Substrate induced changes of the active site electronic states in reduced cytochrome P450cam and the photolysis product of its CO complex. Low-temperature magnetic circular dichroism data.

MCD spectra of camphor-free and camphor-bound reduced cytochrome P450cam have been recorded for the near UV and visible spectral regions at temperatures from 300K down to 2.1K and compared with those of the carbon monoxide photoproducts generated at 4.2K. In the absence of camphor, the reduced P450 is spectroscopically different from the photoproduct. In the presence of camphor, however, the spectra of the reduced P450 and of the photoproduct are almost similar and behave like the photoproduct of the camphor-free enzyme. This behavior indicates that substrate binding induces a higher active site rigidity. From the significant alteration of the temperature dependence of the MCD intensity for the reduced enzyme induced by camphor binding it is concluded that the near degeneracy of the electronic ground state in the substrate-free enzyme is removed by substrate binding.