The iron oxidation and hydrolysis chemistry of Escherichia coli bacterioferritin

Bacterioferritins are members of a class of spherical shell-like iron storage proteins that catalyze the oxidation and hydrolysis of iron at specific sites inside the protein shell, resulting in formation of a mineral core of hydrated ferric oxide within the protein cavity. Electrode oximetry/pH stat was used to study iron oxidation and hydrolysis chemistry in E. coli bacterioferritin. Consistent with previous UV-visible absorbance measurements, three distinct kinetic phases were detected, and the stoichiometric equations corresponding to each have been determined. The rapid phase 1 reaction corresponds to pairwise binding of 2 Fe2+ ions at a dinuclear site, called the ferroxidase site, located within each of the 24 subunits, viz., 2Fe(2+) + P-Z --> [Fe-2-P](Z) + 4H(+), where P-Z is the apoprotein of net charge Z and [Fe-2-P](Z) represents a diferrous ferroxidase complex. The slower phase 2 reaction corresponds to the oxidation of this complex by molecular oxygen according to the net equation: [Fe-2-P](Z) + 1/2 O-2 --> [Fe2O-P](Z) where [Fe2O-P](Z) represents an oxidized diferric ferroxidase complex, probably a mu-oxo-bridged species as suggested by UV-visible and EPR spectrometric titration data. The third phase corresponds to mineral core formation according to the net reaction: 4Fe(2+) + O-2 + 6H(2)O --> 4FeO(OH)((core)) + 8H(+). Iron oxidation is inhibited by the presence of Zn2+ ions, The patterns of phase 2 and phase 3 inhibition are different, though inhibition of both phases is complete at 48 Zn2+ per 24mer, i.e., 2 Zn2+ per ferroxidase center.