Modeling of mammalian P450s on basis of P450cam X-ray structure

Conformational plasticity and structure/function relationships in cytochromes P450

The cytochrome P450s are a superfamily of enzymes that are found in all kingdoms of living organisms, and typically catalyze the oxidative addition of atomic oxygen to an unactivated C-C or C-H bond. Over 8000 nonredundant sequences of putative and confirmed P450 enzymes have been identified, but three-dimensional structures have been determined for only a small fraction of these. While all P450 enzymes for which structures have been determined share a common global fold, the flexibility and modularity of structure around the active site account for the ability of P450 enzymes to accommodate a vast number of structurally dissimilar substrates and support a wide range of selective oxidations. In this review, known P450 structures are compared, and some structural criteria for prediction of substrate selectivity and reaction type are suggested. The importance of dynamic processes such as redox-dependent and effector-induced conformational changes in determining catalytic competence and regio- and stereoselectivity is discussed, and noncrystallographic methods for characterizing P450 structures and dynamics, in particular, mass spectrometry and nuclear magnetic resonance spectroscopy are reviewed.

Covalent modification of Thr302 in cytochrome P450 2B1 by the mechanism-based inactivator 4-tert-butylphenylacetylene

The mechanism of inactivation of cytochrome P450 2B1 (CYP2B1) by 4-tert-butylphenylacetylene (BPA) has been characterized previously to be caused by the covalent binding of a reactive intermediate to the apoprotein rather than heme destruction (J Pharmacol Exp Ther 331:392-403, 2009). The identification of a BPA-glutathione conjugate and the increase in the mass of the BPA-adducted apoprotein have indicated that the mass of adduct is 174 Da, equivalent to the mass of BPA plus one oxygen atom. To identify the adducted residue, BPA-inactivated CYP2B1 was digested with trypsin, and the digest was then analyzed by using capillary liquid chromatography with a LTQ linear ion trap mass spectrometer as the detector. A mass shift of 174 Da was used for a SEQUEST database search. The tandem mass spectrometry fragmentation of the modified peptide and the identity of modified residue were determined. The results revealed a mass increase of 174 Da for the peptide sequence (296)FFAGTSSTTLR(308) in the I-helix of CYP2B1 and that the site of adduction formation is Thr302. Homology modeling and ligand docking studies showed that BPA binds in close proximity to both the heme iron and Thr302 with the distances being 2.96 and 3.42 A, respectively. The identification of Thr302 in the CYP2B1 active site as the site of covalent modification leading to inactivation by BPA supports previous hypotheses that this conserved Thr residue may play a crucial role for various functions in P450s.

Deficiência da 11beta-hidroxilase

A hiperplasia congênita da adrenal devido à deficiência da enzima 11beta-hidroxilase é resultado de uma falha na conversão do 11-desoxicortisol em cortisol na última etapa da via sintética dos glicocorticóides. Em geral, esta forma da doença é responsável por cerca de 5% dos casos. A manifestação clínica do excesso de andrógenos em pacientes do sexo feminino inclui graus de ambigüidade genital que podem variar entre uma clitoromegalia até a virilização completa da genitália. Devido ao acúmulo de mineralocorticóides, aproximadamente 50% dos pacientes desenvolvem hipertensão arterial. Mutações no gene CYP11B1, que codifica a enzima 11beta-hidroxilase, são responsáveis pela doença. As características bioquímicas e moleculares da enzima e suas implicações na apresentação clínica da deficiência da 11beta-hidroxilase são abordadas no presente trabalho de revisão.

Functional interaction of cytochrome P450 with its redox partners: a critical assessment and update of the topology of predicted contact regions

The problem of donor-acceptor recognition has been the most important and intriguing one in the area of P450 research. The present review outlines the topological background of electron-transfer complex formation, showing that the progress in collaborative investigations, combining physical techniques with chemical-modification and immunolocalization studies as well as site-directed mutagenesis experiments, has increasingly enabled the substantiation of hypothetical work resulting from homology modelling of P450s. Circumstantial analysis reveals the contact regions for redox proteins to cluster on the proximal face of P450s, constituting parts of the highly conserved, heme-binding core fold. However, more variable structural components located in the periphery of the hemoprotein molecules also participate in donor docking. The cross-reactivity of electron carriers, purified from pro- and eukaryotic sources, with a diversity of P450 species points at a possible evolutionary conservation of common anchoring domains. While electrostatic mechanisms appear to dominate orientation toward each other of the redox partners to generate pre-collisional encounter complexes, hydrophobic forces are likely to foster electron transfer events by through-bonding or pi-stacking interactions. Moreover, electron-tunneling pathways seem to be operative as well. The availability of new P450 crystal structures together with improved validation strategies will undoubtedly permit the production of increasingly satisfactory three-dimensional donor-acceptor models serving to better understand the molecular principles governing functional association of the redox proteins.

