Study of Water Binding to Low-Spin Fe(III) in Cytochrome P450 by Pulsed ENDOR and Four-Pulse ESEEM Spectroscopies

Cytochrome P450cam (CP450cam) was studied by pulsed ENDOR and two- and four-pulse ESEEM spectroscopies. Spectra were recorded and simulated at the three principal g-values of the rhombic EPR spectrum. The four-pulse ESEEM experiment gave a direct measure of the anisotropic hyperfine interaction for the protons. Using the point dipole approximation this gives a Fe−H distance of 2.6 Å. The measured anisotropic hyperfine interaction reduced the number of hyperfine interaction parameters required to simulate the ENDOR line shapes. Both the four-pulse ESEEM frequencies and the ENDOR spectra at all three principal g-values could be satisfactorily simulated using two magnetically equivalent protons and a water orientation similar to that obtained in our previous 17O ESEEM study. Thus, the pulsed ENDOR and four-pulse ESEEM results are self-consistent with the 17O ESEEM data and indicate that the axial ligand is a water molecule rather than an OH- ligand. The isotropic hyperfine value derived from the numerical simulations is in agreement with previous values derived from proton NMR relaxation studies.