MCQ – 87730

Correct Answer � C Explanation

Ans.C Methionine

Methionine reacts with ATP forming S-adenosylmethionine, “active methionine”. S-adenosyl-methionine is converted to S-adenosyl-homocysteine after transfer of its activated methyl group, and S-adenosyl-homocysteine is then catabolized to form adenosine and homocysteine. Conversion of homocysteine to cystathionine requires the enzyme cystathionine synthase, Vitamin B6 as a cofactor, and the amino acid serine. Alternatively, homocysteine can be converted back to methionine by combining with N-5-methyl-tetrahydrofolate in a reaction requiring vitamin B12 (cobalamin).

In vitamin B12 deficiency, the utilization of the N5-methyl form of tetrahydrofolate in the B12-dependent methylation of homocysteine to methionine is impaired. Because the methylated form cannot be converted directly to other forms of tetrahydrofolate, folate is trapped in the N5-methyl form, which accumulates. The levels of the other forms decrease. Thus, cobalamin deficiency is hypothesized to lead to a deficiency of the tetrahydrofolate forms needed in purine and TMP synthesis, resulting in the symptoms of megaloblastic anemia.