New insights into the structural features and functional relevance of human cytochrome P450 2C9. Part II.

Part I of this article published in the previous issue of Current Drug Metabolism discussed the substrate specificity, inhibitor selectivity and structure-activity relationship (SAR) of human CYP2C9. The features of CYP2C9 pharmacophore and SAR models have been elaborated. Part II of this article will address the homology models of CYP2C9, data from site-directed mutagenesis studies, and crystal structural features of CYP2C9. The heteroactivation of CYP2C9 and its interactions with other CYPs will also be discussed. A number of ligand-based and homology models of CYP2C9 have been reported and this has provided insights into the binding of ligands to the active site of CYP2C9. Site-directed mutagenesis studies have revealed that a number of residues (e.g. R97, F110, F114, R132, R144, D293, F476 and A477) play an important role in ligand binding and determination of substrate specificity. The resolved crystal structures of CYP2C9 have confirmed the importance of these residues in substrate recognition and ligand orientation. Currently, there are three X-ray structures of the human CYP2C9 in Protein Database (PDB): one ligand-free protein (1OG2), and two in complex with S-warfarin (1OG5) or flurbiprofen (1R9O). The published structures of 1OG2 and 1OG5 differ in comparison with 1R9O in residues 30-53 of N-termini, residues 97-121 of B/C-loops, and residues 196-233 of helix F and F/G-loops. CYP2C9 is a two-domain protein with typical fold characteristics of the CYPs. The B-C loop forms part of the active site and contributes to substrate specificity. In the structures of CYP2C9 without ligand bound or with bound S-warfarin, residues 101-106 in the B-C loop form helix B'. In addition, residues 212-222 in the F-G loop form helices F' and G', which was not observed in rabbit CYP2C5 and bacterial CYPs. In the 1OG2 and 1OG5 structures, the heme is stabilized by hydrogen bonds between the propionates and the side chains of W120, R124, H368 and R433. In addition, R97 forms hydrogen bonds to the propionates as well as the carbonyl oxygen atoms of V113 and P367. CYP2C9 is activated by dapsone and its analogues and R-lansoprazole in a stereo-specific and substrate-dependent manner, probably through binding to the active site and inducing positive cooperativity. Further studies are needed to investigate the molecular determinants for ligand-CYP2C9 interactions.