Chemoenzymatic approaches for streamlined detection of active site modifications on thiotemplate assembly lines using mass spectrometry.

For the direct interrogation of peptides harboring covalently modified serines in nonribosomal peptide synthetases, streamlined methodologies described here employ proteolysis and reporter-coenzyme A analogues of four types. The chromophoric and fluorescent coenzyme A analogues pyrene-maleimidyl-S-CoA and BODIPY-FL-N-(2-aminoethyl)maleimidyl-S-CoA were enzymatically loaded onto the active site serines harbored in the ArCP, PCP1, and PCP2 thiolation domains of PchE and PchF, the nonribosomal peptide synthetases responsible for the biosynthesis of the siderophore pyochelin. During the chromatographic separation of cyanogen bromide digests, observation of the absorbance (at 338 and 504 nm) or fluorescence (after irradiation at 365 nm) enabled the selective detection of peptides containing each active site serine. This resulted in quick detection of each active site peptide by Fourier transform mass spectrometry in the fully reconstituted pyochelin system. The loading of short acyl chain reporters in equimolar quantities permitted further insights into digestion heterogeneity and side reactions by virtue of a mass shift signature on each active site peptide. The chromatographic shift of the reporter-loaded peptides relative to peptides carrying on pathway intermediates was 2 min at 7 kDa, providing a general strategy for efficient localization of "carrier" peptides in complex digests of thiotemplate enzymes. Also, the use of the affinity reporter, biotin-maleimidyl-S-coenzyme A, permitted the isolation of intact synthetases at high purity via removal of contaminating Escherichia coli proteins.