Proton Nuclear Magnetic Resonance Studies on Bacitracin A and its Interaction with Zinc Ion

Bacitracin A is a cyclic dodecapeptide antibiotic produced by Bacillus licheniformis. A divalent metal cation, such as Zn²⁺, is required for biological activity. Previous studies have shown one metal ion bound per bacitracin A molecule in solution. A model has been proposed [Scogin, D. A., Mosberg, M. I., Storm, D. R., & Gennis, R. B. (1980) Biochemistry (preceding paper in this issue)] in which the metal is bound to the histidine, thiazoline, and glutamic acid residues of the peptide. The aspartic acid and N-terminal amino group are not directly involved in metal binding, but the pAT of the latter group is lowered in the presence of the bound metal cation. in this paper, high-resolution ¹H NMR is used to examine Zn²⁺ binding to bacitracin A. Assignments were made by monitoring the pH dependence of the ¹H NMR chemical shifts and also by using homonuclear decoupling at 270 MHz. Resonances were assigned to the N-terminal isoleucine, the thiazoline ring, the histidine, the glutamic acid, and the aspartic acid residues. in the presence of 0.025 M Zn²⁺ at pH 4.8, a number of resonances are perturbed. Analysis indicates Zn²⁺ binding to the thiazoline ring, the histidine imidazole, and the glutamic acid carboxyl. The aspartate group is not involved in metal binding, as previously postulated. Data pertaining to the involvement of the Nterminal amino are equivocal but are consistent with the suggested model. All the ¹H NMR data are compatible with this model. To a large extent, this defines the conformation of the zinc-bacitracin A complex in solution.