Studies on the complex formed between bacitracin A and divalent cations.

Bacitracin A is a peptide antibiotic which forms stoichiometric complexes with divalent cations, including Ni2+ and Zn2+. In this paper it is shown that the metal-bacitracin complex contains a group which has a pKa near pH 5.5. Deprotonation of the group is concomitant with the aggregation and precipitation of the metal-bacitracin complex. Bacitracin A, in the absence of metals, does not contain any group which has a pKa in this range. It is postulated that this group is the N-terminal amino of isoleucine, which was previously postulated not to be directly involved in metal coordination based on proton release measurements. An attempt was made to demonstrate directly that the N-terminal amino group is not coordinated to the metal by examining the reactivity of this group with 2,4,6-trinitrobenzene sulfonate. It was clearly shown that bound metals protect the N-terminal amino group from reacting with this reagent. It is speculated that this metal-protection results from a combination of factors, including steric hindrance.