward Southern) involves electrophoresis of restriction-digested DNA, transfer to a membrane, and hybridization with a labeled DNA probe to detect specific sequences.

Question 87: Ribonucleotide reductase reduces ribonucleotides to deoxyribonucleotides using which catalytic mechanism?

A stable tyrosyl radical stabilized by a dinuclear iron center (Correct answer)
NAD+-dependent oxidation
Pyridoxal phosphate-mediated transamination
FAD-mediated hydride transfer

Correct answer: A stable tyrosyl radical stabilized by a dinuclear iron center

Ribonucleotide reductase class I uses a stable tyrosyl radical (generated by a diferric iron center in the R2 subunit) to initiate the radical-based mechanism that removes the 2'-OH of ribonucleotides.

Question 88: Under anaerobic conditions in muscle, pyruvate is converted to lactate to regenerate:

ATP
Acetyl-CoA
FADH₂
NAD⁺ (Correct answer)

Correct answer: NAD⁺

Lactate dehydrogenase reduces pyruvate to lactate, oxidizing NADH back to NAD⁺, which is required for glycolysis to continue producing ATP in the absence of oxygen.

Question 89: The polymerase chain reaction (PCR) requires which enzyme?

DNA ligase
Reverse transcriptase
Thermostable DNA polymerase (Taq polymerase) (Correct answer)
RNA polymerase

Correct answer: Thermostable DNA polymerase (Taq polymerase)

PCR uses a thermostable DNA polymerase (typically Taq from Thermus aquaticus) that retains activity through the high-temperature denaturation cycles required to separate DNA strands.

Question 90: Which of the following statements about the Cori cycle is TRUE?

It involves the direct synthesis of ATP in muscle cells.
It only operates in the presence of oxygen.
It occurs between the liver and brain.
Lactate produced in the muscles is converted back to glucose in the liver. (Correct answer)

Correct answer: Lactate produced in the muscles is converted back to glucose in the liver.

The Cori cycle describes the metabolic pathway where lactate produced during anaerobic glycolysis in active muscles is transported to the liver. In the liver, this lactate is converted back into glucose through gluconeogenesis. This newly synthesized glucose can then be released into the bloodstream to be used by muscles or other tissues, effectively recycling the lactate.

Question 91: Lesch-Nyhan syndrome results from a deficiency of which enzyme in purine metabolism?

Hypoxanthine-guanine phosphoribosyltransferase (HGPRT) (Correct answer)
Purine nucleoside phosphorylase
Xanthine oxidase
Adenosine deaminase

Correct answer: Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)

HGPRT deficiency impairs the purine salvage pathway, leading to massive uric acid overproduction, causing gout, intellectual disability, and self-mutilating behavior characteristic of Lesch-Nyhan syndrome.

Question 92: Two-dimensional gel electrophoresis (2D-PAGE) separates proteins first by isoelectric point, then by:

Charge using ion-exchange
Net charge at pH 7
Molecular weight using SDS-PAGE (Correct answer)
Hydrophobicity using reverse-phase electrophoresis

Correct answer: Molecular weight using SDS-PAGE

2D-PAGE uses isoelectric focusing (IEF) in the first dimension to separate proteins by pI, then SDS-PAGE in the second dimension to separate by molecular weight, giving a two-dimensional protein map.

Question 93: Gel filtration (size-exclusion) chromatography separates proteins such that:

Smaller proteins elute first
Proteins elute in order of hydrophobicity
Proteins elute in order of charge
Larger proteins elute first (Correct answer)

Correct answer: Larger proteins elute first

Larger proteins cannot enter the pores of the gel matrix and pass through the column faster (elute first), while smaller proteins enter the pores and take longer to elute.

Question 94: Pellagra, characterized by the 4 D's (Dermatitis, Diarrhea, Dementia, Death), is caused by deficiency of:

Riboflavin (B2)
Pyridoxine (B6)
Thiamine (B1)
Niacin (B3) (Correct answer)

Correct answer: Niacin (B3)

Pellagra is caused by niacin (vitamin B3) deficiency, which impairs NAD⁺/NADP⁺-dependent redox reactions throughout the body, affecting skin, gut, and nervous system.