The molecular and enzyme kinetic basis for the diminished activity of the cytochrome P450 2D6.17 (CYP2D6.17) variant. Potential implications for CYP2D6 phenotyping studies and the clinical use of CYP2D6 substrate drugs in some African populations

In this study, the basis for the diminished capacity of CYP2D6.17 to metabolise CYP2D6 substrate drugs and the possible implications this might have for CYP2D6 phenotyping studies and clinical use of substrate drugs were investigated in vitro. Enzyme kinetic analyses were performed with recombinant CYP2D6.1, CYP2D6.2, CYP2D6.17 and CYP2D6.T107I using bufuralol, debrisoquine, metoprolol and dextromethorphan as substrates. In addition, the intrinsic clearance of 10 CYP2D6 substrate drugs by CYP2D6.1 and CYP2D6.17 was determined by monitoring substrate disappearance. CYP2D6.17 exhibited generally higher K(m) values compared to CYP2D6.1. The V(max) values were generally not different except for metoprolol alpha-hydroxylation with the V(max) value for CYP2D6.17 being half that of CYP2D6.1. CYP2D6.1 and CYP2D6.2 displayed similar kinetics with all probe drugs except for dextromethorphan O-demethylation with the intrinsic clearance value of CYP2D6.2 being half that of CYP2D6.1. CYP2D6.17 exhibited substrate-dependent reduced clearances for the 10 substrates studied. In a clinical setting, the clearance of some drugs could be affected more than others in individuals with the CYP2D6(*)17 variant. The CYP2D6(*)17 allele might, therefore, contribute towards the poor correlation of phenotyping results when using different probe drugs in African populations. To investigate effects of CYP2D6(*)17 mutations on the structure of the enzyme, a homology model of CYP2D6 was built using the CYP2C5 crystal structure as a template. The results suggest an alteration in position of active-site residues in CYP2D6.17 as a possible explanation for the reduced activity of the enzyme.

Molecular modeling of cytochrome P450 1A1: enzyme-substrate interactions and substrate binding affinities

Human cytochrome P450 1A1, which is present in lungs, plays an important role in the metabolic activation of chemical carcinogens, and in particular, is thought to be linked to lung cancer. The mechanism of carcinogenesis is related to the enzyme's ability to oxidize highly toxic compounds, such as polycyclic aromatic hydrocarbons (PAHs), to their carcinogenic derivatives. In order to better understand P450 1A1 function, a homology model of this enzyme has been constructed. The model has been based on the structure of P450 2C5, the first mammalian P450 to be crystallized. The coordinates of the model have been calculated using a consensus strategy, and the resulting structure has been evaluated with the ProStat and Profiles-3D programs. P450 1A1 substrates, such as benzo[a]pyrene, ethoxyresorufin and methoxyresorufin, were then docked into the active site of the model, and key amino acid residues able to interact with the substrate, have been identified. The analysis of enzyme-substrate interactions indicated that hydrophobic interactions are mainly responsible for binding of these substrates in the active site. Moreover, the non-bond enzyme-substrate interaction energy for ethoxyresorufin was lower than that for methoxyresorufin, which is consistent with higher activity of 1A1 towards the former substrate. Key residue Val-382 may play an important role in these interactions. Additionally, we performed binding free energy calculations for the three substrates. The obtained values were similar to those observed experimentally, which suggests that this approach might be useful for prediction of binding constants.

Cytochrome P450 pattern revision

The pattern suggested for the structure-function superfamily of cytochromes P450 is composed by combining the conserved amino acid motifs. The sizes of P450 cytochromes were estimated according to their length. The empirical coefficients reflecting the peculiarities of the primary structure of these enzymes are calculated. We propose an approach for determining novel proteins sequences to the mentioned superfamily on the ground of the complex of these parameters. A number of the hypothetical proteins from the international databases is related to the cytochromes P450 by means of our pattern.

Roles of cytochrome p450 in development

Cytochrome P450 (CYP) forms are ubiquitous in nature, appearing in almost all phyla, with many forms appearing in any organism. About 50 different forms have been identified in man, and some of these are found in the embryo, some showing temporal dependence. Many of the forms of cytochrome P450 present in one species have homologues in other species. For example, CYP1A2 is present in many species, including man, rabbits, rodents, fish and fowl. The amino acid sequence identity of these homologues is often in excess of 70%. CYP26, too, has more than 61% identity in amino acid sequence between fish, fowl and mammals. In view of the high degree of conservation of sequence as well as of enzymatic activities, it is only reasonable to assume that such strong conservation of sequence also reflects a conservation of function. Since the 'xenobiotic metabolizing' enzymes predate the production of the many xenobiotics they are known to metabolize, perhaps it is reasonable to consider endobiotics as natural substrates for their metabolism. Of the identified forms of cytochrome P450 that are present in embryonic tissue, we consider the possibility that they serve the organism in support of morphogenesis of the embryonic tissue. These forms may either function to generate morphogenic molecules or to keep regions free of them, thereby creating temporal and spatial regions of morphogen action and supporting region-specific changes in cells. One known morphogen, retinoic acid, has the enzymes retinal dehydrogenase (RALDH) and CYP26 maintaining its actions, the former responsible for its generation and the latter for its elimination. Another form of cytochrome P450, CYP1B1 appears also to be involved in differentiation of tissue, with its absence resulting in primary congenital glaucoma. However, the nature of the morphogen it may maintain still remains to be elucidated.

Steroid 11 beta-hydroxylase deficiency and related disorders

Three disorders result from mutations involving two closely linked 11 beta-hydroxylase genes. Steroid 11 beta-hydroxylase deficiency results from mutations in CYP11B1. This is a form of congenital adrenal hyperplasia (CAH) characterized by hypertension and signs of androgen excess. Mutations in CYP11B2 cause aldosterone synthase deficiency, an isolated defect of aldosterone biosynthesis. Recombinations between these two genes cause glucocorticoid suppressible hyperaldosteronism, an autosomal dominant form of hypertension.

