MCAT Biochemistry 5

Question 1

Which of the following correctly describes the function of GLUT2 transporters in pancreatic beta cells?

Accepted Answer

They have high Km and high capacity, allowing beta cells to sense elevated blood glucose

Answer

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:

Accepted Answer

Homogentisate oxidase

Answer

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?

Accepted Answer

Isoleucine

Answer

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?

Accepted Answer

Ketone bodies (acetoacetate and β-hydroxybutyrate)

Answer

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:

Accepted Answer

Positive regulator that enhances transcription when glucose is absent

Answer

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.

MCAT - Medical College Admission Test Practice Test

MCAT - Medical College Admission Test Practice Test

MCAT Biochemistry 5