MCAT Biochemistry 3 — Questions and Answers
Question 1: Which protein secondary structure is stabilized by hydrogen bonds between residues distant in primary sequence?
- Alpha helix
- Beta sheet (Correct answer)
- Random coil
- Both alpha helix and beta sheet
Correct answer: Beta sheet
Beta sheets form hydrogen bonds between backbone atoms of strands that are far apart in the primary sequence, while alpha helices bond residues close together (i and i+4).
Question 2: During the fed state, glucokinase activity in hepatocytes increases because:
- It is activated by insulin-mediated phosphorylation
- High glucose concentrations overcome its low affinity for glucose
- It is released from a nuclear inhibitor protein (Correct answer)
- Glucagon stimulates its gene transcription
Correct answer: It is released from a nuclear inhibitor protein
Glucokinase is sequestered in the nucleus by a regulatory protein at low glucose; when glucose rises, it dissociates and enters the cytoplasm to phosphorylate glucose.
Question 3: Ceramide is the backbone molecule for which class of lipids?
- Glycerophospholipids
- Triacylglycerols
- Sphingolipids (Correct answer)
- Sterols
Correct answer: Sphingolipids
Ceramide consists of sphingosine plus a fatty acid and is the core structure of all sphingolipids, including sphingomyelin and glycosphingolipids.
Question 4: Which amino acid residue is most commonly found at the catalytic site of serine proteases?
- Histidine-Aspartate-Serine triad (Correct answer)
- Lysine-Glutamate-Cysteine triad
- Arginine-Threonine-Serine triad
- Glycine-Proline-Serine triad
Correct answer: Histidine-Aspartate-Serine triad
The catalytic triad of serine proteases consists of His, Asp, and Ser working together to activate the serine nucleophile.
Question 5: In the pentose phosphate pathway, transketolase requires which cofactor?
- NAD⁺
- FAD
- Thiamine pyrophosphate (TPP) (Correct answer)
- Biotin
Correct answer: Thiamine pyrophosphate (TPP)
Transketolase, which transfers two-carbon units in the non-oxidative phase of the pentose phosphate pathway, requires thiamine pyrophosphate (TPP) as a cofactor.
Question 6: A mutation that converts a UAG stop codon to a sense codon is called a:
- Missense mutation
- Nonsense mutation
- Silent mutation
- Suppressor mutation (Correct answer)
Correct answer: Suppressor mutation
A suppressor mutation (typically in a tRNA gene) can read through a stop codon, allowing translation to continue past the original termination point.
Question 7: Which metabolic pathway is uniquely active in the absorptive state to convert excess glucose to fatty acids?
- Glycogenolysis
- Gluconeogenesis
- De novo lipogenesis (Correct answer)
- Beta-oxidation
Correct answer: De novo lipogenesis
De novo lipogenesis converts excess acetyl-CoA (from glucose catabolism) into fatty acids during the absorptive state when insulin is high.
MCAT Biochemistry 3
Which protein secondary structure is stabilized by hydrogen bonds between residues distant in primary sequence?