Congenital adrenal hyperplasia due to 21-hydroxylase deficiency

More than 90% of cases of congenital adrenal hyperplasia (CAH, the inherited inability to synthesize cortisol) are caused by 21-hydroxylase deficiency. Females with severe, classic 21-hydroxylase deficiency are exposed to excess androgens prenatally and are born with virilized external genitalia. Most patients cannot synthesize sufficient aldosterone to maintain sodium balance and may develop potentially fatal "salt wasting" crises if not treated. The disease is caused by mutations in the CYP21 gene encoding the steroid 21-hydroxylase enzyme. More than 90% of these mutations result from intergenic recombinations between CYP21 and the closely linked CYP21P pseudogene. Approximately 20% are gene deletions due to unequal crossing over during meiosis, whereas the remainder are gene conversions--transfers to CYP21 of deleterious mutations normally present in CYP21P. The degree to which each mutation compromises enzymatic activity is strongly correlated with the clinical severity of the disease in patients carrying it. Prenatal diagnosis by direct mutation detection permits prenatal treatment of affected females to minimize genital virilization. Neonatal screening by hormonal methods identifies affected children before salt wasting crises develop, reducing mortality from this condition. Glucocorticoid and mineralocorticoid replacement are the mainstays of treatment, but more rational dosing and additional therapies are being developed.

Threonine 201 in the diiron enzyme toluene 4-monooxygenase is not required for catalysis

The diiron enzyme toluene 4-monooxygenase from Pseudomonas mendocina KR1 catalyzes the NADH- and O(2)-dependent hydroxylation of toluene. A combination of sequence alignments and spectroscopic studies indicate that T4MO has an active site structure closely related to the crystallographically characterized methane monooxygenase hydroxylase. In the methane monooxygenase hydroxylase, active site residue T213 has been proposed to participate in O(2) activation by analogy to certain proposals made for cytochrome P450. In this work, mutagenesis of the comparable residue in the toluene 4-monooxygenase hydroxylase, T201, has been used to investigate the role of an active site hydroxyl group in catalysis. Five isoforms (T201S, T201A, T201G, T201F, and T201K) that retain catalytic activity based on an in vivo indigo formation assay were identified, and detailed characterizations of the purified T201S, T201A, and T201G variants are reported. These isoforms have k(cat) values of 1.2, 1.0, and 0.6 s(-)(1), respectively, and k(cat)/K(M) values that vary by only approximately 4-fold relative to that of the native isoform. Moreover, these isoforms exhibit 80-90% coupling efficiency, which also compares favorably to the >94% coupling efficiency determined for the native isoform. For the T201S, T201A, and T201G isoforms, the regiospecificity of toluene hydroxylation was nearly identical to that of the natural isoform, with p-cresol representing 90-95% of the total product distribution. In contrast, the T201F isoform caused a substantial shift in the product distribution, and gave o- and p-cresol in a 1:1 ratio. In addition, the amount of benzyl alcohol was increased approximately 10-fold with the T201F isoform. For reaction with p-xylene, previous studies have shown that the native isoform reacted to give 4-methybenzyl alcohol and 2, 5-dimethylphenol in a 4:1 ratio [Pikus, J. D., Studts, J. M., McClay, K., Steffan, R. J., and Fox, B. G. (1997) Biochemistry 36, 9283-9289]. For comparison, the T201S, T201A, and T201F isoforms gave a slightly relaxed 3:1 ratio of these products, while the T201G isoform gave a dramatically relaxed 1:1 ratio. On the basis of these studies, we conclude that the hydroxyl group of T201 is not essential to maintaining the turnover rate or the coupling of the toluene 4-monooxygenase complex. However, changing the volume occupied by the side chain at the position of T201 can lead to alterations in the regiospecificity of the hydroxylation, presumably by producing different orientations for substrate binding during catalysis.

Late steps of aldosterone biosynthesis: sheep are not rats

1. The last three steps of aldosterone biosynthesis have been demonstrated to be catalysed by a single enzyme, referred to as CYP11B (or P450(11) beta) in cow, pig, sheep and bullfrog and as CYP11B2 (or P450aldo) in rat, human, mouse and hamster. 2. The related enzyme CYP11B1 (also referred to as P450(11) beta) in rat, human, mouse and hamster does not have aldosterone synthesis activity, but no such enzyme has been reported in the cow, pig or sheep to date. 3. Exclusive aldosterone secretion in the zona glomerulosa (ZG) of the adrenal cortex in species such as rat, human, mouse and hamster could be ascribed to the restricted distribution of CYP11B2 to the same region in the adrenal cortex. 4. In other species, such as cow, pig and sheep, the CYP11B enzyme is expressed throughout the adrenal cortex and, thus, the exclusive aldosterone biosynthesis in the ZG could not be explained simply by the distribution of the enzyme. 5. We have shown in the sheep that potassium loading and acute sodium depletion stimulate the CYP11B transcript levels, which are not further increased by chronic sodium depletion. 6. The predominant CYP11B in the sheep adrenal cortex catalyses the synthesis of aldosterone from deoxycorticosterone (DOC) in vitro, is expressed throughout the adrenal cortex and the corresponding transcript levels are increased by K+ loading or sodium depletion. In short, as far as the last step of aldosterone biosynthesis is concerned, sheep are different from rats. In the rat, the CYP11B2 transcript or protein is elevated by K+ loading or sodium depletion, but not the CYP11B1 transcript or protein. 7. We propose that during severe sodium deficiency there is a switch in the aldosterone pathway to one preferentially involving 18-OH-DOC and not corticosterone.

