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 B<sub>12</sub>) in aerobic bacteria.
As of late 2007, two structures have been solved for this class of enzymes, with PDB accession codes and .