ENZYMATIC REDUCTION OF INORGANIC ANIONS - PRE-STEADY-STATE KINETIC-ANALYSIS OF THE DISSIMILATORY SULFITE REDUCTASE (DESULFOVIRIDIN) FROM DESULFOVIBRIO-VULGARIS (HILDENBOROUGH) - MECHANISTIC IMPLICATIONS

Pre-steady-state kinetic experiments have been performed on the dissimilatory sulfite reductase (desulfoviridin) from Desulfovibrio vulgaris (Hildenborough). Microscopic rate constants for binding (k2) and reductive cleavage of bonds (k(r)) during enzymatic reduction of SO32- and NO2- have been determined. For N02- reduction the reactivity of reaction intermediates has also been measured and a mechanistic scheme has been devised. The experimental rate constants are as follows: k2(SO32-) approximately 4.3 X 10(3) M-1 s-1, kr(SO32-) approximately 12 s-1; k2(NO2-) approximately 3.6 X 10(3) M-1 s-1, kr(NO2-) approximately 14 s-1; k2(NO) approximately 7 X 10(5) M-1 s-1, k(r)(NO) approximately 6.5 s-1, k2(NH2OH) approximately 24 M-1 s-1, kr(NH2OH) approximately 9 s-1. Second-order rate constants for ligand association [k2(AsO2-) approximately 3 X 10(3) M-1 s-1; k2(HS-) approximately 1.8 M-1 s-1] are consistent with the dominance of pi-acceptor or sigma-donor properties, respectively, of substrate molecules. A systematic strategy for the analysis of a multistep enzymatic reduction of an inorganic anion is described.