Enzymatic oxidation of cadmium and lead metals photodeposited on cadmium sulfide

Cadmium and lead metals deposited on CdS particles are shown to act as subs trates - electron donors for enzymes, hydrogenase from Thiocapsa roseopersi cina (HG), NAD-dependent hydrogenase from Alcaligenes eutrophus (NLH), and ferredoxin:NADP oxido-reductase (FNR) from Chlorella in the formation of hy drogen, NADH and NADPH, respectively. Adsorption of the enzyme on the surfa ce of the metallized CdS particle is required for enzymatic oxidation of me tal. The maximum rates for the formation of hydrogen and NADH catalyzed by hydrogenase and NAD-dependent hydrogenase with metals as electron donors ar e comparable with the rates obtained for these enzymes using soluble substr ates. Kinetic analysis of the enzymatic oxidation of cadmium metal has reve aled that the rate decreases mainly due to the formation of a solid product , which is supposed to be Cd(OH)(2). The deceleration of lead oxidation cat alyzed by hydrogenase proceeds at the expense of the inhibitory effect of t he formed Pb2+. The enzymatic oxidation of electrochemically prepared cadmi um metal is also shown. Based on these results, a new mechanism of action o f the enzymes involved in anaerobic biocorrosion is proposed. By this mecha nism, the enzyme accelerates the process of metal dissolution through a med iatorless catalysis of the reduction of the enzyme substrate. (C) 2000 Else vier Science S.A. All rights reserved.