One-carbon Metabolism: Basic Concepts, continued
S-ADENOSYLMETHIONINE (SAM)
SAM is the most prolific donor of one carbon groups in biosynthetic reactions. Its formation is catalyzed by methionine adenosyl transferase. It costs THREE high-energy phosphate bonds to make SAM.
Synthesis of S-adenosylmethionine is depicted in Devlin, Figure 19.41.
The particularly important feature of SAM is that it donates methyl groups (we call them " active ~CH3 " to a large number of acceptors, including DNA, RNA, phospholipids, and many proteins. Donation of these methyl groups is part of a small cycle:
Resynthesis of methionine is depicted in Devlin, Figure 19.44
The step in which methionine is re-generated requires two important vitamin derivatives:
- Methyl-cobalamin, a derivative of vitamin B12, which donates the methyl group to homocysteine
- N5 -methyl-tetrahydrofolate, which donates its methyl group to cobalamin, and allows the reaction to continue. Dietary or other deficiencies of either vitamin can cause serious problems. Notice that all three of the one-carbon carriers are involved in this cycle. SAM is also used to make cysteine.
Synthesis of cysteine from SAM is depicted in Devlin, Figure 19.42