Cytochrome P450cam substrate specificity: relationship between structure and catalytic oxidation of alkylbenzenes

The oxidation by cytochrome P450cam (CYP101) of ethylbenzene and a series of substrates derived from it by addition of one, two, three, or four carbon atoms has been examined. For each of the 18 substrates, the shift in spin state due to substrate binding, the extent of coupled turnover to give organic products and uncoupled turnover to give H2O2 and H2O, and the identities of the organic products have been determined. The same studies have been carried out with the T185L and T185F mutants of P450cam in which the active site volume is decreased. The results show that no detectable correlation exists between the observed spin state change and any other parameter studied. For substrates of equal size, coupled and uncoupled turnover vary widely but both are maximized when the phenyl ring bears one large alkyl substituent or a methyl ortho to the largest alkyl substituent. The presence of substituents in addition to these decreases activity. In the absence of other changes, coupled turnover is correlated with the size of the largest substituent, but no such correlation exists for uncoupled turnover. Decreasing the size of the active site cavity by a T185L mutation generally increases coupled turnover without altering the dependence on the alkyl group size. A T185F mutation causes too great an active site perturbation for structure-activity studies. Substrate oxidation occurs preferentially at 2 degrees or 3 degrees C-H bonds of the largest substituent or on the benzylic methyl ortho to it. Aromatic hydroxylation only competes with the oxidation of nonbenzylic 1 degree C-H bonds. The extent of coupled turnover is a function of substrate shape, substrate size, and cavity size, but still elusive parameters control the extent of uncoupled turnover.

Epitope mapping of cytochrome P450cam (CYP101)

Eighteen linear antigenically active sites were revealed in cytochrome P450 from Pseudomonas putida (P450cam) by hexapeptide scanning. These sites occupy about 31% of the protein sequence. Hexapeptide epitope sequences of P450cam are not found in other cytochromes P450. However, several cytochromes P450 contain shorter fragments of P450cam epitope sequences which may cause weak immune cross-reactions. P450cam antigenic determinants are located generally at the boundaries of secondary structure elements. Mapping of P450cam antigenic determinants on the three-dimensional structure of this protein reveals 14 highly water-accessible antigenic sites and only 1 site (No. 322-327, QMLSGL) which is inaccessible to water. Several functionally important sites and amino acid residues of P450cam are localized within revealed linear epitopes or very close to them. These sites include substrate-binding regions, residues responsible for the putidaredoxin interaction (Arg72, Arg112, Lys314, and Arg364), heme binding (Gln108, Arg112, Asp297, Arg299, and Cys357), and proton translocation (Lys178, Arg186, and Glu366).

P450BM-3; a tale of two domains--or is it three?

Over 400 P450s have been identified to date in prokaryotes and eukaryotes, plants and animals, mitochondria and endoplasmic reticulum. These enzymes function in areas such as metabolism and steroidogenesis. The eukaryotic members of this gene superfamily of proteins have proved difficult to study because of the hydrophobic nature of their substrates, their various redox partners, and membrane association. To better understand the structure/function relationship of P450s-what determines substrate specificity and selectivity, what determines redox-partner binding, and which regions are involved in membrane binding-we have compared the three crystallized, soluble bacterial P450s (two class I and one class II) and a model of a steroidogenic, eukaryotic P450 (P450arom), to define which structural elements form a conserved structural fold for P450s, what determines specificity of substrate binding and redox-partner binding, and which regions are potentially involved in membrane association. We believe that there is a conserved structural fold for all P450s that can be used to model those P450s that prove intransigent to structural determination. However, although there appears to be a conserved structural core among P450s, there is sufficient sequence variability that no two P450s are structurally identical. NADPH-P450 reductase transfers electrons from NADPH to P450 during the P450 catalytic cycle. This enzyme has usually been thought of as a simple globular protein; however, sequence analysis has shown that NADPH-P450 reductase is related to two separate flavoprotein families, ferredoxin nucleotide reductase (FNR) and flavodoxin. Recent studies by Wolff and his colleagues have shown that the FAD-binding FNR domain and FMN-binding flavodoxin domain of human NADPH-P450 reductase can be independently expressed in Escherichia coli. The subdomains can be used to reconstitute, however poorly, the monooxygenase activity of the P450 system. We have been utilizing the reductase domain of P450BM-3 to study the mechanism of electron transfer from NADPH to P450 in this complex multidomain protein. We have overexpressed both the FNR subdomain and the flavodoxin subdomain in E. coli and fully reconstituted the cytochrome c reductase activity of this enzyme. Our studies have shown that electron transfer from NADPH through the reductase domain to the P450 requires shuttling of the FMN subdomain between the reductase subdomain and the P450. Studies of the factors that control the molecular recognition and interaction among these three proteins are complicated by the weakness of the association and changes in the strength of the interaction depending on the redox state of each of the components. How these structural and mechanistic studies of a soluble bacterial P450 can be extended to gain a better understanding of the control of membrane-bound eukaryotic P450-dependent redox systems is discussed.

