RDN Nutritional Biochemistry 5

Question 1

What is the consequence of biotin deficiency on fatty acid synthesis?

Accepted Answer

Reduced activity of acetyl-CoA carboxylase, decreasing malonyl-CoA production

Answer

Biotin is the covalently attached cofactor of acetyl-CoA carboxylase; deficiency impairs carboxylation of acetyl-CoA to malonyl-CoA, blocking the first committed step in de novo fatty acid synthesis.

Question 2

Which enzyme deficiency leads to the accumulation of branched-chain amino acids (leucine, isoleucine, valine) as seen in maple syrup urine disease?

Accepted Answer

Branched-chain alpha-keto acid dehydrogenase complex

Answer

Maple syrup urine disease results from deficiency of branched-chain alpha-keto acid dehydrogenase, which oxidatively decarboxylates the keto acids derived from BCAA transamination.

Question 3

In the fed state, which transcription factor is activated by insulin signaling to upregulate lipogenic gene expression?

Accepted Answer

SREBP-1c (Sterol Regulatory Element Binding Protein-1c)

Answer

Insulin activates SREBP-1c, which translocates to the nucleus and upregulates transcription of lipogenic enzymes including fatty acid synthase and acetyl-CoA carboxylase.

Question 4

Which reaction in the TCA cycle generates GTP (or ATP) directly via substrate-level phosphorylation?

Accepted Answer

Succinyl-CoA synthetase reaction

Answer

Succinyl-CoA synthetase couples the hydrolysis of the thioester bond in succinyl-CoA to the phosphorylation of GDP (or ADP) to form GTP (or ATP) in substrate-level phosphorylation.

Question 5

Selenium is an essential component of which enzyme critical for thyroid hormone metabolism?

Accepted Answer

Iodothyronine deiodinase

Answer

Iodothyronine deiodinases are selenoproteins that convert T4 (thyroxine) to the active T3 (triiodothyronine) by removing an iodine atom; selenium deficiency can impair thyroid hormone activation.

(RDN) Registered Dietitian Nutritionist Practice Test

(RDN) Registered Dietitian Nutritionist Practice Test

RDN Nutritional Biochemistry 5