The iron storage protein bacterioferritin (BFR) consists of 24 identical subunits, each containing a dinuclear metal binding site called the ferroxidase center, which is essential for fast iron core formation. Cobalt(II) binding to wild-type and site-directed variants of Escherichia coli BFR was studied by optical and magnetic techniques. Data from absorption spectroscopy demonstrate the binding of two cobalt(II) ions per subunit of wild-type and heme-free BFR, each with a pseudotetrahedral or pentacoordinate geometry, and EPR studies show that the two cobalt(II) ions are weakly magnetically coupled. Studies of variants of BFR in which a single glutamic acid residue at the ferroxidase center is replaced by alanine confirm that this is the site of cobalt(II) binding, since the altered centers bind only one cobalt(II) ion. This work shows that the electroneutrality of the ferroxidase center is preserved on binding a pair of divalent metal ions. Optical and EPR data show that cobalt(II) binding to BFR exhibits positive cooperativity, with an average Kd of approximately 1 x 10(-5) M. The favored filling of the ferroxidase center with pairs of metal ions may have mechanistic implications for the iron(II) binding process. Discrimination against oxidation of single iron(II) ions avoids odd electron reduction products of oxygen.