CHARGE COMPENSATED BINDING OF DIVALENT METALS TO BACTERIOFERRITIN - H+ RELEASE ASSOCIATED WITH COBALT(II) AND ZINC(II) BINDING AT DINUCLEARMETAL SITES

Divalent metal ion binding to the bacterial iron-storage protein, bact erioferritin (BFR), which contains a dinuclear metal binding site with in each of its 24 subunits, was investigated by potentiometric and spe ctrophotometric methods, Cobalt(II) and zinc(II) were found to bind at both high- and low-affinity sites, Cobalt(II) binding at the high-aff inity site was observed at a level of two per subunit with the release of similar to 1.6 protons per metal ion, thus confirming the dinuclea r metal centre as the high-affinity site, Zinc(II) binding at the dinu clear centre (high-affinity site) resulted in the release of similar t o 2 protons per metal ion, but exhibited a binding stoichiometry which indicated that not all dinuclear centres were capable of binding two zinc(II) ions. Competition data showed that binding affinities for the dinuclear centre mere in the order zinc(II) > cobalt(II), and also co nfirmed the unexpected stoichiometry of zinc(II) binding, This work em phasises the importance of charge neutrality at the dinuclear centre.