| Bacitracin A is a cyclic dodecapeptide antibiotic produced by Bacillus licheniformis. Bacteriocidal activity requires the presence of divalent metal cations such as Zn('2+). The metal-bacitracin A complex binds to bactoprenylpyrophosphate, a lipid intermediate required for cell wall biosynthesis which is found within the bacterial membrane. In this thesis, the pH dependence of the metal binding to bacitracin A has been investigated in an effort to define the sites of metal coordination. Most of the studies described in this report were performed with Ni('2+) and Zn('2+). Metal binding was monitored by observing changes in the ultraviolet spectrum of bacitracin A, by monitoring the proton release which is concomitant with metal binding to the peptide, and by observing changes in the proton nuclear magnetic resonance spectrum. It was determined that both Ni('2+) and Zn('2+) 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 here 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 ion coordinates directly to the glutamate carboxyl, the histidine imidazole, and the thiazoline ring. The asparate 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(,a) of the N-terminal amino is shifted from 7.7 to about 5.7 upon metal binding. Deprotonation of this group within the complex is suggested to cause precipitation of the metal-bacitracin A complex. This model is consistent with all the metal binding data and, furthermore, is consistent with the high resolution proton nuclear resonance data obtained for zinc-bacitracin A. To a large extent, this model defines the conformation of the metal-bacitracin A complex in solution. |