Inherited forms of mineralocorticoid hypertension

Aldosterone, the most important mineralocorticoid, regulates electrolyte excretion and intravascular volume mainly through its effects on renal distal convoluted tubules and cortical collecting ducts. Excess secretion of aldosterone or other mineralocorticoids or abnormal sensitivity to mineralocorticoids may result in hypertension, suppressed plasma renin activity, and hypokalemia. Such conditions often have a genetic basis, and studies of these conditions have provided valuable insights into the normal and abnormal physiology of mineralocorticoid action. Deficiencies of steroid 11 beta-hydroxylase or 17 alpha-hydroxylase are types of congenital adrenal hyperplasia, the autosomal recessive inability to synthesize cortisol. These two defects often cause hypertension because of overproduction of cortisol precursors that are, or are metabolized to, mineralocorticoid agonists. These disorders result from mutations in the CYP11B1 and CYP17 genes encoding the corresponding enzymes. Glucocorticoid-suppressible hyperaldosteronism is an autosomal dominant form of hypertension in which aldosterone secretion is abnormally regulated by corticotropin. It is caused by recombinations between linked genes encoding closely related isozymes, 11 beta-hydroxylase (CYP11B1) and aldosterone synthase (CYP11B2), generating a dysregulated chimeric gene with aldosterone synthase activity. Apparent mineralocorticoid excess is a loss of functional ligand specificity of the mineralocorticoid receptor caused by a deficiency of the kidney isozyme of 11 beta-hydroxysteroid dehydrogenase, an enzyme that normally metabolizes cortisol to cortisone to prevent cortisol from occupying the receptor. This autosomal recessive form of severe hypertension results from mutations in the HSD11K (HSD11B2) gene.

Bacterial cytochromes P-450

The cytochromes P-450 (P-450s) constitute an extremely large family ('superfamily') of haemoproteins that catalyse the oxidation of a wide range of physiological and non-physiological compounds. A remarkable feature of the P-450s is the manipulation of the same basic structure and chemistry to achieve an enormous range of functions in organisms as diverse as bacteria and man. Indeed, the P-450s have been described as 'the most versatile biological catalyst known'. Much research is focussed on mammalian P-450s, with their roles in such processes as steroid transformations and the metabolism of carcinogens and other xenobiotics. However, our knowledge of the structure and function of the P-450s has been advanced by analysis of a limited number of its bacterial members, primarily P-450cam from Pseudomonas putida. Four P-450 structures have been solved to date, all of which are from bacterial sources. The aim of this review is to assess current knowledge of the many bacterial P-450s, with emphasis on their diverse biological roles and on the advances in our knowledge of this extremely important enzyme class, which have been made feasible through their study.

Engineering a mineralocorticoid- to a glucocorticoid-synthesizing cytochrome P450

Site-directed mutagenesis of a domain (amino acids 299-338) aligning to the I-helix region of P450cam, P450BM3 and P450terp was used to investigate the different regioselectivities displayed in the hydroxylation reactions performed by human aldosterone synthase (P450aldo) and 11beta-hydroxylase (P45011beta). The two enzymes are 93% identical and are essential for the synthesis of mineralocorticoids and glucocorticoids in the human adrenal gland. Single replacement of P450aldo residues for P45011 beta-specific residues at positions 296, 301, 302, 320, and 335 only gave rise to slightly increased 11beta-hydroxylase activities. However, a L301P/A320V double substitution increased 11beta-hydroxylase activity to 60% as compared with that of P45011 beta. Additionally substituting Ala-320 for Val-320 of P45011 beta further enhanced this activity to 85%. The aldosterone synthase activities of the mutant P450aldo proteins were suppressed to a varying degree, with triple replacement mutant L301P/E302D/A320V retaining only 10% and double replacement mutant L301P/A320V retaining only 13% of the P450aldo wild type activity. These results demonstrate a switch in regio- and stereoselectivities of the engineered P450aldo enzyme due to manipulation of residues at three critical positions, and we attribute the determination of these features in P450aldo to the structure of a region analogous to the I-helix in P450cam.

Structural flexibility and functional versatility of cytochrome P450 and rapid evolution

P450 represents a large group of heme-thiolate enzymes that exhibit remarkably diverse activities for the metabolism of numerous endogenous and exogenous chemicals. Recent site-directed mutagenesis studies indicate that a single mutation at any of the key residues can be enough to alter the substrate and/or product specificities in the P450 activities. Molecular modeling predicts that these key residues are located within the substrate heme pocket. Structural elements involved in diversifying P450 activity appear to correspond to the B' helix, the F helix and the F/G interhelical loop in the bacterial P450s. Structures represented by these regions are extremely variable despite the fact that the core of the P450 substrate pocket is well conserved. A mutation within these regions may result in a significant geometrical alteration of the pocket and lead to diversify the P450 activity. Phylogenetical analysis shows a relatively high rate of nonsynonymous substitution within these substrate binding regions. The functional versatility of P450 can thus be largely accounted for in terms of pocket change brought about by rapid mutations.

Antigenic mapping of bacterial and animal cytochromes P-450

A peptide scanning (PEPSCAN) approach was used for antigenic mapping of two hepatic microsomal cytochromes P450 (rab1A2 and rab2B4) and the microbial cytochrome from Pseudomonas putida (P450 101 or P450cam). This approach includes simultaneous synthesis of pin-linked overlapping hexapeptides covering the whole sequences of three P450s and testing them by ELISA with corresponding polyclonal antisera. Microsomal cytochrome P450 maps were shown to vary depending on an antiserum used for testing the peptides, however, the most active linear B-epitopes were revealed with antisera from two animal species used. P450 linear B-epitopes were classified into individual and group-specific epitopes. While almost all P450 101 linear antigenic determinants are unique for this protein, rab1A2 and rab2B4 contain epitopes both individual for each protein, and subfamily- or even family-specific epitopes. These results point out the possibility of producing both monospecific and group-specific antipeptide antibodies against different P450s. The antigenic map of P450 101 was superimposed on the structural-functional map of this protein. Its linear B-epitopes were shown to coincide with boundaries of secondary structure elements, with surface-located, water accessible regions and with sites responsible for intermolecular interactions in the Pseudomonas putida monooxygenase system. Several known or predicted functionally active sites in microsomal cytochrome P450 rab1A2 and rab2B4 were also shown to coincide with linear B-epitopes. The peculiarities of epitope locations in the protein tertiary structure will allow to predict antigenic regions starting from protein structural information and vice versa, to structural protein models in accordance with antigenic mapping results. Antigenic regions which coincide with sites responsible for intermolecular interactions in monooxygenase systems may be synthesized as separate peptides and used as blockers of such interactions.

