ABSTRACT:Predicting binding affinities for receptor-ligand complexes is still one of the challenging processes in computational structurebased ligand design. Many computational methods have been developed to achieve this goal, such as docking and scoring methods, the linear interaction energy (LIE) method, and methods based on statistical mechanics. In the present investigation, we started from an LIE model to predict the binding free energy of structurally diverse compounds of cytochrome P450 1A2 ligands, one of the important human metabolizing isoforms of the cytochrome P450 family. The data set includes both substrates and inhibitors. It appears that the electrostatic contribution to the binding free energy becomes negligible in this particular protein and a simple empirical model was derived, based on a training set of eight compounds. The root mean square error for the training set was 3.7 kJ/mol. Subsequent application of the model to an external test set gives an error of 2.1 kJ/mol, which is remarkably good, considering the simplicity of the model. The structures of the proteinligand interactions are further analyzed, again demonstrating the large versatility and plasticity of the cytochrome P450 active site.The cytochromes P450 (P450s) are ubiquitous heme-containing enzymes, which play a vital role in the detoxification of xenobiotics and procarcinogen activation. This family shares a common structural feature of the enzymatic active site, including a planar ferric-protoporphyrin IX heme complex, in which the iron is attached through a cysteine link to the protein.The metabolism of a drug is commonly a process of converting a hydrophobic compound into a hydrophilic one by introducing hydrophilic moieties, very often hydroxyl groups, to the parent compounds. More than 50 human isoforms of P450s have been characterized and only seven of them are responsible for Ͼ90% of metabolism of currently marketed drugs (Williams et al., 2004;Nelson, 2009). The most important isoforms are CYP1A2, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4. Some of the P450 isoforms also show polymorphism (e.g., CYP1A2, CYP2C9, and CYP2D6), which leads to a poor pharmacokinetic profile. Early prediction of pharmacokinetic properties such as absorption, distribution, metabolism, elimination, and toxicity of new compounds in drug discovery has become essential these days (Dimasi, 2001). This has been achieved by in vivo, in vitro, and in silico methods and should lead to a reduction in failure rates in the drug development process (van de Waterbeemd and Gifford, 2003;Baranczewski et al., 2006;Afzelius et al., 2007;Stjernschantz et al., 2008).Cytochrome P450 1A2 is an inducible member of the P450 superfamily. It is induced by some polycyclic aromatic hydrocarbons and heterocyclic amines, some of which are found in cigarette smoke and charred food (Fontana et al., 1999). Activation and overexpression of this isoform are linked with high risk of various cancers (Seow et al., 2001;Jernström et al., 2008). The importance of CYP1A2 in...