A vitamin B12 deficiency may affect heme synthesis by reducing the concentration of which of the following? Choose the one best answer.
1) Acetyl-CoA
2) Succinyl-CoA
3) Glycine
4) Alanine
📝 Explanation:
Vitamin B12 participates in two reactions in the body-conversion of homocysteine to methionine and conversion ofmethylmalonyl-CoA to succinyl-CoA.
Methylmalonyl-CoA is produced via various amino acid degradation pathways, and from oddcarbon chain fatty acid oxidation. In the absence of B12, succinyl-CoA would only be produced as an intermediate of the TCA cycle, and if it were removed from the cycle for heme synthesis, energy production may suffer.
Glycine is usually obtained from the diet, although in a B12 deficiency a functional folate deficiency may also develop, leading to an inhibition of serine hydroxymethyltransferase, the enzyme that converts serine to glycine, and requires free tetrahydrofolate.
Succinyl-CoA and glycine are the precursors for heme synthesis. A B12 deficiency would not inhibit the production of acetyl-CoA, succinate, or alanine.
MCQ