Cytochrome P450, both in the presence and absence of substrate, is shown to be a mixture of high- and lowtemperature spectral forms, with the suggestion that these two forms correspond to the S = 1/2 and S = 5/2 spin states of the heme iron. The equilibrium constants and thermodynamic parameters describing the spin transition have been quantitated using high-resolution optical spectroscopy and are in excellent agreement with high-temperature Móssbauer and electron paramagnetic resonance spectroscopy results. The spin-state equilibrium is shown to modulate both the substrate binding and the oxidation-reduction potential of the cytochrome. These three interacting equilibria are presented in terms of a general thermodynamic model of regulation which provides a clear and concise description of the multicomponent reaction scheme.
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