Epimerization of an l-Cysteinyl to a d-Cysteinyl Residue during Thiazoline Ring Formation in Siderophore Chain Elongation by Pyochelin Synthetase from Pseudomonas aeruginosa

The thiazoline-containing siderophores pyochelin, yersiniabactin, and Micacocidin A all have d-thiazoline rings, participating in high-affinity chelation of ferric iron. However, studies with pyochelin (Pch) synthetase and yersiniabactin (Ybt) synthetase reconstituted from pure protein components have shown that only l-cysteine is activated and tethered as a covalent aminoacyl−S−enzyme intermediate. Nor are any of the canonical epimerase domains of nonribosomal peptide synthetase (NRPS) assembly lines found in the Ybt or Pch synthetase modules. Here, we report that the PchE subunit of the Pch synthetase exchanges solvent deuterium into the C2 center of the thiazoline moieties during siderophore chain elongation. Both PchE and HMWP2, from Ybt synthetase, subunits have a 310−360-residue insert in their amino acid activation domains that look like defective methyltransferase (MT) domains. We suggest these inserts are noncanonical epimerase domains, reversibly deprotonating and reprotonating acyl−S−enzyme intermediates at the C2 locus. The PchE subunit does not epimerize the Cys−S−enzyme intermediate, but once amide bond formation from a benzoyl−S−PchE donor is catalyzed by the cyclization (Cy) domain of PchE, the N-benzoyl-Cys−S−PchE intermediate is present as a d,l-mixture. The subsequent phenylthiazolinyl−S−PchE intermediate, arising from cyclodehydration of the N-benzoyl-Cys−S−PchE intermediate, is likewise a d,l-mixture on hydrolytic release and enantiomer analysis. These results suggest a default role for MT domains of NRPS assembly lines in generating α-carbanionic species from thioester intermediates during siderophore chain elongation.