Question 95: In oxidative phosphorylation, ATP synthesis is powered by:

A proton gradient across the mitochondrial inner membrane (Correct answer)
The transfer of electrons directly to ATP synthase
The release of carbon dioxide from the citric acid cycle
Substrate-level phosphorylation

Correct answer: A proton gradient across the mitochondrial inner membrane

During oxidative phosphorylation, ATP synthesis is powered by a proton gradient established across the mitochondrial inner membrane. The electron transport chain pumps protons into the intermembrane space, creating an electrochemical gradient. Protons then flow back into the matrix through ATP synthase, and the energy released drives the phosphorylation of ADP to ATP.

Question 96: Which enzyme catalyzes the committed (first unique) step of pyrimidine biosynthesis?

Carbamoyl phosphate synthetase II (CPS II)
Dihydroorotase
PRPP synthetase
Aspartate transcarbamoylase (ATCase) (Correct answer)

Correct answer: Aspartate transcarbamoylase (ATCase)

ATCase (aspartate transcarbamoylase) catalyzes the condensation of carbamoyl phosphate with aspartate, forming carbamoyl aspartate, which is the committed step unique to pyrimidine biosynthesis.

Question 97: Which level of protein structure refers to the sequence of amino acids in a polypeptide chain?

Primary structure (Correct answer)
Quaternary structure
Secondary structure
Tertiary structure

Correct answer: Primary structure

Primary structure is the linear sequence of amino acids linked by peptide bonds, which ultimately determines all higher levels of protein structure.

Question 98: Ketone bodies are produced in the liver primarily from excess:

Glucose
Acetyl-CoA (Correct answer)
Glycerol
Amino acids

Correct answer: Acetyl-CoA

When acetyl-CoA from fatty acid oxidation exceeds the capacity of the TCA cycle (e.g., during fasting), the liver converts excess acetyl-CoA into ketone bodies (acetoacetate, β-hydroxybutyrate, acetone).

Question 99: A signal peptide on a newly synthesized protein directs it to which cellular location for processing?

Rough endoplasmic reticulum (Correct answer)
Mitochondria
Nucleus
Cytosol

Correct answer: Rough endoplasmic reticulum

Signal peptides are N-terminal hydrophobic sequences recognized by signal recognition particle (SRP), which directs the ribosome to the rough ER for co-translational insertion.

Question 100: Which of the following nitrogenous bases is classified as a purine?

Adenine (Correct answer)
Cytosine
Uracil
Thymine

Correct answer: Adenine

Adenine and guanine are purines, characterized by a bicyclic structure of a pyrimidine ring fused to an imidazole ring, while cytosine, uracil, and thymine are monocyclic pyrimidines.

Question 101: The fluid mosaic model of the cell membrane was proposed by:

Lipmann and Krebs
Mitchell and Moyle
Watson and Crick
Singer and Nicolson (Correct answer)

Correct answer: Singer and Nicolson

Singer and Nicolson (1972) proposed the fluid mosaic model, describing the membrane as a dynamic lipid bilayer with embedded proteins that can move laterally.

Question 102: Which monosaccharide is the primary fuel for the brain under normal conditions?

Ribose
Glucose (Correct answer)
Galactose
Fructose

Correct answer: Glucose

Glucose is the brain's primary energy substrate under normal fed conditions, as the blood-brain barrier limits the transport of most other fuels.

Question 103: The enzyme lactase cleaves lactose into which two monosaccharides?

Galactose + fructose
Glucose + galactose (Correct answer)
Glucose + fructose
Glucose + glucose

Correct answer: Glucose + galactose

Lactase (beta-galactosidase) hydrolyzes the beta(1→4) glycosidic bond of lactose to release glucose and galactose, which are absorbed in the small intestine.

Question 104: Which of the following processes requires energy input in the form of ATP?

Osmosis
Active transport (Correct answer)
Facilitated diffusion
Simple diffusion

Correct answer: Active transport

Active transport is the process that requires energy input in the form of ATP to move molecules across a cell membrane. Unlike passive transport, active transport moves substances against their concentration gradient, from an area of lower concentration to an area of higher concentration. This energy-dependent process is crucial for maintaining specific ion concentrations and nutrient uptake.

Question 105: Glycogen synthase is activated when it is in which phosphorylation state?

Dephosphorylated (active) (Correct answer)
Phosphorylated (active)
Unrelated to phosphorylation
Phosphorylated (inactive) and glycogen-bound

Correct answer: Dephosphorylated (active)

Glycogen synthase is active in its dephosphorylated form; phosphorylation by kinases such as GSK-3 inhibits the enzyme, reducing glycogen synthesis.

Question 106: Which of the following statements about competitive inhibition is TRUE?

