FORMATION OF IRON(III)-TYROSINATE IS THE FASTEST REACTION OBSERVED IN FERRITIN

Rapid mineralization of ferritin, characteristic of protein with H-type subunits, coincides with formation of a specific Fe(III)-tyrosinate complex. The pseudo-first-order rate constant for Fe(II) oxidation by H-subunit-type ferritin has now been shown to be 700-900 times greater than any previously reported for ferritin; k(ox) = 1000 s-1 for formation of the specific Fe(III)-tyrosinate complex (A550nm) or formation of less defined Fe(III)-oxo multinuclear complexes (A420nm). Formation of multinuclear Fe(III)-oxo complexes and O2 consumption were biphasic. In the first phase, up to 50 Fe atoms/ferritin molecule were rapidly oxidized, accompanied by formation of the Fe(III)-tyrosinate complex; saturation of the sites which formed the Fe(III)-tyrosinate complex also required 50 Fe/ferritin molecule. The sigmoidal shape of the curve obtained by plotting the initial rate of oxidation during the rapid phase of mineralization versus added [Fe(II)] suggested a more complex reaction pathway of ferroxidation than previously described. During the second phase of mineralization, Fe(III)-tyrosinate decreased, but multinuclear Fe(III)-oxo complexes and O2 consumption continued to increase at a slower rate. Recovery of the rapid oxidation pathway paralleled recovery of the site for Fe(III)-tyrosinate formation; full regeneration of the Fe(III)-tyrosinate sites was gradual over a period of 12 h, as if the movement of Fe(III) along the biomineralization pathway in the protein was slow and was accompanied by conformational changes which affected the Fe(III)-tyrosinate site. The rapid mineralization rates characteristic of ferritin with H-type subunits clearly involves Fe(III)-tyrosinate at a very early, and possibly the first, stop along the route of Fe through the protein coat to the central core.