Bacitracin A is a cyclic dodecapeptide antibiotic produced by Bacillus licheniformis. Bacteriocidal activity requires the presence of divalent cations such as Zn2+. The metal-bacitracin A complex binds to bactoprenyl pyrophosphate, a lipid intermediate required for cell wall biosynthesis which is found within the bacterial membrane. in this paper, the pH dependence of the metal binding to bacitracin A is investigated in an effort to define the sites of metal coordination. Most of the studies described in this report were performed with Ni2+ and Zn2+. Metal binding was monitored by observing changes in the ultraviolet absorption spectrum of bacitracin A and by monitoring the proton release which is concomitant with metal binding to the peptide. It was determined that both Ni2+ and Zn2+ form 1:1 complexes with bacitracin A in solution. These complexes are soluble in acidic solutions, but above approximately pH 5.5 they become insoluble. On the basis of the data reported as well as results previously reported from other laboratories, a model for divalent metal ion binding to bacitracin is suggested. It is proposed that the metal coordinates directly to the glutamate carboxyl, the histidine imidazole, and the thiazoline ring. The aspartate carboxyl and N-terminal amino group are not directly involved in metal binding. It is further proposed that due to the proximity of the metal, the pK of the N-terminal amino is shifted from 7.7 to 5.7 upon metal binding. Deprotonation of this group is suggested to cause precipitation of the bacitracin A-metal complex. This model is consistent with all the metal binding data and, furthermore, is consistent with the *H NMR data presented in the accompanying paper [Mosberg, H. I., Scogin, D. A., Storm, D. R., & Gennis, R. B. (1980) Biochemistry (following paper in this issue)].
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