IDENTIFICATION OF A FUNCTIONAL WATER CHANNEL IN CYTOCHROME-P450 ENZYMES

Cytochrome P450 enzymes are monooxygenases that contain a functional h eme b group linked to a conserved cysteine with a thiolate bond, In th e native state, the central iron atom is hexacoordinated with a covale ntly bound water molecule, The exclusion of solvent molecules from the active site is essential for efficient enzymatic function, Upon subst rate binding, water has to be displaced from the active site to preven t electron uncoupling that results in hydrogen peroxide or water, In c ontrast to typical hemoproteins, the protein surface is not directly a ccessible from the heme of cytochromes P450, We postulate a two-state model in which a conserved arginine, stabilizing the heme propionate i n all known cytochrome P450 crystal structures, changes from the initi al, stable side-chain conformation to another rotamer (metastable), In this new state, a functional water channel (aqueduct) is formed from the active site to a water cluster located on the thiolate side of the heme, close to the protein surface, This water cluster communicates w ith the surface in the closed state and is partly replaced by the flip ping arginine side chain in the open state, allowing water molecules t o exit to the surface or to reaccess the active site, This two-state m odel suggests the presence of an exit pathway for water between the ac tive site and the protein surface.