The crystal structure of the resting state of cytochrome P450(cam) (CYP101), a heme thiolate protein, shows a cluster of six water molecules in the substrate binding pocket, one of which is coordinating to iron(III) as sixth ligand. The resting state is low-spin and changes to high-spin when substrate camphor binds and H2O is removed. In contrast to the protein, previously synthesised enzyme models such as H2O-Fe-III (porph)(ArS-) were shown to be purely high-spin. Iron(S-)porphyrins with different distal sites mimicking Proposed remote effects have been prepared and studied by cw-EPR. The results indicate that the low-spin of the resting state of P450(cam) is due to the fact that the water molecule coordinating to iron has an OH-like character because of hydrogen bonding and polarisation of the water cluster, respectively.
