Structure and mechanism in the bacterial dihaem cytochrome c peroxidases

The bacterial cytochrome c peroxidases contain an electron-transferring haem c (E) and a peroxidatic haem c (P). Many are isolated in an inactive oxidised state. Reduction of the E haem promotes Ca²⁺-dependent spin state and coordination changes at the P haem rendering it accessible to ligand. Recent crystallographic work on the oxidised and mixed valence enzymes has suggested a mechanism by which an electron entering the E haem remotely triggers this activation of the P haem. Binding of hydrogen peroxide at the activated P haem leads to an intermediate catalytic form containing two oxidising equivalents, one of which is a ferryl oxene. This form of the enzyme is then reduced by two single electron transfers to the E haem delivered by small redox proteins such as cytochromes or cupredoxins. The binding of these small redox proteins is dominated by global electrostatic forces but the interfaces of the electron transfer complexes that are formed are largely hydrophobic and relatively non-specific. These features allow very high electron transfer rates in the steady state.