Kinetic studies on the XXXX reductase of @entity23585 .

@entity72890 @entity3542 @entity6857 @entity23585 @entity734
The six haem groups of the @entity3542 reductase enzyme isolated from @entity23585 are rapidly reduced by the addition of @entity6857 ( @entity72890 ). 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 @entity6857 -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 @entity6857 -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 @entity6857 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 @entity734 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.
