Roles of vitamin B12 and folate in metabolic pathways

Both folate and the vitamin B12 derivative methylcobalamin are used in methyl transfer to convert homocysteine to methionine. This is why deficiency of vitamin B12 or folate causes increased homocysteine.

Only vitamin B12 is involved in hydrogen transfer to convert methylmalonic acid (MMA) to succinyl-CoA. This is why only vitamin B12 deficiency causes increased MMA.