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

Question 1: Which metabolic pathway is activated during prolonged starvation to supply energy to the brain?

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

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

Mitchell and Moyle
Watson and Crick
Lipmann and Krebs
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 3: In enzyme kinetics, the Michaelis constant (Km) is defined as:

The maximum reaction velocity
The substrate concentration at half-maximal velocity (Correct answer)
The rate constant for product release
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 4: Which of the following best describes passive transport?

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

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 5: 2,3-Bisphosphoglycerate (2,3-BPG) affects hemoglobin oxygen binding by:

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

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

Ionic bonds only
Peptide bonds
Covalent disulfide bonds only
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 7: Which amino acid can form disulfide bonds that stabilize protein structure?

Threonine
Serine
Cysteine (Correct answer)
Methionine

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

Secondary (Correct answer)
Tertiary
Quaternary
Primary

Correct answer: Secondary

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

Question 9: In the DNA double helix, adenine forms hydrogen bonds with which complementary base?

Cytosine
Uracil
Thymine (Correct answer)
Guanine

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

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

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

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

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

Cellobiose
Maltose
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 13: Which class of proteins contains immunoglobulin-fold domains and is central to adaptive immunity?

Antibodies (immunoglobulins) (Correct answer)
Nuclear receptors
Fibrous proteins
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 14: De novo purine nucleotide synthesis begins with which metabolite as the core scaffold?

Inosine monophosphate (IMP)
Ribose-5-phosphate (Correct answer)
Xanthine
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 15: Which prosthetic group is found in myoglobin and hemoglobin, enabling oxygen binding?

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

Correct answer: Heme group

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

Question 16: Which level of protein structure refers to the sequence of amino acids in a polypeptide chain?

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

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

Riboflavin (B2)
Thiamine (B1)
Pyridoxine (B6)
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 18: Folate (Vitamin B9) deficiency causes megaloblastic anemia primarily because folate is required for synthesis of:

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

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

Peroxisome only
Mitochondrial matrix (Correct answer)
Smooth ER
Cytosol

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

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

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

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

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

Pyridoxal phosphate (B6) (Correct answer)
Folate (B9)
Biotin
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 23: Lipoic acid (lipoate) functions as a cofactor in alpha-keto acid dehydrogenase complexes by:

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

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 24: Which type of fatty acid contains one double bond?

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

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

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

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 26: The Bohr effect refers to which property of hemoglobin?

Decreased O₂ affinity at lower pH and higher CO₂ (Correct answer)
Increased O₂ affinity at low temperature
Increased O₂ affinity when 2,3-BPG binds
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 27: Protein denaturation by heat or chemicals disrupts all levels of structure EXCEPT which?

Tertiary
Secondary
Primary (Correct answer)
Quaternary

Correct answer: Primary

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

Question 28: Lesch-Nyhan syndrome results from a deficiency of which enzyme in purine metabolism?

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

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 29: Carnitine plays an essential role in fatty acid oxidation by:

Transporting long-chain fatty acyl groups across the inner mitochondrial membrane (Correct answer)
Activating fatty acids to acyl-CoA in the cytosol
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 30: Which of the following statements about the Cori cycle is TRUE?

It occurs between the liver and brain.
It only operates in the presence of oxygen.
It involves the direct synthesis of ATP in muscle cells.
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 31: Hemoglobin is an example of a protein with which level of structural organization?

Secondary
Quaternary (Correct answer)
Tertiary
Primary

Correct answer: Quaternary

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

Question 32: What is the primary function of the phospholipid bilayer in the cell membrane?

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

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

Simple diffusion
Facilitated diffusion
Osmosis
Active transport (Correct answer)

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

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

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 35: Which technique separates proteins based on their isoelectric points?

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

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

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

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

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

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

5' capping
Splicing out introns
Addition of a primer (Correct answer)
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 39: Which sugar transporter (GLUT) is expressed in pancreatic beta-cells and serves as a glucose sensor?

GLUT4
GLUT1
GLUT2 (Correct answer)
GLUT5

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

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

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

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

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

Nucleus
Cytosol
Mitochondria
Rough endoplasmic reticulum (Correct answer)

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

Triglycerides
Cholesterol
Sphingomyelin
Phospholipids (Correct answer)

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 44: Fatty acid synthesis occurs in which cellular compartment?

Peroxisome
Cytosol (Correct answer)
Mitochondrial matrix
Smooth ER

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

Thymine
Hypoxanthine
Cytosine
Uracil (Correct answer)

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

Lipogenesis
Gluconeogenesis (Correct answer)
Ketogenesis
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 47: CRISPR-Cas9 gene editing creates targeted double-strand DNA breaks using which component to guide it to the correct genomic location?

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

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

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

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 49: 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 50: Which enzyme catalyzes the committed (first unique) step of pyrimidine biosynthesis?

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

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

The pH at which the protein denatures
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 has maximum enzymatic activity

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

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

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 53: 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 54: The saponification number (saponification value) of a fat is a measure of:

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

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

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

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 56: Which of the following statements about enzymes is TRUE?

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

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

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

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

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

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 59: What distinguishes a nucleotide from a nucleoside?

