PCAT Biochemistry 5 — Questions and Answers
Question 1: Which coenzyme is required for the transamination reactions that transfer amino groups from amino acids to alpha-keto acids?
- FAD
- NAD+
- Pyridoxal phosphate (PLP) (Correct answer)
- Coenzyme A
Correct answer: Pyridoxal phosphate (PLP)
Pyridoxal phosphate (PLP), derived from vitamin B6, is the essential coenzyme for all aminotransferase (transaminase) reactions, shuttling amino groups via a Schiff base intermediate.
Question 2: What is the net ATP yield from complete aerobic oxidation of one glucose molecule (using the malate-aspartate shuttle)?
- 30-32 ATP (Correct answer)
- 36-38 ATP
- 24 ATP
- 38-40 ATP
Correct answer: 30-32 ATP
Modern estimates calculate approximately 30-32 ATP net from aerobic glucose oxidation, accounting for the actual P/O ratios of NADH (~2.5 ATP) and FADH2 (~1.5 ATP) rather than the older theoretical values.
Question 3: Which of the following is a characteristic of allosteric enzymes?
- They follow simple Michaelis-Menten kinetics
- They show sigmoidal (S-shaped) velocity vs. substrate concentration curves (Correct answer)
- They are always inhibited by their own products
- They require a covalent modification to change activity
Correct answer: They show sigmoidal (S-shaped) velocity vs. substrate concentration curves
Allosteric enzymes display sigmoidal kinetics (positive cooperativity) rather than hyperbolic Michaelis-Menten kinetics, because binding of substrate at one subunit increases affinity at other subunits.
Question 4: Ceramide is the backbone of which class of sphingolipids?
- Triglycerides
- Phosphatidylcholines
- Sphingomyelins and glycosphingolipids (Correct answer)
- Cardiolipins
Correct answer: Sphingomyelins and glycosphingolipids
Ceramide (sphingosine + fatty acid) is the hydrophobic core of all sphingolipids; addition of phosphocholine gives sphingomyelin, while addition of sugars gives glycosphingolipids (cerebrosides, gangliosides).
Question 5: During translation, which site on the ribosome holds the tRNA carrying the growing polypeptide chain?
- A (aminoacyl) site
- P (peptidyl) site (Correct answer)
- E (exit) site
- Decoding center
Correct answer: P (peptidyl) site
The P (peptidyl) site holds the tRNA attached to the growing polypeptide chain; the incoming aminoacyl-tRNA enters the A site, and the uncharged tRNA exits through the E site.
Question 6: Which enzyme deficiency is responsible for Phenylketonuria (PKU)?
- Tyrosine aminotransferase
- Phenylalanine hydroxylase (Correct answer)
- Homogentisate oxidase
- Fumarylacetoacetate hydrolase
Correct answer: Phenylalanine hydroxylase
PKU results from a deficiency in phenylalanine hydroxylase (PAH), which normally converts phenylalanine to tyrosine, causing toxic accumulation of phenylalanine and its metabolites.
Question 7: Which process is described as the movement of acetyl groups from the mitochondria to the cytoplasm for fatty acid synthesis?
- Beta-oxidation
- The citrate shuttle (Correct answer)
- The malate-aspartate shuttle
- Ketogenesis
Correct answer: The citrate shuttle
Acetyl-CoA cannot cross the inner mitochondrial membrane directly; it condenses with oxaloacetate to form citrate, which is exported and then cleaved by ATP-citrate lyase in the cytoplasm to release acetyl-CoA for fatty acid synthesis.
PCAT Biochemistry 5
Which coenzyme is required for the transamination reactions that transfer amino groups from amino acids to alpha-keto acids?