Erythromycin 3''-O-methyltransferase (, EryG) is an enzyme with systematic name S-adenosyl-L-methionine:erythromycin C 3''-O-methyltransferase. This enzyme catalyses two related chemical reactions. The first is the final step in the biosynthesis of the antibiotic, erythromycin A:
This is a methylation reaction at the 3-position of the mycarose group; the methyl group comes from the cofactor, S-adenosyl methionine (SAM), which becomes S-adenosyl-L-homocysteine (SAH). The enzyme was characterised from Saccharopolyspora erythraea.
The enzyme can also convert erythromycin D to erythromycin B.
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