Remote Effects Modulating the Spin Equilibrium of the Resting State of Cytochrome P450cam –An Investigation Using Active Site Analogues

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 H 2 O is removed. In contrast to the protein, previously synthesised enzyme models such as H 2 O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.