Computer modeling of 3D structures of cytochrome P450s

The understanding of structure-function relationship of enzymes requires detailed information of their three-dimensional structure. Protein structure determination by X-ray and NMR methods, the two most frequently used experimental procedures, are often difficult and time-consuming. Thus computer modeling of protein structures has become an increasingly active and attractive option for obtaining predictive models of three-dimensional protein structures. Specifically, for the ubiquitous metabolizing heme proteins, the cytochrome P450s, the X-ray structures of four isozymes of bacterial origin, P450cam, P450terp, P450BM-3 and P450eryF have now been determined. However, attempts to obtain the structure of mammalian forms by experimental means have thus far not been successful. Thus, there have been numerous attempts to construct models of mammalian P450s using homology modeling methods in which the known structures have been used to various extents and in various strategies to build models of P450 isozymes. In this paper, we review these efforts and then describe a strategy for structure building and assessment of 3D models of P450s recently developed in our laboratory that corrects many of the weaknesses in the previous procedures. The results are 3D models that for the first time are stable to unconstrained molecular dynamics simulations. The use of this method is demonstrated by the construction and validation of a 3D model for rabbit liver microsomal P450 isozyme 2B4, responsible for the oxidative metabolism of diverse xenobiotics including widely used inhalation anesthetics. Using this 2B4 model, the substrate access channel, substrate binding site and plausible surface regions for binding with P450 redox partners were identified.

The roles of individual amino acids in altering substrate specificity of the P450 2a4/2a5 enzymes

A single amino acid substitution is sufficient to alter substrate specificity of P450 enzymes. Mouse P450 2a5, for example, has its substrate specificity converted from coumarin 7- to testosterone 15 alpha-hydroxylase activity by the substitution of Phe at position 209 to Leu. Furthermore, placing Asn at this position confers a novel corticosterone 15 alpha-hydroxylase activity to this P450. Recent site-directed mutational studies show the presence of the topologically common residues, each of which can determine the specificities of various mammalian P450s. For instance, residue 209 (in 2a5) corresponds to a residue at position 206 in rat P4502B1 that regulates its steroid hydroxylase activity. High substrate specificity often observed in an individual P450, therefore, can be determined and altered by the identities of a few critical residues. The structural flexibility of the substrate-heme pocket may also provide P450 enzymes with the ability to display a broad range of substrate specificities. Understanding the underlying principles whereby the flexible pocket determines P450 activities may lead us to the prediction of P450 activities based on the identities of key amino acid residues.

Altering the regiospecificity of androstenedione hydroxylase activity in P450s 2a-4/5 by a mutation of the residue at position 481

Mouse P450 2a-5 (coumarin 7-hydroxylase) acquires androstenedione (AD) hydroxylase activity by substituting Phe at position 209 with Asn. However, this mutant P450 2a-5 (F209N) and the corresponding mutant P450 2a-4 (L209N) exhibit different regiospecificites of androstenedione (AD) hydroxylase activity. While the former mutant catalyzes both AD 15 alpha- and 7 alpha-hydroxylase activities at similar rates, the latter mutant maintains the original high specificity of AD 15 alpha-hydroxylase activity. The AD hydroxylase activities in chimeric enzymes of the mutants L209N and F209N show that the regiospecificites are determined by the carboxy-terminal halves of the P450 molecules. Mutations at each of the four different residues within the carboxy-terminal halves indicate that the differences in regiospecificity are determined by the Val/Ala mutation at position 481. As the size of the hydrophobic amino acid at position 481 becomes larger (Ala < Val < Ile), the regiospecificities toward the C15 position of the AD molecule are dramatically increased. The regiospecificity is also increased by placing positively-charged Arg at position 481, although the remaining 15 alpha-hydroxylase activity in this mutant is considerably lower than the other P450s. The results indicate that the size of the residue at position 481 is a key factor in regulating the regiospecificity of AD hydroxylase activity in the P450s. Modeling AD in the substrate-heme pocket of bacterial P450 101A provided further support that residue 481 may reside near the steroid molecule so as to possibly affect the AD hydroxylase activity.

Multiple steroid-binding orientations: alteration of regiospecificity of dehydroepiandrosterone 2- and 7-hydroxylase activities of cytochrome P-450 2a-5 by mutation of residue 209

The mutation of Ala-117 to Val conferred dehydroepiandrosterone (DHEA) hydroxylase activity on cytochrome P-450 2a-4, with the production of both 2 alpha- and 7 alpha-hydroxyDHEA at similar rates. P-450 2a-5 which has Val at position 117, acquired high DHEA hydroxylase activity by mutation of Phe-209. Mutant F209L of P-450 2a-5 exhibited strong regiospecificity at the 2-position of the DHEA molecule with the production of 2 alpha-hydroxy DHEA as the major metabolite. On the other hand, mutant F209V of P-450 2a-5 showed the 7-position to be the major hydroxylation site, 7 beta-hydroxyDHEA and 7 alpha-OHDHEA being produced. Therefore the regiospecificity of DHEA hydroxylase activity of P-450 2a-5 is altered between the 2- and 7-position depending on the amino acid at position 209. Modelling of the DHEA molecule in the pocket of bacterial P-450cam showed that the steroid can be accommodated in at least two orientations for which the 2- or 7- position is near the sixth axial position of the haem. Moreover, these two orientations, which are of similar energy, can be interconverted by a 180 degrees rotation of the steroid molecule around its long axis. These results support the hypothesis that the steroid molecule in the pocket is in dynamic equilibrium with multiple binding orientations and that the equilibrium is apparently determined by a few critical residues including those at positions 117 and 209.

