Characterization of SafC, a catechol 4-O-methyltransferase involved in saframycin biosynthesis

Members of the saframycin/safracin/ecteinascidin family of peptide natural products are potent antitumor agents currently under clinical development. Saframycin MX1, from Myxococcus xanthus, is synthesized by a nonribosomal peptide synthetase, SafAB, and an O-methyltransferase, SafC, although other proteins are likely involved in the pathway. SafC was overexpressed in Escherichia coli, purified to homogeneity, and assayed for its ability to methylate a variety of substrates. SafC was able to catalyze the O-methylation of catechol derivatives but not phenols. Among the substrates tested, the best substrate for SafC was L-dihydroxyphenylalanine (L-dopa), which was methylated specifically in the 4'-O position (k(cat)/K-m = 5.5 x 10(3) M-1 s(-1)). SafC displayed less activity on other catechol derivatives, including catechol, dopamine, and caffeic acid. The more labile L-5'-methyldopa was an extremely poor substrate for SafC (k(cat)/K-m = similar to 2.8 x 10(-5) M-1 s(-1)). L-Dopa thioester derivatives were also much less reactive than L-dopa. These results indicate that SafC-catalyzed 4'-O-methylation Of L-dopa occurs prior to 5'-C-methylation, suggesting that 4'-O-methylation is likely the first committed step in the biosynthesis of saframycin MX1. SafC has biotechnological potential as a methyltransferase with unique regioselectivity.