by one of the two classes of methionine synthases (i.e. cobalamin-dependent or cobalamin-independent). This methionine can then be converted back to SAM, completing the cycle. In the rate-limiting step of the SAM cycle, MTHFR (methylenetetrahydrofolate reductase) irreversibly reduces 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate.

Radical SAM enzymes

A large number of enzymes cleave SAM reductively to produce radicals: 5´-deoxyadenosyl 5´-radical,

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