Mixed Deck — All Biochemistry Topics Flashcards

Ketone bodies are produced in the liver primarily from excess:

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).

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

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.

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

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.

Methotrexate inhibits nucleotide synthesis by targeting which enzyme?

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.

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

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.

Which of the following best describes passive transport?

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.

The Bohr effect refers to which property of hemoglobin?

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.

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

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.

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

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.

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

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.

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

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.

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

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.

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

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.

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

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.

Which of the following statements about the Cori cycle is TRUE?

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.

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

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.

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

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.

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

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).

Chaperone proteins assist in which process?

Answer: Correct protein folding

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

SDS-PAGE separates proteins primarily based on:

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.