Precorrin-3B C17-methyltransferase () is an enzyme that catalyzes the chemical reaction
The two substrates of this enzyme are the chlorin, precorrin-3B and S-adenosyl methionine. Its products are precorrin-4 and S-adenosylhomocysteine.
This 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-3B C17-methyltransferase. Other names in common use include precorrin-3 methyltransferase, and CobJ. This enzyme is part of the biosynthetic pathway to cobalamin (vitamin B<sub>12</sub>) in aerobic bacteria and during this step the macrocycle ring-contracts so that the corrin core of the vitamin is formed.