Mechanism of mercury(II) reductase and influence of ligation on the reduction of mercury(II) by a water soluble 1,5-dihydroflavin.

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Abstract
The nature and rate of reduction of Hg2+ to Hg0 by 1,5-dihydro-3,(3-sulfopropyl)lumiflavin (FlH2) in buffered aqueous solutions (pH 4.7) is dependent on the ligation of Hg2+. In the presence of N,N-bis(2-hydroxyethyl)glycine or when ligated to ethylenediaminetetraacetic acid, the reduction is first order in Hg2+ and FH2. The apparent second-order rate constant with N,N-bis(2-hydroxyethyl)glycine (2.2 .times. 106 M-1 .cntdot. s-1) is much greater than that in the presence of ligating ethylenediaminetetraacetic acid (1.5 .times. 102 M-1 .cntdot. s-1). When ligated by mercaptoethanesulfonate, reduction of Hg2+ by FH2 is characterized by a pronounced lag phase, which is dependent on the concentration of mercaptoethanesulfonate. The rate decreases with increase in mercaptoethanesulfonate, and with an excess of 10 equivalents, Hg2+ is not reduced by FH2. These observations show that bis-ligation by thiolate greatly decreases the reducibility of Hg2+ and that further ligation by thiolate further retards the reaction. Comparision of oxidation-reduction potentials at various pH values shows that bis-ligation (or greater) of Hg2+ by thiolate substantially lowers the reduction potential of Hg2+ below that of 3(3-sulfopropyl)lumiflavin (Fox). Thus, the ease of reduction of Hg2+ complexes by FH2 decreases with increasing thermodynamic stability of the complex. These results do not support the proposed role of the thiol functionalities in facilitating the mercury (II) reductase (Hg:NADP+ oxidoreductase, EC 1.16.1.1)-catalyzed reduction of Hg2+ through tris- or tetraligation of Hg2+.