In enzymology, a magnesium protoporphyrin IX methyltransferase (EC 2.1.1.11) is an enzyme that catalyzes the chemical reaction

S-adenosyl-L-methionine + magnesium protoporphyrin IX ${\displaystyle \rightleftharpoons }$ S-adenosyl-L-homocysteine + magnesium protoporphyrin IX 13-methyl ester

The two substrates of this enzyme are S-adenosyl methionine and magnesium protoporphyrin IX; its two products are S-adenosylhomocysteine and magnesium protoporphyrin IX 13-methyl ester.

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:magnesium-protoporphyrin-IX O-methyltransferase. This enzyme is part of the biosynthetic pathway to chlorophylls.^[1][2]

• Biosynthesis of chlorophylls