Haitao Ji

The three-dimensional structure of lanosterol 14alpha-demethylase (P450(14DM), CYP51) of Candida albicans was modeled on the basis of crystallographic coordinates of four prokaryotic P450s: P450BM3, P450cam, P450terp, and P450eryF. The P450(14DM) sequence was aligned to those of known proteins using a knowledge-based alignment method. The main chain coordinates of the core regions were transferred directly from the corresponding coordinates of P450BM3. The side chain conformations of the core regions were determined by the conformations of the equivalent residues with the highest homologous scores in four crystal structures. The model was then refined using molecular mechanics and molecular dynamics. The reliability of the resulting model was assessed by Ramachandran plots, Profile-3D, hydropathy plot analysis, and by analyzing the consistency of the model with the experimental data. The structurally and functionally important residues such as the heme binding residues, the residues interacting with redox-partner protein and/or involved in electron transfer, the residues lining substrate access channel, and the substrate binding residues were identified from the model. These residues are candidates for further site-directed mutagenesis and site-specific antipeptide antibody binding experiments. The active analogue approach was employed to search the pharmacophoric conformations for 14 azole antifungals. The resulting bioactive conformations were docked into the active site of lanosterol 14alpha-demethylase of Candida albicans. All 14 azole antifungals are shown to have a similar docking mode in the active site. The halogenated phenyl group of azole inhibitors is deep in the same hydrophobic binding cleft as the 17-alkyl chain of substrate. The pi-pi stacking interaction might exist between halogenated phenyl ring of inhibitors and the aromatic ring of residue Y132. The long side chains of some inhibitors such as itraconazole and ketoconazole surpass the active site and interact with the residues in the substrate access channel. To compare with mammalian enzymes, structurally selective residues of the active site of fungal lanosterol 14alpha-demethylase are distributed in the C terminus of F helix, beta6-1 sheet and beta6-2 sheet.

In a continuing effort to develop highly potent azole antifungal agents, the three-dimensional quantitative structure-activity relationship methods, CoMFA and CoMSIA, were applied using a set of novel azole antifungal compounds. The binding mode of the compounds at the active site of lanosterol 14alpha-demethylase was further explored using the flexible docking method. Various hydrophobic, van der Waals, pi-pi stacking, and hydrogen bonding interactions were observed between the azoles and the enzyme. Based on results from the molecular modeling, a receptor-based pharmacophore model was established to guide the rational optimization of the azole antifungal agents. Thus, a total of 57 novel azoles were designed and synthesized by a three-step optimization process. In vitro antifungal assay revealed that the antifungal activities of these novel azoles were greatly improved, which confirmed the reliability of the model from molecular modeling.