title: Kinetic studies on the nitrite reductase of Wolinella succinogenes.
abstract: The six haem groups of the nitrite reductase enzyme isolated from Wolinella succinogenes are rapidly reduced by the addition of dithionite (S2O4(2-)). The reduction, however, is not homogeneous. Two of the haem groups, namely those that show spectral characteristics typical of five-co-ordinated haem groups, are reduced in a dithionite-concentration-dependent fashion with a rate limit of 1.5 S-1. The other four haem groups, which show spectral characteristics very similar to those of normal six-co-ordinate c-haem groups, reduce in a linear dithionite-concentration-dependent manner with a second-order rate constant of 150 M-1/2 X S-1. The ratio of the amplitudes of the two reduction phases observed in stopped-flow studies is found to be dependent on the concentration of dithionite used. A model is proposed to account for these observations, and computer simulations show that the model represents a good fit to the experimental data. The two haem groups with five-co-ordinate spectral characteristics bind CO. Flash photolysis of the CO complex exhibits one major recombination process with a linear dependence in rate on CO concentration with a second-order rate constant of 2 X 10(6) M-1 X S-1. By contrast, stopped-flow mixing of the reduced protein with CO shows a very complex pattern of combination, with most of the observed absorbance change associated with a concentration-independent step. These findings are rationalized in terms of structural changes in the protein consequent to ligand binding.
mesh_terms: D001438:Bacteroidaceae / Q000201:enzymology*; D002248:Carbon Monoxide / Q000378:metabolism; D003201:Computers; D004227:Dithionite / Q000494:pharmacology; D007700:Kinetics; D008954:Models, Biological; D010084:Oxidation-Reduction; D010088:Oxidoreductases / Q000378:metabolism*; D013053:Spectrophotometry
chemical_list: D004227:Dithionite; D002248:Carbon Monoxide; D010088:Oxidoreductases; C048228:nitrite reductase (heme)