Coordination Chemistry of Microbial Iron Transport Compounds. 21. Kinetics and Mechanism of Iron Exchange in Hydroxamate Siderophore Complexes

Article abstract of DOI:10.1021/ja00412a015

The kinetics of iron exchange and iron removal from two siderophore complexes have been examined, using 55Fe labeling techniques and UV-vis spectrophotometric methods, respectively.Iron exchange between the ferric complexes of ferrioxamine B (FeHDFO⁺) and ferrichrome A (FeDFC^3-), two trihydroxamate siderophores from microbial cultures, is extremely slow; under conditions where there is a 5percent excess of H4DFO⁺ per FeHDFO⁺, the half-time for exchange for equimolar concentrations (4.0 mM) of the two complexes at 25 deg C and pH 7.4 is approximately 220 h.The kinetic reveal an apparent first-order dependence of the rate on each of the competing metal complexes, with the exchange proceeding through a chain reaction involving free ligand.Furthermore, pH dependence studies demonstrate that the exchange rate is accelerated as a function of increasing hydrogen ion concentration.Kinetics of iron removal from ferrioxamine B with use of ethylenediaminetetraacetic acid (EDTA) show first-order dependence on the concentrations of the iron complex and hydrogen ion at 25 deg C, with a pseudo-first-order rate constant of 4.8*10^-5 s^-1 at pH 5.4.Variation of the initial rate with EDTA concentration shows saturation kinetics at high ligand concentrations.The results are interpreted in terms of a two-step mechanism involving: (1) protonation of the ferrioxamine B complex and (2) subsequent bimolecular reaction with EDTA.Results of kinetic studies of the reverse process are consistent with the known equilibrium constants and microscopic reversibility.The reaction is first-order in the FeEDTA complex and desferrioxamine B.Although essentially no pH dependence is observed for this reverse process between pH 4 and 6, the reaction rate varies inversely with hydrogen ion concentration above pH 6.This is explained by consideration of the acid-base equilibria associated with the reactants, which give rise to multiple pathway for product formation.Specifically, the deprotonated form of the ferric-EDTA complex, Fe(OH)EDTA^2-, displays faster reaction kinetics with desferrioxamine B than does its conjugate acid form.A comparison of observed rate constants for the forward and reverse processes with known equilibrium constants shows good agreement.The postulated mechanisms for siderophore mediated microbial iron transport are evaluared in terms of the rates of iron exchange observed in these experiments.