First principles investigation of singly reduced cytochrome P450

1. The application of novel ab initio quantum mechanical methods to the states in the cycle of cytochrome P450 following the first reduction step is described. 2. A good correlation was found between the calculated energy of reduction and the experimentally determined redox potential for a range of substrate- and substrate analogue-bound systems. 3. On reduction of the haem system, the ground state of Fe remains Fe3+. On binding of a CO molecule, Fe adopts a low-spin Fe2+ state, in agreement with experiment. However, on binding of an O-2 molecule, calculations indicate that the system adopts a ferric superoxide ground state, in which the Fe is in a low-spin Fe3+ state.