Communication between the active sites in dimeric mercuric ion reductase: an alternating sites hypothesis for catalysis

Mercuric reductase, a flavoprotein disulfide oxidoreductase, catalyzes the two-electron reduction of Hg(II) to Hg(0) by NADPH. As with all the members of this class of proteins, the enzyme is a dimer of identical subunits with two active sites per dimer, each composed of one FAD and catalytically essential residues from both subunits. In the enzyme from Tn501, these residues include, at a minimum, FAD and cysteines 135 and 140 from one subunit and cysteines 558' and 559' from the other. With this sort of active site arrangement, the enzyme seems perfectly set up for some type of subunit communication. In this report, we present results from several titrations, as well as kinetics studies, that, taken together, are consistent with the occurrence of subunit communication. In particular, the results indicate that pyridine nucleotide complexed dimers of the enzyme are asymmetric. Since the EH2-NADPH complex of the enzyme is the relevant reductant of Hg(II), these observations suggest that the enzyme may function asymmetrically during catalysis. An alternating sites model is proposed for the catalytic reduction of Hg(II), where both subunits of the dimer function in catalysis, but the steps are staggered and the subunits reverse roles after part of the reaction. An attractive feature of this proposal is that it provides a reasonable solution to the thermodynamic dilemma the enzyme faces in needing to both bind Hg(II) very tightly and reduce it.