Competitive inhibitors bind to the active site of the enzyme. (Correct answer)
Competitive inhibitors reduce the Vmax of an enzyme.
Competitive inhibitors form covalent bonds with the enzyme.
Competitive inhibition cannot be overcome by increasing substrate concentration.

Correct answer: Competitive inhibitors bind to the active site of the enzyme.

Competitive inhibitors function by binding directly to the active site of an enzyme, competing with the natural substrate for access. Because they occupy the active site, they prevent the substrate from binding and undergoing catalysis. This type of inhibition can often be overcome by increasing the substrate concentration.

Question 107: During fasting, which process provides glucose for the body when glycogen stores are depleted?

Ketogenesis
Lipogenesis
Gluconeogenesis (Correct answer)
Glycogenesis

Correct answer: Gluconeogenesis

During fasting, when the body's glycogen stores are depleted, gluconeogenesis becomes the primary process to synthesize new glucose. This pathway, mainly in the liver, produces glucose from non-carbohydrate precursors like amino acids, lactate, and glycerol. Gluconeogenesis is vital for maintaining blood glucose levels to supply energy to glucose-dependent tissues, such as the brain.

Question 108: In the DNA double helix, adenine forms hydrogen bonds with which complementary base?

Thymine (Correct answer)
Guanine
Uracil
Cytosine

Correct answer: Thymine

Adenine pairs with thymine in DNA via two hydrogen bonds (A=T), following Chargaff's rules, while guanine pairs with cytosine via three hydrogen bonds.

Question 109: S-adenosylmethionine (SAM) is the principal methyl donor in biochemical methylation reactions. How is SAM synthesized?

From ATP and methionine by methionine adenosyltransferase (Correct answer)
From GTP and methionine by methionine adenosyltransferase
From AMP and homocysteine by methyltransferase
From PRPP and methionine by a transferase

Correct answer: From ATP and methionine by methionine adenosyltransferase

Methionine adenosyltransferase transfers the adenosyl group of ATP to methionine, forming SAM and releasing inorganic pyrophosphate and phosphate; SAM then donates its methyl group in reactions from DNA methylation to creatine synthesis.

Question 110: Insulin stimulates glucose uptake in muscle and adipose tissue primarily by:

Inhibiting glucagon receptor signaling
Promoting translocation of GLUT4 transporters to the plasma membrane (Correct answer)
Increasing hexokinase gene transcription directly
Activating GLUT2 transporters already in the membrane

Correct answer: Promoting translocation of GLUT4 transporters to the plasma membrane

Insulin signaling via PI3K/Akt promotes the translocation of GLUT4-containing vesicles from intracellular stores to the plasma membrane, rapidly increasing glucose uptake.

Question 111: What is the term for the movement of a phospholipid from one leaflet of the bilayer to the other?

Rotational diffusion
Vesicle budding
Lateral diffusion
Flip-flop (transverse diffusion) (Correct answer)

Correct answer: Flip-flop (transverse diffusion)

Flip-flop (transverse diffusion) is the movement of a lipid across the bilayer, which is thermodynamically unfavorable and requires flippase enzymes to occur rapidly.

Question 112: In the integration of metabolism, which pathway is the primary source of ATP during intense physical activity?

Oxidative phosphorylation
Glycolysis (Correct answer)
Fatty acid oxidation
Pentose phosphate pathway

Correct answer: Glycolysis

During intense physical activity, oxygen supply to muscles can become limited, leading to anaerobic conditions. In this scenario, glycolysis becomes the primary source of ATP, rapidly producing a small amount of energy without requiring oxygen. While less efficient than oxidative phosphorylation, it provides the quick bursts of energy necessary for high-intensity exercise.

Question 113: In a non-competitive inhibition mechanism, the inhibitor:

Covalently modifies the active site irreversibly
Binds only to the enzyme-substrate complex
Competes with substrate for the active site
Binds an allosteric site on either free enzyme or ES complex, reducing Vmax without changing Km (Correct answer)

Correct answer: Binds an allosteric site on either free enzyme or ES complex, reducing Vmax without changing Km

A non-competitive inhibitor binds an allosteric site independent of substrate binding, reducing Vmax without affecting Km because substrate binding and inhibitor binding are independent events.

