Substrate-Induced Conformational Changes of the Tyrocidine Synthetase 1 Adenylation Domain Probed by Intrinsic Trp Fluorescence

Nonribosomal peptide synthetases (NRPS) are multifunctional proteins that catalyze the synthesis of the peptide products with enormous biological potential. The process of biosynthesis starts with the adenylation (A) domain, which during the catalytic cycle undergoes extensive structural rearrangements. In this paper, we present the first study of the tyrocidine synthetase 1 A-domain (TycA-A) fluorescence properties. The TycA-A protein contains five potentially fluorescent Trp residues at positions 227, 301, 323, 376 and 406. The contribution of each Trp to the TycA-A emission was determined using protein variants bearing single Trp to Phe substitutions. The accessibility of the Trp side chains during adenylation showed that only W227 is affected by substrate binding. The protein variant containing solely fluorescent W227 residue was constructed and further used as a probe to explore the binding effect of different non-cognate amino acid substrates. The results indicate a different accessibility of W227 residue in the presence of non-cognate amino acids, which might offer an explanation for the higher aminoacyl-adenenylate leakage. Overall, our results suggest that intrinsic tryptophan fluorescence could be used as a method to probe the effect of substrate binding on the local structure in NRPS adenylation domains.