ELECTRONIC-PROPERTIES OF THE DISSIMILATORY SULFITE REDUCTASE FROM DESULFOVIBRIO-VULGARIS (HILDENBOROUGH) - COMPARATIVE-STUDIES OF OPTICAL-SPECTRA AND RELATIVE REDUCTION POTENTIALS FOR THE [FE4S4]-SIROHAEM PROSTHETIC CENTERS

The dissimilatory sulphite reductase (desulfoviridin) from the sulphate-reducing bacterium Desulfovibrio vulgaris (Hildenborough) displays distinct optical and redox characteristics relative to the haem subunit of Escherichia coli assimilatory sulphite reductase. For high-spin pentaco-ordinate desulfoviridin there is minimal change in the absorbance of the oxidized chromophores both after reduction or after addition of exogenous ligands. A ligand-metal charge-transfer band similar to 702 nm is observed in both the oxidized and one-electron-reduced enzyme. E.p.r. spectroscopy has been used to define the relative reduction potentials for sirohaem and [Fe4S4] centres (Delta E(0) = E(s)(0) - E(c)(0)) as a function of sirohaem axial co-ordination. Typically Delta E(0) lies in a range from - 10 to - 50 mV. These results show a correlation with the sigma-donor or pi-acceptor properties of the ligand and stand in sharp contrast with estimates for the E. coli enzyme. The electronic properties of the coupled [Fe4S4]sirohaem redox centre common to both nitrite- and sulphite-reducing enzymes are apparently strongly dependent on the environment generated by protein side chains.