Functional and DNA sequence divergence of the CYP71 gene family in higher plants

During ripening of avocado (Persea americana), the CYP71A1 mRNA and protein accumulate to relatively high levels. Although the CYP71A1 gene was the first plant P450 to be cloned and sequenced, the functional role of this P450 remains obscure. Substrate studies have shown that CYP71A1 will metabolize various monoterpenes (nerol and geraniol), although these have not been detected in ripening fruit. Using DNA from a conserved domain of the CYP71A1 gene, we have explored the scope of the CYP71 (or related) gene family in avocado using low stringency DNA hybridization. This analysis suggests that there are approximately 10-12 genes in the CYP71 family. An alternative approach using PCR gave essentially identical results.

Modeling cytochrome P450 14 alpha demethylase (Candida albicans) from P450cam

The tertiary structure of cytochrome P450 14 alpha demethylase--Candida albicans (P450 CA) is modeled on the basis of sequence alignment with two closely related proteins and the crystallographic structure of Pseudomonas putida P450cam. The secondary structure prediction system used combines the information from several algorithms and trains the data to offer an optimized prediction of the known P450cam. The trained algorithm was then used to predict the secondary structure of the other P450 sequences. The prediction of the surface coil regions was aided by an alignment between P450 CA and the homologous sequences P450 14 alpha demethylase--Saccharomyces cerevisiae (66 SD) and P450 14 alpha demethylase--Candida tropicalis (72 SD). The prediction and alignment information was combined to establish an alignment between P450 CA and P450cam, and to assign full secondary structure to the target protein. This secondary structure was folded from the template of P450cam and the predicted structure was relaxed by molecular dynamics. Model checking highlighted minor adjustments in the alignment, correctly orienting hydrophobic and hydrophilic side chains. The model offers explanations for several known experimental results and suggests further investigations that may prove fruitful in understanding the structure and mechanisms of the P450 family (Porter, T.D. and Coon, M.J. Minireview cytochrome P450. J. Biol. Chem. 1991, 266, 13469-13472. Waterman, M.R. Cytochrome P450 cellular distribution and structural considerations. Current Opinion in Structural Biology 1992, 2, 384-387. Aoyama, Y., Yoshida, Y., Sonohdo, Y. and Sato, Y. Structural analysis of the interaction between the side-chain of substrates and the active site of lanosterol 14 alpha demethylase (P450 14DM) of yeast. Biochim. Biophys. Acta 1992, 1122, 251-255.).

Differential effects of mutations in substrate recognition site 1 of cytochrome P450 2C2 on lauric acid and progesterone hydroxylation

Mutations at amino acid positions 107-120, which are part of a predicted substrate recognition site [Gotoh, O. (1992) J. Biol. Chem. 267, 83-90], were analyzed in C2MstC1, a chimera of P450 2C2 and P450 2C1. This hybrid protein has a new activity for progesterone C21-hydroxylation in addition to the lauric acid (omega-1)hydroxylase activity present in both parent proteins. Various substitutions for highly conserved glycines at positions 111 and 117 and tryptophan at position 120 strongly decreased the lauric acid hydroxylase activity of P450 2C2 and C2MstC1 and the progesterone hydroxylase activity of C2MstC1. Activities of mutant proteins with substitutions at 107, 108, and 112-115 were also strongly reduced. Modest or no decreases in activity were observed for substitutions at 109, 110, 116, 118, and 119. Lauric acid hydroxylase activity decreased more in most C2MstC1 mutants than in those of P450 2C2, particularly at positions 107 and 108. A substitution of phenylalanine for valine-112 reduced progesterone hydroxylation by 30-fold while only moderately reducing lauric acid hydroxylase by 40%. This differential effect on two dissimilar substrates demonstrates the importance of residue 112 for substrate interactions. The results are consistent with a model in which residues 107-110 align with the B'-helix of the bacterial proteins P450cam and P450BM-3. This helix is followed by a substrate-contacting loop from 111 to 116, and residues 117-120 align with the C-helices of the bacterial proteins In this alignment, Trp-120 is positioned behind the heme such that it could participate in electron transfer from reductase.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural determinants of cytochrome P450 2B1 specificity: evidence for five substrate recognition sites

