In enzymology, precorrin-3B C17-methyltransferase (EC 2.1.1.131) is an enzyme that catalyzes the chemical reaction The two substrates of this enzyme are S-adenosyl methionine and precorrin 3B, and its two products are S-adenosylhomocysteine and precorrin 4.
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 B12) in aerobic bacteria and during this step the macrocycle ring-contracts so that the corrin core of the vitamin is formed.