Question 114: An ELISA (enzyme-linked immunosorbent assay) detects a target molecule using:

Radioactively labeled antibodies (RIA method)
Enzyme-conjugated antibodies producing a colorimetric signal (Correct answer)
Electric current to detect protein migration
Fluorescent dyes that stain all proteins nonspecifically

Correct answer: Enzyme-conjugated antibodies producing a colorimetric signal

ELISA uses antibodies conjugated to enzymes (e.g., HRP or alkaline phosphatase) that convert a substrate into a colored product, allowing quantitative detection of antigens.

Question 115: De novo purine nucleotide synthesis begins with which metabolite as the core scaffold?

Ribose-5-phosphate (Correct answer)
Xanthine
Inosine monophosphate (IMP)
Hypoxanthine

Correct answer: Ribose-5-phosphate

De novo purine synthesis starts with ribose-5-phosphate (from the pentose phosphate pathway), which is converted to PRPP (5-phosphoribosyl-1-pyrophosphate), the activated ribose donor for ring assembly.

Question 116: Which technique is used to determine the three-dimensional structure of proteins at atomic resolution using X-rays?

X-ray crystallography (Correct answer)
Cryo-electron microscopy (cryo-EM)
Circular dichroism (CD) spectroscopy
Nuclear magnetic resonance (NMR) spectroscopy

Correct answer: X-ray crystallography

X-ray crystallography diffracts X-rays through protein crystals and uses the resulting diffraction pattern to calculate electron density maps, from which atomic coordinates are determined.

Question 117: The net ATP yield from complete oxidation of one molecule of glucose through glycolysis and oxidative phosphorylation is approximately:

36-38 ATP
8 ATP
30-32 ATP (Correct answer)
2 ATP

Correct answer: 30-32 ATP

Using current estimates of the P/O ratio, complete oxidation of glucose yields approximately 30–32 ATP: 2 from glycolysis, 2 from the TCA cycle substrate-level phosphorylation, and ~26–28 from oxidative phosphorylation.

Question 118: Which hormone primarily regulates the metabolic processes in the fed state?

Cortisol
Insulin (Correct answer)
Epinephrine
Glucagon

Correct answer: Insulin

Insulin is the primary hormone that regulates metabolic processes in the fed state, when blood glucose levels are elevated after a meal. It promotes the uptake of glucose by cells, stimulates the synthesis of glycogen in the liver and muscles, and encourages fat storage. Insulin effectively lowers blood glucose and directs nutrients towards storage.

Question 119: Which of the following statements about enzymes is TRUE?

Enzyme activity is independent of temperature and pH.
Enzymes are consumed during the reaction.
Enzymes lower the activation energy of a reaction. (Correct answer)
All enzymes require cofactors to function.

Correct answer: Enzymes lower the activation energy of a reaction.

Enzymes are biological catalysts that accelerate biochemical reactions by lowering their activation energy. They achieve this by binding to specific substrates at their active site, stabilizing the transition state, and facilitating product formation. Enzymes are not consumed during the reaction and can be reused multiple times.

Question 120: Which technique measures real-time amplification of DNA during PCR by detecting fluorescent signal proportional to DNA amount?

Gel electrophoresis
Quantitative PCR (qPCR) (Correct answer)
DNA microarray
Southern blot

Correct answer: Quantitative PCR (qPCR)

Quantitative PCR (qPCR) monitors fluorescent signal (from SYBR Green or TaqMan probes) after each amplification cycle, allowing real-time quantification of starting template DNA or cDNA.

Question 121: Pyridoxal phosphate (PLP), the active form of Vitamin B6, is a cofactor for which class of enzymes?

Kinases
Aminotransferases (transaminases) and other amino acid-metabolizing enzymes (Correct answer)
Oxidoreductases only
Carboxylases

Correct answer: Aminotransferases (transaminases) and other amino acid-metabolizing enzymes

PLP forms a Schiff base with amino acid substrates and is essential for aminotransferases, amino acid decarboxylases, and many other enzymes involved in amino acid metabolism.

Question 122: 5-Fluorouracil (5-FU) exerts its anticancer activity primarily by inhibiting which enzyme?

Thymidylate synthase (Correct answer)
Ribonucleotide reductase
Dihydrofolate reductase
CTP synthetase

Correct answer: Thymidylate synthase

5-FU is converted to 5-FdUMP, which forms a covalent ternary complex with thymidylate synthase and N5,N10-methyleneTHF, irreversibly blocking dTMP synthesis and depleting the thymidine pool for DNA replication.

Question 123: Which enzyme is responsible for unwinding the DNA double helix replication?

