MCAT Biochemistry 4 — Questions and Answers
Question 1: Which of the following best describes the role of ubiquitin in protein degradation?
- Ubiquitin unfolds proteins to expose hydrophobic cores
- Ubiquitin marks proteins for degradation by the 26S proteasome (Correct answer)
- Ubiquitin is the catalytic subunit of the proteasome
- Ubiquitin activates lysosomal proteases
Correct answer: Ubiquitin marks proteins for degradation by the 26S proteasome
Polyubiquitin chains attached to substrate proteins serve as a recognition signal for the 26S proteasome, which then unfolds and degrades the tagged protein.
Question 2: Phenylketonuria (PKU) results from accumulation of phenylalanine due to deficiency of:
- Tyrosine hydroxylase
- Phenylalanine hydroxylase (Correct answer)
- Homogentisate oxidase
- Fumarylacetoacetate hydrolase
Correct answer: Phenylalanine hydroxylase
Phenylalanine hydroxylase converts phenylalanine to tyrosine; its deficiency causes phenylalanine accumulation and toxic metabolite formation, leading to intellectual disability.
Question 3: The Bohr effect describes which phenomenon in hemoglobin physiology?
- Increased O₂ affinity at high CO₂ concentrations
- Decreased O₂ affinity at low pH (Correct answer)
- Increased O₂ affinity upon 2,3-BPG binding
- Sigmoidal oxygen-binding curve due to cooperativity
Correct answer: Decreased O₂ affinity at low pH
The Bohr effect states that increased H⁺ (lower pH) and CO₂ decrease hemoglobin's oxygen affinity, promoting O₂ release in metabolically active tissues.
Question 4: Which step of fatty acid synthesis requires the direct input of CO₂ (as bicarbonate)?
- Condensation of acetyl-CoA with malonyl-CoA
- Carboxylation of acetyl-CoA to malonyl-CoA (Correct answer)
- Reduction of 3-ketoacyl-ACP
- Dehydration of 3-hydroxyacyl-ACP
Correct answer: Carboxylation of acetyl-CoA to malonyl-CoA
Acetyl-CoA carboxylase adds CO₂ to acetyl-CoA to form malonyl-CoA, the activated two-carbon donor for fatty acid chain elongation.
Question 5: Which base is found in RNA but NOT in DNA?
- Adenine
- Guanine
- Uracil (Correct answer)
- Cytosine
Correct answer: Uracil
RNA contains uracil instead of thymine; uracil lacks the methyl group at position 5 that thymine possesses.
Question 6: A patient with G6PD deficiency is at risk for hemolytic anemia because red blood cells cannot adequately:
- Perform glycolysis to generate ATP
- Regenerate NADPH to maintain glutathione in reduced form (Correct answer)
- Synthesize heme for hemoglobin
- Transport oxygen across the membrane
Correct answer: Regenerate NADPH to maintain glutathione in reduced form
G6PD produces NADPH via the pentose phosphate pathway; without NADPH, glutathione cannot be reduced to protect RBCs from oxidative stress, causing hemolysis.
Question 7: In DNA replication, which enzyme removes RNA primers and fills in the resulting gaps with DNA?
- DNA polymerase I (Correct answer)
- DNA polymerase III
- DNA ligase
- Primase
Correct answer: DNA polymerase I
In prokaryotes, DNA polymerase I has 5'→3' exonuclease activity to remove RNA primers and then fills in the gaps using its 5'→3' polymerase activity.
MCAT Biochemistry 4
Which of the following best describes the role of ubiquitin in protein degradation?