MCAT Biochemistry 5 — Questions and Answers
Question 1: Which of the following correctly describes the function of GLUT2 transporters in pancreatic beta cells?
- They are insulin-dependent and import glucose only after insulin is secreted
- They have low Km and high capacity, acting as glucose sensors
- They have high Km and high capacity, allowing beta cells to sense elevated blood glucose (Correct answer)
- They actively pump glucose against its concentration gradient
Correct answer: They have high Km and high capacity, allowing beta cells to sense elevated blood glucose
GLUT2 has a high Km (~15-20 mM) and high Vmax, so glucose uptake into beta cells is proportional to blood glucose concentration, coupling glucose sensing to insulin secretion.
Question 2: Alkaptonuria is caused by accumulation of homogentisate due to deficiency of:
- Phenylalanine hydroxylase
- Tyrosine aminotransferase
- Homogentisate oxidase (Correct answer)
- Fumarylacetoacetate hydrolase
Correct answer: Homogentisate oxidase
Homogentisate oxidase deficiency leads to accumulation of homogentisic acid, which polymerizes and deposits in connective tissues causing ochronosis and dark urine.
Question 3: Which of the following amino acids is both glucogenic and ketogenic?
- Alanine
- Leucine
- Isoleucine (Correct answer)
- Lysine
Correct answer: Isoleucine
Isoleucine is both glucogenic and ketogenic, as its catabolism yields succinyl-CoA (glucogenic) and acetyl-CoA (ketogenic).
Question 4: During prolonged fasting, the brain adapts by using which alternative fuel source?
- Free fatty acids
- Glucagon
- Ketone bodies (acetoacetate and β-hydroxybutyrate) (Correct answer)
- Glycogen
Correct answer: Ketone bodies (acetoacetate and β-hydroxybutyrate)
After several days of fasting, the brain shifts to using ketone bodies derived from hepatic fatty acid oxidation, reducing its glucose requirement by up to 75%.
Question 5: In the lac operon, the CAP-cAMP complex acts as a:
- Negative regulator that represses transcription when glucose is present
- Positive regulator that enhances transcription when glucose is absent (Correct answer)
- Competitive inhibitor of the lac repressor
- Post-translational modifier of beta-galactosidase
Correct answer: Positive regulator that enhances transcription when glucose is absent
When glucose is low, cAMP rises and binds CAP (catabolite activator protein), and the complex binds the lac promoter to enhance RNA polymerase binding and increase transcription.
Question 6: Zymogen activation of pepsinogen to pepsin in the stomach occurs via:
- Enterokinase cleavage in the small intestine
- Autocatalytic cleavage at low pH (Correct answer)
- Trypsin-mediated cleavage
- Chymotrypsin activation
Correct answer: Autocatalytic cleavage at low pH
Pepsinogen undergoes autocatalytic activation at gastric pH below 5, cleaving its inhibitory propeptide to form active pepsin.
Question 7: Which of the following correctly pairs a metabolic pathway with its subcellular location in eukaryotes?
- Beta-oxidation of fatty acids — cytoplasm
- Gluconeogenesis — exclusively mitochondria
- Heme synthesis — both mitochondria and cytoplasm (Correct answer)
- Cholesterol synthesis — mitochondria
Correct answer: Heme synthesis — both mitochondria and cytoplasm
Heme synthesis begins and ends in the mitochondria (ALA synthesis and final steps), with intermediate steps occurring in the cytoplasm.
MCAT Biochemistry 5
Which of the following correctly describes the function of GLUT2 transporters in pancreatic beta cells?