Helicase (Correct answer)
Topoisomerase
Ligase
DNA polymerase

Correct answer: Helicase

Helicase is the enzyme responsible for unwinding the DNA double helix during replication. It breaks the hydrogen bonds between complementary base pairs, separating the two strands to create a replication fork. This unwinding is essential to expose the single-stranded DNA templates, allowing DNA polymerase to synthesize new DNA strands.

Question 124: Vitamin K is required for the gamma-carboxylation of which proteins critical for blood clotting?

Fibrinogen and thrombin
Clotting factors II, VII, IX, and X (Correct answer)
Von Willebrand factor
Platelet glycoproteins

Correct answer: Clotting factors II, VII, IX, and X

Vitamin K-dependent gamma-glutamyl carboxylase modifies prothrombin (factor II) and factors VII, IX, and X, adding extra carboxyl groups that allow Ca²⁺ binding essential for clot formation.

Question 125: PRPP amidotransferase, which catalyzes the first committed step of de novo purine synthesis, is allosterically inhibited by which molecules?

Orotate and UMP
Carbamoyl phosphate and aspartate
IMP and XMP only
AMP, ADP, ATP, GMP, GDP, and GTP (Correct answer)

Correct answer: AMP, ADP, ATP, GMP, GDP, and GTP

PRPP amidotransferase is subject to cumulative feedback inhibition by the end-product purine nucleotides (AMP/ADP/ATP and GMP/GDP/GTP), ensuring synthesis shuts down when purine pools are replete.

Question 126: SDS-PAGE separates proteins primarily based on:

Hydrophobicity
Molecular weight (size) (Correct answer)
Isoelectric point
Net charge at physiological pH

Correct answer: Molecular weight (size)

SDS denatures proteins and coats them with uniform negative charge proportional to mass, so migration in the polyacrylamide gel reflects molecular weight only.

Question 127: Which pyrimidine base is present in RNA but absent from DNA?

Hypoxanthine
Uracil (Correct answer)
Cytosine
Thymine

Correct answer: Uracil

Uracil replaces thymine in RNA; thymine contains a methyl group at C-5 that uracil lacks, and uracil pairs with adenine in RNA just as thymine does in DNA.

Question 128: Coenzyme A (CoA) is derived from which vitamin?

Pantothenic acid (B5) (Correct answer)
Thiamine (B1)
Niacin (B3)
Riboflavin (B2)

Correct answer: Pantothenic acid (B5)

Coenzyme A contains pantothenic acid (vitamin B5) as a core component, along with adenosine 3',5'-bisphosphate and a phosphopantetheine moiety that provides the reactive thiol group.

Question 129: Which of the following is NOT involved in post-transcriptional modification of eukaryotic mRNA?

Splicing out introns
Addition of a primer (Correct answer)
5' capping
Polyadenylation at the 3' end

Correct answer: Addition of a primer

Post-transcriptional modification of eukaryotic mRNA involves several crucial steps, including the addition of a 5' cap, splicing out introns, and polyadenylation at the 3' end. These modifications protect the mRNA, facilitate its export from the nucleus, and enhance its translation efficiency. The addition of a primer is a step in DNA replication, not mRNA processing.

Question 130: Which chromatographic technique separates molecules based on their specific binding interaction with an immobilized ligand?

Size-exclusion chromatography
Affinity chromatography (Correct answer)
Reverse-phase HPLC
Ion-exchange chromatography

Correct answer: Affinity chromatography

Affinity chromatography exploits specific, reversible binding between a target molecule and an immobilized ligand (e.g., His-tag/Ni-NTA, antibody/antigen) to achieve high-purity protein isolation.

Question 131: Competitive inhibition of an enzyme:

Increases both Km and Vmax
Increases apparent Km without changing Vmax (Correct answer)
Decreases Vmax without changing Km
Decreases both Km and Vmax

Correct answer: Increases apparent Km without changing Vmax

A competitive inhibitor competes with substrate for the active site, increasing apparent Km (reduced affinity) but leaving Vmax unchanged because excess substrate can overcome inhibition.

GRE Biochemistry, Cell and Molecular Biology Subject Test

The GRE Biochemistry, Cell and Molecular Biology Subject Test assesses graduate-level knowledge across biochemistry, cell biology, and molecular biology and genetics for students applying to life sciences graduate programs. The exam consists of 180 multiple-choice questions drawn proportionally from three core content areas.

180

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170 min

Time Limit

70%

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