B12 is needed in which step of folate synthesis-

Ans. is 'b' i.e., Methyl tetrahvdrofolate to tetrahydrofolate o When acting as a methyl donor, S-adenosylmethionine forms homocystein, wrhich may be remethylated by methyl- tetrahvdrofolate catalyzed by methionine synthase, a vitamin B12 dependent enzyme,o In the reaction methyl-THF is converted to tetrahydrofolate (THF) and methvl-THF acts as the major source of THF for tissues.o Deficiency of vitamine-B12 also causes folate trapQ.o This is because homocysteine cannot be converted to methionine and in the same reaction N5-methyl THF (N5-methyl-H4 folate) cannot be converted to free THF (H4 folate), due to deficiency of vitamin B12.o Thus, most of the folic acid is trapped as its methyl derivative, i.e. N5-methyl THF (Note : THF stands for tetrahydrofolate).o Thus a vitamin B12 deficiency can lead to a folate deficiency.o Due to this action of vitamin B12. (Le., help to generate free THF for reuse), it is involved in nucleic acid metabolism indirectly as folic acid (THF) is required for one carbon unit transfer in nucleic acid (DNA and RNA) metabolism.