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

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

GM2 ganglioside (Correct answer)
Sphingomyelin
Glucocerebroside
Ceramide

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

Lanosterol synthase
HMG-CoA synthase
HMG-CoA reductase (Correct answer)
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 62: Which enzyme is responsible for unwinding the DNA double helix replication?

Helicase (Correct answer)
Ligase
DNA polymerase
Topoisomerase

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

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

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

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

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

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

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

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

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

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

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 68: Southern blotting detects:

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

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 69: 5-Fluorouracil (5-FU) exerts its anticancer activity primarily by inhibiting which enzyme?

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

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

Uracil
Thymine
Adenine (Correct answer)
Cytosine

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 71: SDS-PAGE separates proteins primarily based on:

Isoelectric point
Hydrophobicity
Molecular weight (size) (Correct answer)
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 72: Two-dimensional gel electrophoresis (2D-PAGE) separates proteins first by isoelectric point, then by:

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

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 73: Sphingolipids differ from glycerophospholipids in that their backbone is:

Sphingosine (Correct answer)
Glycerol
Cholesterol
Serine

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

Debranching enzyme
Glycogen phosphorylase
Glycogen synthase
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 75: Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors. Which of the following is NOT a gluconeogenic precursor?

Glycerol
Acetyl-CoA (Correct answer)
Lactate
Oxaloacetate

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 76: Scurvy results from Vitamin C deficiency primarily because without ascorbate:

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

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

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

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 78: The anomeric carbon of glucose is carbon number:

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

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

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

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

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

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

Hexokinase (Correct answer)
Phosphofructokinase
Pyruvate kinase
Aldolase

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

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

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

Ribose
Galactose
Fructose
Glucose (Correct answer)

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

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

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

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

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

Beta(1→3) linkages at branch points
Alpha(1→2) linkages throughout
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 87: In the integration of metabolism, which pathway is the primary source of ATP during intense physical activity?

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

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 88: 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 89: Competitive inhibition of an enzyme:

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

Question 90: Vitamin K is required for the gamma-carboxylation of which proteins critical for blood clotting?

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

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

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

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

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

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

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

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

Transcription occurs in the cytoplasm of eukaryotic cells.
RNA polymerase synthesizes RNA in the 5' to 3' direction. (Correct answer)
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 95: Which of the following best describes the function of tRNA in translation?

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

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

Protein kinase A (PKA) (Correct answer)
Protein kinase C (PKC)
Calmodulin-dependent kinase
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 97: Gel filtration (size-exclusion) chromatography separates proteins such that:

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

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

Methyl groups
CO₂ (carboxyl groups) (Correct answer)
Amino groups
Acyl 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 99: In a non-competitive inhibition mechanism, the inhibitor:

Covalently modifies the active site irreversibly
Competes with substrate for the active site
Binds only to the enzyme-substrate complex
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 100: In oxidative phosphorylation, ATP synthesis is powered by:

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

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 101: Chaperone proteins assist in which process?

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

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 102: The enzyme lactase cleaves lactose into which two monosaccharides?

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

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

Glucagon
Insulin (Correct answer)
Cortisol
Epinephrine

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

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

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

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

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

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

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

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

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

Lysine (Correct answer)
Serine
Aspartate
Cysteine

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

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

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 110: The Bradford assay measures protein concentration using which dye?

Lowry reagent
Coomassie Brilliant Blue G-250 (Correct answer)
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 111: Vitamin C (ascorbate) is required for collagen synthesis because it is needed for:

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

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

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

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

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

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 114: 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 115: Cholesterol in animal cell membranes functions to:

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

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

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

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 117: 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 AMP and homocysteine by methyltransferase
From PRPP and methionine by a transferase
From GTP and methionine by methionine adenosyltransferase

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

Acetyl-CoA
NAD⁺ (Correct answer)
FADH₂
ATP

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

Beriberi (Correct answer)
Rickets
Scurvy
Pellagra

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 120: Familial hypercholesterolemia is caused by a defect in:

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

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 121: Mass spectrometry in proteomics identifies proteins by measuring:

Their fluorescence emission wavelength
Their migration speed in an electric field without gel
The mass-to-charge ratio (m/z) of peptide fragments (Correct answer)
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 122: The net ATP yield from complete oxidation of one molecule of glucose through glycolysis and oxidative phosphorylation is approximately:

30-32 ATP (Correct answer)
36-38 ATP
8 ATP
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 123: Methotrexate inhibits nucleotide synthesis by targeting which enzyme?

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

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 124: Which of the following is true about the DNA replication?

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

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

Acetyl-CoA
AMP (Correct answer)
Citrate
ATP

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

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

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

Inhibiting glucagon receptor signaling
Increasing hexokinase gene transcription directly
Promoting translocation of GLUT4 transporters to the plasma membrane (Correct answer)
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 128: In the pyrimidine salvage pathway, uridine is converted directly to UMP by which enzyme?

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

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

Niacin (B3) (Correct answer)
Riboflavin (B2)
Pyridoxine (B6)
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 130: Ketone bodies are produced in the liver primarily from excess:

Acetyl-CoA (Correct answer)
Amino acids
Glucose
Glycerol

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

Allantoin
Hypoxanthine
Uric acid (Correct answer)
Xanthine

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.

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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