Source:http://linkedlifedata.com/resource/umls/id/C1152311
GO: Catalysis of the reaction: S-adenosyl-L-methionine(1+) + magnesium protoporphyrin IX = S-adenosyl-L-homocysteine + H(+) + magnesium protoporphyrin IX 13-monomethyl ester. [EC:2.1.1.11, RHEA:17812]