Characterization of iturin synthetase in the wild-type Bacillus subtilis strain producing iturin and in an iturin deficient mutant

Assembly line enzymology by multimodular nonribosomal peptide synthetases: the thioesterase domain of E. coli EntF catalyzes both elongation and cyclolactonization

BACKGROUND

EntF is a 142 kDa four domain (condensation-adenylation-peptidyl carrier protein-thioesterase) nonribosomal peptide synthetase (NRPS) enzyme that assembles the Escherichia coli N-acyl-serine trilactone siderophore enterobactin from serine, dihydroxybenzoate (DHB) and ATP with three other enzymes (EntB, EntD and EntE). To assess how EntF forms three ester linkages and cyclotrimerizes the covalent acyl enzyme DHB-Ser-S-PCP (peptidyl carrier protein) intermediate, we mutated residues of the proposed catalytic Ser-His-Asp triad of the thioesterase (TE) domain.

RESULTS

The Ser1138-->Cys mutant (kcat decreased 1000-fold compared with wild-type EntF) releases both enterobactin (75%) and linear (DHB-Ser)2 dimer (25%) as products. The His 1271-->Ala mutant (kcat decreased 10,000-fold compared with wild-type EntF) releases only enterobactin, but accumulates both DHB-Ser-O-TE and (DHB-Ser)2-O-TE acyl enzyme intermediates. Electrospray ionization and Fourier transform mass spectrometry of proteolytic digests were used to analyze the intermediates.

CONCLUSIONS

These results establish that the TE domain of EntF is both a cyclotrimerizing lactone synthetase and an elongation catalyst for ester-bond formation between covalently tethered DHB-Ser moieties, a new function for chain-termination TE domains found at the carboxyl termini of multimodular NRPSs and polyketide synthases.

Biosynthetic systems for nonribosomal peptide antibiotic assembly

Modular peptide synthetases, which act as the protein templates for the synthesis of a large number of peptide antibiotics and siderophores, hold great potential for the development of novel compounds. Recently, significant progress has been made towards understanding their molecular architecture and substrate specificity. The first crystal structure of a peptide synthetase has been solved, and the enzymes responsible for post-translational modification of peptide synthetases have recently been discovered. These will allow addressing important yet poorly understood mechanistic aspects.