title: Studies on the complex formed between bacitracin A and divalent cations.
abstract: 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.
mesh_terms: D001414:Bacitracin / Q000031:analogs & derivatives*; D002413:Cations, Divalent; D003619:Dansyl Compounds; D007700:Kinetics; D009532:Nickel; D011485:Protein Binding; D014302:Trinitrobenzenesulfonic Acid; D015032:Zinc
chemical_list: D002413:Cations, Divalent; D003619:Dansyl Compounds; D001414:Bacitracin; D009532:Nickel; D014302:Trinitrobenzenesulfonic Acid; C026461:bacitracin A; D015032:Zinc; C029038:dansyl chloride