Substrate Specificity of BaeL KS

2 nESI mass spectra of (T B) Psy KSl, KS2, KS3 and Bae KS5, KS5(M237A) sprayed from 80 20 MeCNiHiO 0.1 % TEA. Theoretical and measured molecular weights are annotated on the spectra 

Initial rate was calculated from a ln[KS]/[KSo] versus t plot, given to 2 significant figures 

The (/ )/(5)-P-hydroxyl SNACs 25 and 26 were found to acylate KS5, but at a far reduced rate to that of the unbranched SNACs. Interestingly, the P-keto SNAC (24) acylated KS5 reasonably well, this may be due to the reduced steric bulk of the sp fi-keto compared to that of the sp fl-hydroxyl group. Another explanation 

4 Deconvoluted ESI mass spectrum of Bae KS5 after incubation with SNAC derivative (22). Two attachments of the full SNAC to the KS were observed, highlighted by successive mass shifts of + 187 Da 

This initial set of data clearly demonstrates that simple, short-chain SNAC mimics of the acyl-phosphopantetheine chain are able to probe the substrate specificity of BaeL KS5. This KS domain is situated before a key -branching step in the biosynthesis of 18, and is shown to from these assays to be highly specific for an unbranched substrate, providing evidence that examining the specificity of KS domains may aid biosynthetic assignment of PKSs. 

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