Second derivative spectroscopy was used to determine the percentage of tyrosine residues that are exposed to solvent in cytochrome P-450cam isolated from Pseudomonas putida. The ratio between two peak to trough second derivative absorbance differences has been shown to be dependent on the polarity of the microenvironment surrounding tyrosine residues [Ragone, R., Colonana, G., Balestrieri, C, Servillo, L., & Irace, G. (1984) Biochemistry 23, 1871]. With a number of camphor analogues that independently vary the spin equilibrium of the ferric cytochrome P-450cam, experiments have demonstrated that the percentage of tyrosine residues exposed to solvent is linearly dependent on the percentage of ferric high-spin species present. This is not simply a function of the extent of substrate binding since in all cases the substrate concentration was sufficient to ensure saturation of the cytochrome. The local microenvironment of approximately one tyrosine residue appears to be linearly correlated with the percentage of ferric high-spin cytochrome. Structural studies of cytochrome P-450cam using small-angle X-ray scattering [Lewis, B. A., & Sligar, S. G. (1983) J. Biol. Chem. 258, 3599] and high-pressure difference spectroscopy [Fisher, M. T., Scarlata, S. F., & Sligar, S. G. (1985) Arch. Biochem. Biophys. 240,456] imply that global conformational changes linked to the spin equilibria are small. Together with the data reported herein, these results suggest that one tyrosine residue is involved in a conformational change that is directly linked with the spin equilibrium.
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