EC 2.1.1.254     
Accepted name: erythromycin 3′′-O-methyltransferase
Reaction: (1) S-adenosyl-L-methionine + erythromycin C = S-adenosyl-L-homocysteine + erythromycin A
(2) S-adenosyl-L-methionine + erythromycin D = S-adenosyl-L-homocysteine + erythromycin B
Other name(s): EryG
Systematic name: S-adenosyl-L-methionine:erythromycin C 3′′-O-methyltransferase
Comments: The enzyme methylates the 3 position of the mycarosyl moiety of erythromycin C, forming the most active form of the antibiotic, erythromycin A. It can also methylate the precursor erythromycin D, forming erythromycin B, which is then converted to erythromycin A by EC 1.14.13.154, erythromycin 12 hydroxylase.
References:
1.  Paulus, T.J., Tuan, J.S., Luebke, V.E., Maine, G.T., DeWitt, J.P. and Katz, L. Mutation and cloning of eryG, the structural gene for erythromycin O-methyltransferase from Saccharopolyspora erythraea, and expression of eryG in Escherichia coli. J. Bacteriol. 172 (1990) 2541–2546. [PMID: 2185226]
2.  Summers, R.G., Donadio, S., Staver, M.J., Wendt-Pienkowski, E., Hutchinson, C.R. and Katz, L. Sequencing and mutagenesis of genes from the erythromycin biosynthetic gene cluster of Saccharopolyspora erythraea that are involved in L-mycarose and D-desosamine production. Microbiology 143 (1997) 3251–3262. [PMID: 9353926]
[EC 2.1.1.254 created 2012]