PCAT Biochemistry 4 — Questions and Answers
Question 1: During beta-oxidation of a 16-carbon saturated fatty acid (palmitate), how many acetyl-CoA molecules are produced?
- 6
- 7
- 8 (Correct answer)
- 9
Correct answer: 8
A 16-carbon fatty acid undergoes 7 rounds of beta-oxidation, yielding 8 acetyl-CoA molecules, 7 FADH2, and 7 NADH.
Question 2: Which of the following best describes the role of calmodulin in cellular signaling?
- It directly activates adenylyl cyclase upon ligand binding
- It is a calcium-binding protein that activates target enzymes when Ca2+ levels rise (Correct answer)
- It inhibits phosphodiesterase to maintain cAMP levels
- It phosphorylates protein kinase A upon G-protein activation
Correct answer: It is a calcium-binding protein that activates target enzymes when Ca2+ levels rise
Calmodulin binds four calcium ions when intracellular Ca2+ rises, undergoes a conformational change, and then activates target proteins such as calmodulin-dependent protein kinases.
Question 3: Which type of RNA polymerase transcribes mRNA in eukaryotes?
- RNA polymerase I
- RNA polymerase II (Correct answer)
- RNA polymerase III
- RNA polymerase IV
Correct answer: RNA polymerase II
RNA polymerase II transcribes protein-coding genes to produce pre-mRNA (hnRNA) in eukaryotes; it also produces most snRNAs.
Question 4: In the lac operon, which molecule acts as the inducer to relieve repression?
- Lactose
- Allolactose (Correct answer)
- Galactose
- cAMP
Correct answer: Allolactose
Allolactose, a metabolite of lactose formed by beta-galactosidase, binds the lac repressor and causes a conformational change that releases it from the operator.
Question 5: Which of the following correctly describes a Schiff base in enzyme catalysis?
- A hydrogen bond between lysine and a carbonyl substrate
- A covalent imine bond formed between an amino group and a carbonyl compound (Correct answer)
- An ester bond between serine and a phosphate group
- An ionic interaction between aspartate and a positively charged substrate
Correct answer: A covalent imine bond formed between an amino group and a carbonyl compound
A Schiff base (aldimine) is a covalent C=N bond formed when a primary amine (often enzyme lysine) reacts with a carbonyl group of a substrate, important in aminotransferase and aldolase reactions.
Question 6: Glucokinase (hexokinase IV) differs from hexokinase I primarily because it:
- Has a lower Km for glucose and is inhibited by glucose-6-phosphate
- Has a high Km for glucose and is not inhibited by its product glucose-6-phosphate (Correct answer)
- Is expressed in muscle and brain rather than the liver
- Phosphorylates fructose preferentially over glucose
Correct answer: Has a high Km for glucose and is not inhibited by its product glucose-6-phosphate
Glucokinase has a high Km (~10 mM) for glucose and is not subject to product inhibition, allowing hepatocytes to act as a glucose buffer by phosphorylating glucose only when blood glucose is high.
Question 7: Which bond links nucleotides together in a DNA strand?
- N-glycosidic bond
- 3',5'-phosphodiester bond (Correct answer)
- Hydrogen bond
- Peptide bond
Correct answer: 3',5'-phosphodiester bond
Adjacent nucleotides in a DNA strand are connected by phosphodiester bonds formed between the 3'-hydroxyl of one sugar and the 5'-phosphate of the next.
PCAT Biochemistry 4
During beta-oxidation of a 16-carbon saturated fatty acid (palmitate), how many acetyl-CoA molecules are produced?