Cytochrome P450s (CYPs) are membrane-associated heme proteins and hepatic microsomal enzymes, which participate in drug metabolism and detoxification. Among more than 50 human CYPs, an isoform, CYP3A4, is a major enzyme responsible for drug-drug interactions that is concerned in at least 50% of drug metabolism.1) In addition, CYP3A4 is expressed at a high level in the human liver.
2)Thus, it is important to know the CYP3A4 inhibitory potency of new drug candidates at an early stage of drug discovery. Although many detailed studies about CYP3A4 inhibitory pharmacophores have been published, 3) precise interactions between inhibitors (or substrates) and CYP3A4 have not been defined well. Recently, the crystal structure of CYP2C5 was determined by Williams et al.,4) which is the first structure of a mammalian microsomal CYP. To understand the molecular basis of CYP3A4 inhibition well, a CYP3A4 homology model was constructed, and inhibitors and substrates were docked into the model. In this report we will describe some interesting features of the putative active site of CYP3A4.
ExperimentalHomology Modeling The amino acid sequence of CYP3A4 was taken from the ExPASy web site (http://tw.expasy.org/). The amino acid sequence alignment used for the homology modeling was obtained as follows. The amino acid sequences for which 3D structures were available at the time (Pseudomonas-Putida P450 CAM ,9,10) and mammalian CYP2C5 4) ) were aligned according to their 3D structures using the homology module of Insight II (ver. 2000, Accelrys Inc., San Diego, California, U.S.A.). Subsequently, amino acid sequences of human metabolic CYPs were aligned by sequence homology using ClustalW.11) The combined alignments were manually revised so that insertions and deletions did not get into the portions corresponding to probable secondary structures, as far as possible, and the following four characteristic motifs of CYPs were aligned: WXXXR (where W is tryptophan, R is arginine, and X is any amino acid) in the alpha-helix C, EXXR (where E is glutamate, R is arginine, and X is any amino acid) in the alphahelix K, ZXXPXXZXPXXZ (where P is proline, Z is aromatic amino acid, and X is any amino acid) after the alpha-helix KЈ and FXXGXXXCXG (where F is phenylalanine, G is glycine, C is cysteine, and X is any amino acid) before the alpha-helix L.
12)According to the alignment (Fig. 1), a homology model of CYP3A4 was constructed based on the crystal structure of mammalian CYP2C5 (PDB code:1DT6) using the homology module of Insight II. Although the sequence identities between CYP102 (BM3) and CYP3A4 versus CYP2C5 and CYP3A4 are very similar (22%), CYP2C5 was chosen as a template since the mammalian CYP was presumed to be more suitable than a prokaryotic one. Using the Search/Generate-Loops function of Insight II, conformations of the insertion and deletion parts in the alignment were obtained referring to the known 3-D structures. After some manual adjustments to remove large steric hindrances, the whole structure was subjected to energy minimiz...