Biochemistry Lipids and Membranes 2 — Questions and Answers
Question 1: Beta-oxidation of fatty acids occurs primarily in which cellular compartment?
- Cytosol
- Smooth ER
- Mitochondrial matrix (Correct answer)
- Peroxisome only
Correct answer: Mitochondrial matrix
Beta-oxidation of most fatty acids occurs in the mitochondrial matrix, where fatty acyl-CoA undergoes sequential rounds of oxidation to produce acetyl-CoA, NADH, and FADHâ‚‚.
Question 2: Carnitine plays an essential role in fatty acid oxidation by:
- Activating fatty acids to acyl-CoA in the cytosol
- Transporting long-chain fatty acyl groups across the inner mitochondrial membrane (Correct answer)
- Serving as a coenzyme for beta-oxidation enzymes
- Regenerating CoA from acetyl-CoA
Correct answer: Transporting long-chain fatty acyl groups across the inner mitochondrial membrane
Carnitine shuttles long-chain fatty acyl groups across the inner mitochondrial membrane as acylcarnitine, which is essential because the inner membrane is impermeable to acyl-CoA.
Question 3: During fatty acid synthesis, malonyl-CoA is formed from acetyl-CoA by which enzyme?
- Fatty acid synthase (FAS)
- Acetyl-CoA carboxylase (ACC) (Correct answer)
- HMG-CoA reductase
- Citrate lyase
Correct answer: Acetyl-CoA carboxylase (ACC)
Acetyl-CoA carboxylase (ACC) catalyzes the ATP-dependent carboxylation of acetyl-CoA to malonyl-CoA, the committed step in fatty acid synthesis.
Question 4: Fatty acid synthesis occurs in which cellular compartment?
- Mitochondrial matrix
- Peroxisome
- Cytosol (Correct answer)
- Smooth ER
Correct answer: Cytosol
Fatty acid synthesis by fatty acid synthase (FAS) occurs in the cytosol, using NADPH as the reducing agent and malonyl-CoA as the two-carbon donor.
Question 5: Ketone bodies are produced in the liver primarily from excess:
- Glucose
- Amino acids
- Acetyl-CoA (Correct answer)
- Glycerol
Correct answer: Acetyl-CoA
When acetyl-CoA from fatty acid oxidation exceeds the capacity of the TCA cycle (e.g., during fasting), the liver converts excess acetyl-CoA into ketone bodies (acetoacetate, β-hydroxybutyrate, acetone).
Question 6: Which enzyme is the rate-limiting step in cholesterol biosynthesis?
- Squalene synthase
- HMG-CoA synthase
- HMG-CoA reductase (Correct answer)
- Lanosterol synthase
Correct answer: HMG-CoA reductase
HMG-CoA reductase catalyzes the conversion of HMG-CoA to mevalonate and is the primary regulated, rate-limiting step in cholesterol synthesis; statins inhibit this enzyme.
Biochemistry Lipids and Membranes 2
Beta-oxidation of fatty acids occurs primarily in which cellular compartment?