PCAT Biochemistry 2 — Questions and Answers
Question 1: Which enzyme catalyzes the committed step of glycolysis?
- Hexokinase
- Phosphofructokinase-1 (Correct answer)
- Pyruvate kinase
- Phosphoglucose isomerase
Correct answer: Phosphofructokinase-1
Phosphofructokinase-1 (PFK-1) catalyzes the phosphorylation of fructose-6-phosphate to fructose-1,6-bisphosphate, which is the committed and rate-limiting step of glycolysis.
Question 2: What is the primary function of ubiquinone (CoQ) in the electron transport chain?
- Pumps protons across the inner mitochondrial membrane
- Transfers electrons from complexes I and II to complex III (Correct answer)
- Accepts electrons from cytochrome c
- Generates ATP via substrate-level phosphorylation
Correct answer: Transfers electrons from complexes I and II to complex III
Ubiquinone (CoQ) is a mobile electron carrier that collects electrons from NADH (via Complex I) and FADH2 (via Complex II) and transfers them to Complex III.
Question 3: Which amino acid serves as the direct precursor for dopamine biosynthesis?
- Phenylalanine
- Tryptophan
- Tyrosine (Correct answer)
- Histidine
Correct answer: Tyrosine
Tyrosine is hydroxylated to L-DOPA by tyrosine hydroxylase, then decarboxylated to dopamine by DOPA decarboxylase.
Question 4: A patient with a deficiency in glucose-6-phosphatase would be unable to perform which process?
- Glycolysis
- Glycogenolysis releasing free glucose to blood (Correct answer)
- Fatty acid oxidation
- Gluconeogenesis from amino acids
Correct answer: Glycogenolysis releasing free glucose to blood
Glucose-6-phosphatase, found mainly in the liver, removes the phosphate from glucose-6-phosphate so free glucose can be exported into the bloodstream; its deficiency causes Von Gierke disease.
Question 5: In the urea cycle, which nitrogen-containing compound directly donates one of the two nitrogen atoms incorporated into urea?
- Glutamine
- Aspartate (Correct answer)
- Ammonia
- Alanine
Correct answer: Aspartate
Aspartate donates its amino group to argininosuccinate, contributing the second nitrogen atom of urea; ammonia provides the first nitrogen via carbamoyl phosphate.
Question 6: Which of the following best describes the Km value of an enzyme?
- The substrate concentration at which reaction velocity is maximal
- The substrate concentration at which velocity is half of Vmax (Correct answer)
- The product concentration that causes 50% inhibition
- The enzyme concentration needed for maximum activity
Correct answer: The substrate concentration at which velocity is half of Vmax
Km (Michaelis constant) is the substrate concentration at which the reaction velocity equals half of Vmax, and it reflects the affinity of the enzyme for its substrate.
Question 7: Which vitamin is required as a cofactor for pyruvate dehydrogenase complex activity?
- Vitamin B12
- Vitamin B1 (thiamine) (Correct answer)
- Vitamin B6 (pyridoxine)
- Vitamin B3 (niacin)
Correct answer: Vitamin B1 (thiamine)
Thiamine pyrophosphate (TPP), derived from vitamin B1, is an essential cofactor for pyruvate dehydrogenase, enabling oxidative decarboxylation of pyruvate to acetyl-CoA.
PCAT Biochemistry 2
Which enzyme catalyzes the committed step of glycolysis?