Precorrin-3B C17-methyltransferase

{{infobox enzyme

| Name = precorrin-3B C17-methyltransferase

| EC_number = 2.1.1.131

| CAS_number = 152787-64-9

| GO_code = 0030789

| image =

| width =

| caption =

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In enzymology, precorrin-3B C17-methyltransferase ({{EC number|2.1.1.131}}) is an enzyme that catalyzes the chemical reaction

:S-adenosyl-L-methionine + precorrin-3B $\backslash rightleftharpoons$ S-adenosyl-L-homocysteine + precorrin-4

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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.