KINETIC-STUDIES ON THE ELECTRON-TRANSFER BETWEEN BACTERIAL C-TYPE CYTOCHROMES AND METAL-OXIDES

Cyclic voltammetry was used to investigate the kinetics of the electro n transfer between various soluble or solid metal oxides, and polyheme c-type cytochromes from Desulfuromonas acetoxidans and Desulfovibrio. The second order rate constant for the catalytic reduction of soluble chromate ions by Desulfuromonas acetoxidans cytochrome c(7) was found to be 6 x 10(5) M-1 s(-1). By using the membrane electrode technology , it has been shown that the catalytic process for Cr(VI) reduction is efficient even when the cytochrome is entrapped in the close vicinity of the electrode surface. Moreover, this proceeding allowed the catal ytic reduction of solid metal oxides such as manganese(IV), vanadium(V ) and iron(III) oxides to be performed. Results suggest that the metal reductase activity of a microorganism is governed by its c-type cytoc hrome content. Furthermore, only cytochromes with bishistidinyl heme i ron coordination act as metal reducers whereas mitochondrial c-type cy tochromes do not. This approach opens new pathways for the use of sulf ur or sulfate bacteria in the bioremediation of metal contaminated wat ers and waste streams. Processes involving the use of entrapped enzyme s reactors could be developed according to the metal reducing activity of their polyheme c-type cytochromes. (C) 1998 Elsevier Science S.A. All rights reserved.