RDN Nutritional Biochemistry 2 — Questions and Answers
Question 1: Which enzyme is responsible for the rate-limiting step in fatty acid synthesis?
- Fatty acid synthase
- Acetyl-CoA carboxylase (Correct answer)
- Malonyl-CoA decarboxylase
- Citrate lyase
Correct answer: Acetyl-CoA carboxylase
Acetyl-CoA carboxylase catalyzes the conversion of acetyl-CoA to malonyl-CoA, which is the committed, rate-limiting step in de novo fatty acid synthesis.
Question 2: During prolonged fasting, the liver produces ketone bodies primarily from which substrate?
- Glucose
- Glycerol
- Acetyl-CoA from fatty acid beta-oxidation (Correct answer)
- Amino acids from muscle protein
Correct answer: Acetyl-CoA from fatty acid beta-oxidation
During fasting, elevated fatty acid beta-oxidation generates excess acetyl-CoA that exceeds the TCA cycle capacity, leading to ketone body synthesis (ketogenesis) in liver mitochondria.
Question 3: Which B vitamin serves as a cofactor for transaminase (aminotransferase) enzymes involved in amino acid metabolism?
- Thiamin (B1)
- Riboflavin (B2)
- Pyridoxine (B6) (Correct answer)
- Cobalamin (B12)
Correct answer: Pyridoxine (B6)
Pyridoxal phosphate (PLP), the active form of vitamin B6, is an essential cofactor for all transaminase reactions that transfer amino groups between amino acids and alpha-keto acids.
Question 4: What is the primary function of the pentose phosphate pathway in erythrocytes?
- ATP production via substrate-level phosphorylation
- Generation of NADPH to maintain glutathione in reduced form (Correct answer)
- Production of ribose-5-phosphate for DNA synthesis
- Conversion of glucose to fructose
Correct answer: Generation of NADPH to maintain glutathione in reduced form
In red blood cells, the pentose phosphate pathway generates NADPH needed to keep glutathione reduced, protecting cells from oxidative damage caused by reactive oxygen species.
Question 5: Homocysteine accumulation in the blood is associated with deficiencies in which combination of nutrients?
- Vitamin D, calcium, and phosphorus
- Folate, vitamin B6, and vitamin B12 (Correct answer)
- Iron, copper, and zinc
- Vitamin C, vitamin E, and selenium
Correct answer: Folate, vitamin B6, and vitamin B12
Folate, B6 (as PLP), and B12 are all required in the methionine cycle; deficiency of any one leads to impaired homocysteine remethylation or transsulfuration, causing hyperhomocysteinemia.
Question 6: Which metabolic pathway is most active in the postprandial state in adipose tissue?
- Beta-oxidation of fatty acids
- Ketogenesis
- De novo lipogenesis and triglyceride synthesis (Correct answer)
- Gluconeogenesis
Correct answer: De novo lipogenesis and triglyceride synthesis
After a meal, elevated insulin promotes glucose uptake and de novo lipogenesis in adipose tissue, converting excess acetyl-CoA into fatty acids that are esterified into triglycerides for storage.
Question 7: The urea cycle requires which amino acid as a direct nitrogen donor to form carbamoyl phosphate?
- Glutamine (via glutamate) (Correct answer)
- Aspartate
- Alanine
- Glycine
Correct answer: Glutamine (via glutamate)
Glutamine donates its amide nitrogen (via glutaminase yielding glutamate, then glutamate dehydrogenase yielding NH4+) to carbamoyl phosphate synthetase I to initiate the urea cycle.
RDN Nutritional Biochemistry 2
Which enzyme is responsible for the rate-limiting step in fatty acid synthesis?