This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Vitamins, Subject name: Biochemistry
All are true about Vitamin B|2, except - 
Select and explain. 
A. Active form is methylcobalamine, B. Requires for conversion of homocysteine to methionine, C. Requires in metabolism of methylmalonyl Co A, D. Requires for conversion of pyruvate to lactate
D: Requires for conversion of pyruvate to lactate
Answer: D: Requires for conversion of pyruvate to lactate 
Explanation: Ans. is 'd' i.e., Requires for conversion of pyruvate to lactate o Active form of vitamin B12 are methylcobalamine and deoxyadenosylcobalamine. Following reactions require vitamin B12 coenzyme:i) Isomerization of methylmalonyl CoA to succinyl CoA : In this reaction, active form of vitamin B12 is deoxyadenosyl cobalamine. Propionyl-CoA is produced as catabolic end product of some alipathic amino acids and b-oxidation of odd chain fatty acids. Propionyl CoA is then converted to succinyl CoA through methyimalonyi-CoA. Thus methylmalonyl-CoA is accumulated and excreted in urine as methylmalonic acid (methylmalonate) in vitamin B12 deficiencyQ, i.e. methylmalonic aciduriaQ.ii) Conversion of homocystein to methionine : - In this reaction, active form is methylcobalamine. This is the only reaction which requires both vitamin B12, (as methylcobalamine) and folic acid (as N2-methyl- H4-folate). The reaction is catalyzed by the enzyme cobalamin-dependent methionine synthase also called 5-methyltetrahydrofolate-homocysteine methyltransferaseQ.iii) Methylation of pyrimidine ring to form thymine.iv) Conversion of ribonucleotides to deoxyribonucleotides in DNA synthesis.v) Required for metabolism of diols :vi) In bacteria for interconversion of glutamate and b methyl asparate