PCAT Biochemistry 3 — Questions and Answers
Question 1: In competitive inhibition, increasing the substrate concentration will:
- Decrease Vmax and increase Km
- Leave Vmax unchanged and increase apparent Km (Correct answer)
- Decrease both Vmax and Km
- Increase Vmax and decrease Km
Correct answer: Leave Vmax unchanged and increase apparent Km
Competitive inhibitors bind the active site reversibly; adding excess substrate outcompetes the inhibitor, so Vmax is unchanged but the apparent Km is increased.
Question 2: Which lipoprotein is primarily responsible for transporting dietary (exogenous) triglycerides from the intestine to peripheral tissues?
- LDL
- HDL
- VLDL
- Chylomicrons (Correct answer)
Correct answer: Chylomicrons
Chylomicrons are assembled in intestinal epithelial cells and carry absorbed dietary triglycerides through the lymphatic system to peripheral tissues.
Question 3: Sickle cell disease results from a single amino acid substitution in hemoglobin. What change occurs?
- Glutamate is replaced by lysine at position 6 of the beta chain
- Glutamate is replaced by valine at position 6 of the beta chain (Correct answer)
- Valine is replaced by glycine at position 6 of the alpha chain
- Aspartate is replaced by glutamate at position 6 of the beta chain
Correct answer: Glutamate is replaced by valine at position 6 of the beta chain
A point mutation converts the GAG codon to GTG, replacing the hydrophilic glutamate with hydrophobic valine at position 6 of the beta-globin chain, causing HbS polymerization under low oxygen conditions.
Question 4: Which fatty acid is classified as an omega-3 fatty acid?
- Arachidonic acid (20:4)
- Linoleic acid (18:2)
- Alpha-linolenic acid (18:3) (Correct answer)
- Oleic acid (18:1)
Correct answer: Alpha-linolenic acid (18:3)
Alpha-linolenic acid (ALA, 18:3Δ9,12,15) is an omega-3 fatty acid because its first double bond is at the 3rd carbon from the methyl end; it is an essential fatty acid.
Question 5: Which enzyme is responsible for unwinding the DNA double helix during replication?
- DNA polymerase III
- Primase
- Helicase (Correct answer)
- Topoisomerase II
Correct answer: Helicase
Helicase uses ATP hydrolysis to break hydrogen bonds between base pairs and unwind the double helix at the replication fork.
Question 6: The rate-limiting enzyme of the HMP shunt (pentose phosphate pathway) is:
- Transketolase
- 6-Phosphogluconate dehydrogenase
- Glucose-6-phosphate dehydrogenase (Correct answer)
- Phosphogluconate dehydratase
Correct answer: Glucose-6-phosphate dehydrogenase
Glucose-6-phosphate dehydrogenase (G6PD) catalyzes the first and rate-limiting step of the pentose phosphate pathway, producing NADPH and 6-phosphoglucono-delta-lactone.
Question 7: Which amino acids are considered ketogenic only (not glucogenic)?
- Leucine and lysine (Correct answer)
- Isoleucine and phenylalanine
- Tyrosine and tryptophan
- Threonine and methionine
Correct answer: Leucine and lysine
Leucine and lysine are the only purely ketogenic amino acids; their catabolism yields only acetyl-CoA and/or acetoacetate, which cannot be used for gluconeogenesis.
PCAT Biochemistry 3
In competitive inhibition, increasing the substrate concentration will: