Microsomal cytochrome P450 family 1 enzymes play prominent roles in xenobiotic detoxication and procarcinogen activation. P450 1A2 is the principal cytochrome P450 family 1 enzyme expressed in human liver and participates extensively in drug oxidations. This enzyme is also of great importance in the bioactivation of mutagens, including the N-hydroxylation of arylamines. P450-catalyzed reactions involve a wide range of substrates, and this versatility is reflected in a structural diversity evident in the active sites of available P450 structures. Here, we present the structure of human P450 1A2 in complex with the inhibitor ␣-naphthoflavone, determined to a resolution of 1.95 Å . ␣-Naphthoflavone is bound in the active site above the distal surface of the heme prosthetic group. The structure reveals a compact, closed active site cavity that is highly adapted for the positioning and oxidation of relatively large, planar substrates. This unique topology is clearly distinct from known active site architectures of P450 family 2 and 3 enzymes and demonstrates how P450 family 1 enzymes have evolved to catalyze efficiently polycyclic aromatic hydrocarbon oxidation. This report provides the first structure of a microsomal P450 from family 1 and offers a template to study further structure-function relationships of alternative substrates and other cytochrome P450 family 1 members. Enzymes of the cytochrome P450 (CYP)5 superfamily play a significant physiologic role in the detoxication of foreign compounds and the biosynthesis of endogenous compounds, including steroid hormones, bile acids, and cholesterol. The enzymes comprising P450 families 1, 2, and 3 contribute most extensively to the biotransformation of xenobiotics to more polar metabolites that are more readily excreted. In humans and most mammals, family 1 contains three well characterized P450 monooxygenases; 1A1, 1A2, and 1B1. These enzymes are generally distinguished from P450s in other families by their capacity to oxidize a variety of polynuclear aromatic hydrocarbons (PAHs).6 Moreover, the expression levels of the three enzymes are induced by exposure to PAHs (1). The induction is mediated by a ligand-activated transcription factor, the aryl hydrocarbon receptor, which is a basic-loop-helix PAS domain protein that binds to enhancer elements flanking the CYP1A1, CYP1A2, and CYP1B1 genes and stimulates transcription.The oxidation of PAHs is generally protective. However, some P450-catalyzed reactions can transform these relatively inert compounds into genotoxic metabolites that can initiate mutagenesis and cancer. Human P450 1A2 is notable among family 1 enzymes for the capacity to N-oxidize arylamines, the major metabolic process in the bioactivation of arylamines to potent mutagenic or carcinogenic compounds (2). ␣-Naphthoflavone (ANF), a prototype flavonoid, is known to competitively inhibit P450s of family 1, albeit at different concentrations, and has been used to discriminate between P450 family 1 enzymes (3). Flavonoids have gained recent interest in vi...

Plants developed a diverse battery of defense mechanisms in response to continual challenges by a broad spectrum of pathogenic microorganisms. Their defense arsenal includes inhibitors of cell wall-degrading enzymes, which hinder a possible invasion and colonization by antagonists. The structure of Triticum aestivum xylanase inhibitor-I (TAXI-I), a first member of potent TAXI-type inhibitors of fungal and bacterial family 11 xylanases, has been determined to 1.7-Å resolution. Surprisingly, TAXI-I displays structural homology with the pepsin-like family of aspartic proteases but is proteolytically nonfunctional, because one or more residues of the essential catalytical triad are absent. The structure of the TAXI-I⅐Aspergillus niger xylanase I complex, at a resolution of 1.8 Å, illustrates the ability of tight binding and inhibition with subnanomolar affinity and indicates the importance of the C-terminal end for the differences in xylanase specificity among different TAXI-type inhibitors.

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Although a crystal structure and a pharmacophore model are available for cytochrome P450 2C8, the role of protein flexibility and specific ligand-protein interactions that govern substrate binding are poorly understood. X-ray crystal structures of P450 2C8 complexed with montelukast (2.8 Å ), troglitazone (2.7 Å ), felodipine (2.3 Å ), and 9-cis-retinoic acid (2.6 Å ) were determined to examine ligand-protein interactions for these chemically diverse compounds. Montelukast is a relatively large anionic inhibitor that exhibits a tripartite structure and complements the size and shape of the active-site cavity. The inhibitor troglitazone occupies the upper portion of the active-site cavity, leaving a substantial part of the cavity unoccupied. The smaller neutral felodipine molecule is sequestered with its dichlorophenyl group positioned close to the heme iron, and water molecules fill the distal portion of the cavity. The structure of the 9-cis-retinoic acid complex reveals that two substrate molecules bind simultaneously in the active site of P450 2C8. A second molecule of 9-cis-retinoic acid is located above the proximal molecule and can restrain the position of the latter for more efficient oxygenation. Solution binding studies do not discriminate between cooperative and noncooperative models for multiple substrate binding. The complexes with structurally distinct ligands further demonstrate the conformational adaptability of active site-constituting residues, especially Arg-241, that can reorient in the active-site cavity to stabilize a negatively charged functional group and define two spatially distinct binding sites for anionic moieties of substrates.Cytochrome P450 2C8 is one of the principal drug-metabolizing P450 4 monooxygenases expressed in human liver. It is the predominant hepatic P450 catalyzing the 6␣-hydroxylation of paclitaxel (1) and the epoxidation of arachidonic acid (2, 3). Additionally, P450 2C8 contributes extensively to the metabolism of drugs such as pioglitazone, rosiglitazone, troglitazone, amodiaquine, amiodarone, and cerivastatin (4) as well as to the oxidation of retinoic acid (5-8). A screen of 209 commonly used drugs and related compounds identified several potent inhibitors of P450 2C8, including montelukast, candesartan cilexetil, mometasone furoate, clotrimazole, and felodipine (9). In addition, the glucuronide of gemfibrozil is a potent inhibitor of P450 2C8 (10, 11), and this inhibition is thought to underlie a drug-drug interaction between gemfibrozil and cerivastatin that can increase risk of rhabdomyolysis. Interestingly, glucuronides of 17␤-estradiol (12) and diclofenac (13) are also P450 2C8 substrates.A pharmacophore model for substrates of P450 2C8 was proposed by Mansuy and co-workers (14) based on the observation that many substrates are large organic anions at physiologic pH and exhibit sites of oxidation that are located ϳ13 Å from the anionic group. Additionally, it was noted that several polar moieties are often present at intermediate distances between the site o...

Background: Knowledge of the structural features of P450 2C19 that underlie its distinct roles in human drug metabolism is lacking. Results: The structure of P450 2C19 was determined by x-ray crystallography. Conclusion:The structure of the enzyme exhibits features that distinguish it from closely related P450s 2C8 and 2C9. Significance: Structural characterization of P450 2C19 contributes to a better understanding of its role in drug clearance.