The binuclear iron centers of uteroferrin and the purple acid phosphatases

Abstract

The purple acid phosphatases comprise a group of proteins distinguished by their enzymic activity, distinctive color and, most importantly, the presence of a spin-coupled binuclear iron center. This center exists in two stable interconvertible states: pink, reduced, EPR-visible and enzymically active, with an antiferromagnetically exchanged Fe(II)-Fe(III) cluster, and purple, oxidized, EPR-silent and inert, with the binuclear pair as Fe(III)-Fe(III). In uteroferrin, the purple acid phosphatase of uterine secretions, a transient intermediate species has also been identified. Engendered by the interaction of phosphate with the pink form of uteroferrin, this transient intermediate is purple, EPR-silent and devoid of the contact-shifted proton resonances seen in its pink parent. Nevertheless, it is paramagnetic, with an Fe(II)-Fe(III) couple demonstrable by Mossbauer spectroscopy. Although considerable progress toward characterizing the properties of uteroferrin and the purple acid phosphatases has been achieved in recent years, enigmas persist. The identity of the ligands of each iron atom and how these change in nature or arrangement during redox transitions is still unknown. Perhaps most interestingly, the mechanism of enzymic activity, and the relation of the redox and enzymatic properties of the purple acid phosphatases to their yet uncertain physiological roles, remain to be established.