MCAT Biochemistry 4

Question 1

Which of the following best describes the role of ubiquitin in protein degradation?

Accepted Answer

Ubiquitin marks proteins for degradation by the 26S proteasome

Answer

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:

Accepted Answer

Phenylalanine hydroxylase

Answer

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?

Accepted Answer

Decreased O₂ affinity at low pH

Answer

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)?

Accepted Answer

Carboxylation of acetyl-CoA to malonyl-CoA

Answer

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?

Accepted Answer

Uracil

Answer

RNA contains uracil instead of thymine; uracil lacks the methyl group at position 5 that thymine possesses.

MCAT - Medical College Admission Test Practice Test

MCAT - Medical College Admission Test Practice Test

MCAT Biochemistry 4