Mechanism of mercury(II) reductase and influence of ligation on the reduction of mercury(II) by a water soluble 1,5-dihydroflavin.
The nature and rate of reduction of Hg2+ to Hg0 by 1,5-dihydro-3,(3-sulfopropyl)lumiflavin (FIH2) 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 FIH2. The apparent second-order rate constant with N,N-bis(2-hydroxyethyl)glycine (2.2 x 10(6) M-1.s-1) is much greater than that in the presence of ligating ethylenediaminetetraacetic acid (1.5 x 10(2) M-1.s-1). When ligated by mercaptoethanesulfonate, reduction of Hg2+ by FIH2 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 FIH2. These observations show that bis-ligation by thiolate greatly decreases the reducibility of Hg2+ and that further ligation by thiolate further retards the reaction. Comparison 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 (FIox). Thus, the ease of reduction of Hg2+ complexes by FIH2 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+.
Gopinath
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Department of Chemistry, University of California, Santa Barbara 93116.
Kaaret
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Bruice
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D004492:Edetic Acid; D005415:Flavins / Q000378:metabolism*; D005998:Glycine / Q000031:analogs & derivatives; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008628:Mercury / Q000378:metabolism*; D015080:Mesna; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000378:metabolism*; D012996:Solutions; D013053:Spectrophotometry; D014494:Unithiol
D005415:Flavins; D012996:Solutions; C027494:N,N-bis(2-hydroxyethyl)glycine; D014494:Unithiol; C059461:1,5-dihydro-3-(3-sulfopropyl)lumiflavin; D004492:Edetic Acid; D010088:Oxidoreductases; C021551:mercuric reductase; D008628:Mercury; D015080:Mesna; D005998:Glycine