3 years ago

Elucidation of the Stereospecificity of C-Methyltransferases from trans-AT Polyketide Synthases

Elucidation of the Stereospecificity of C-Methyltransferases from trans-AT Polyketide Synthases
David E. Cane, Chaitan Khosla, Xinqiang Xie
S-Adenosyl methionine (SAM)-dependent C-methyltransferases are responsible for the C2-methylation of 3-ketoacyl-acyl carrier protein (ACP) intermediates to give the corresponding 2-methy-3-ketoacyl-ACP products during bacterial polyketide biosynthesis mediated by trans-AT polyketide synthases that lack integrated acyl transferase (AT) domains. A coupled ketoreductase (KR) assay was used to assign the stereochemistry of the C-methyltransferase-catalyzed reaction. Samples of chemoenzymatically generated 3-ketopentanoyl-ACP (9) were incubated with SAM and BonMT2 from module 2 of the bongkrekic acid polyketide synthase. The resulting 2-methyl-3-ketopentanoyl-ACP (10) was incubated separately with five (2R)- or (2S)-methyl specific KR domains. Analysis of the derived 2-methyl-3-hydroxypentanoate methyl esters (8) by chiral GC-MS established that the BonMT2-catalyzed methylation generated exclusively (2R)-2-methyl-3-ketopentanoyl-ACP ((2R)-10). Identical results were also obtained with three additional C-methyltransferases—BaeMT9, DifMT1, and MupMT1—from the bacillaene, difficidin, and mupirocin trans-AT polyketide synthases

Publisher URL: http://dx.doi.org/10.1021/jacs.7b02911

DOI: 10.1021/jacs.7b02911

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