In enzymology, precorrin-2 C20-methyltransferase () is an enzyme that catalyzes the chemical reaction
This methylation reaction converts precorrin-2 into precorrin-3A. The methyl group comes from the cofactor, S-adenosyl methionine (SAM), which gives S-adenosyl-L-homocysteine (SAH).
The enzyme belongs to the family of transferases, specifically those transferring one-carbon group methyltransferases. The systematic name of this enzyme class is S-adenosyl-L-methionine:precorrin-4 C20-methyltransferase and another names in common use is CobI. It is part of the biosynthetic pathway to cobalamin (vitamin B12) in aerobic bacteria.
Structural studies As of late 2007, two structures have been solved for this class of enzymes, with PDB accession codes and .
See also Cobalamin biosynthesis Cobalt-factor II C20-methyltransferase for enzyme that gives the same reaction in anaerobic organisms
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