Twelve site-directed mutants of rat cytochrome P450 2B1 distributed over seven positions and four putative substrate recognition sites (SRS) were constructed and expressed in COS cells. Function was examined using androstenedione and testosterone as substrates. Substitutions at positions 303, 360, and 473 did not markedly affect the regio- or stereoselectivity of androgen metabolism, whereas mutants in positions 206 (SRS-2), 302 (SRS-4), and 363 and 367 (SRS-5) exhibited markedly different steroid metabolite profiles compared with parental P450 2B1. In particular, the Phe-206-->Leu substitution conferred androgen 6 alpha- and testosterone 7 alpha-hydroxylase activities, and the Thr-302-->Ser substitution suppressed androgen 16 beta-hydroxylation in favor of androstenedione 16 alpha- and testosterone 15 alpha-hydroxylation. Replacement of Val-363 or Val-367 with Ala conferred androgen 15 alpha-hydroxylase and 6 beta-hydroxylase activities, respectively, and suppressed susceptibility to mechanism-based inactivation by the P450 2B1-selective chloramphenicol analog N-(2-p-nitrophenethyl)chlorofluoroacetamide. The Val-367-->Ala mutant was also resistant to chloramphenicol itself. The Leu mutant at position 363 exhibited increased specificity for androstenedione and testosterone 16 beta-hydroxylation, whereas the Leu mutant at position 367 exhibited decreased stereospecificity. Most interestingly, the size of key residues identified plays a critical role in governing steroid hydroxylation from the alpha-face or beta-face and hydroxylation on the D-ring or the B-ring.(ABSTRACT TRUNCATED AT 250 WORDS)

Crystal structure and refinement of cytochrome P450terp at 2.3 A resolution

Cytochrome P450terp is a class I (mitochondrial/bacterial) P450 that catalyzes the hydroxylation of alpha-terpineol as part of the catabolic assimilation of this compound by a pseudomonad species. Crystals grown from the purified protein have the symmetry of space group P6(1)22, and cell dimensions a = b = 69.4 A, c = 456.6 A, alpha = beta = 90 degrees, gamma = 120 degrees. Diffraction data were collected at the Cornell High Energy Synchrotron Source, and the structure of P450terp was solved by a combination of molecular replacement and multiple isomorphous replacement techniques. A model of P450terp was built and refined against native data, to an R-factor of 18.9% for data with I > or = sigma(I) between 6.0 A and 2.3 A resolution. This model contains 412 of the 428 P450terp amino acid residues; the loop between helices F and G is disordered in the crystal. While the overall fold of P450terp is very similar to that of P450cam, only three-quarters of the C alpha positions can be superimposed, to a root-mean-square deviation of only 1.87 A. The mode of substrate binding by P450terp can be predicted, and probable substrate contact residues identified. The heme environment and side-chain positions in the adjacent I-helix suggest possible modes of proton delivery in the catalytic cycle of the enzyme.

Structural alteration of mouse P450coh by mutation of glycine-207 to proline: spin equilibrium, enzyme kinetics, and heat sensitivity

Mouse cytochrome P450coh is a high-spin haem protein which specifically catalyses coumarin 7-hydroxylase activity. A mutation of Gly-207 to Pro shifts the P450coh completely to the low-spin form, indicating that the sixth axial position of the haem is hexaco-ordinated with a water molecule in the mutant G207P. Moreover, the G207P mutation increases the Km value for coumarin 7-hydroxylase activity 100-fold and the Kd value for coumarin binding 200-fold. Conversely, the mutation decreases the Ki and Kd values 10- and 20-fold respectively when testosterone, a larger molecule, is used as a substrate. The results, therefore, are consistent with an idea that the substrate pocket may be larger in the mutant G207P than in the wild-type cytochrome P-450. A Gly-207 to Ala mutation (G207A) of P450coh (G207A), on the other hand, affects neither the spectral nor the enzymic properties of P450coh. Pro-207, through cis/trans isomerization or formation of a kink, may confer on the G207P a structural alteration of its substrate-haem pocket. Our previous studies [Iwasaki, Juvonen, Lindberg and Negishi (1991) J. Biol. Chem. 266, 3380-3382; Juvonen, Iwasaki and Negishi (1991) J. Biol. Chem. 266, 16431-16435] show that the residue at position 209 in P450coh resides close to the sixth axial position of the haem, and the spin equilibrium of the cytochrome P-450 shifts toward the high-spin state as residue 209 becomes more hydrophobic and larger. A Gly-207 to Pro mutation, therefore, results in the creation of a larger substrate pocket in the mutant cytochrome P-450 by altering the protein structure around residue 209 so that a water molecule and testosterone can be accommodated.

Crystal structure of hemoprotein domain of P450BM-3, a prototype for microsomal P450's

Cytochrome P450BM-3, a bacterial fatty acid monoxygenase, resembles the eukaryotic microsomal P450's and their flavoprotein reductase in primary structure and function. The three-dimensional structure of the hemoprotein domain of P450BM-3 was determined by x-ray diffraction and refined to an R factor of 16.9 percent at 2.0 angstrom resolution. The structure consists of an alph and a beta domain. The active site heme is accessible through a long hydrophobic channel formed primarily by the beta domain and the B' and F helices of the alpha domain. The two molecules in the asymmetric unit differ in conformation around the substrate binding pocket. Substantial differences between P450BM-3 and P450cam, the only other P450 structure available, are observed around the substrate binding pocket and the regions important for redox partner binding. A general mechanism for proton transfer in P450's is also proposed.

Spectroscopic characterization of a newly isolated cytochrome P450 from Rhodococcus rhodochrous

Cytochrome P450 (P450) from Rhodococcus rhodochrous have been characterized through circular dichroism and nuclear magnetic resonance (NMR) spectroscopy, both in the substrate-free and substrate-bound forms. The data are compared with those of P450cam and indicate a close similarity of the structure of the active site in the two proteins. The substrate-free species contains low-spin iron(III), while the 2-ethoxyphenol bound species contains high-spin iron(III). The substrate is in slow exchange on the NMR time scale. The binding of CN- has been investigated and the final adduct characterized through NMR spectra. Nuclear relaxation times of the isotropically shifted signals turn out to be shorter than in other heme proteins, both in the high- and in the low-spin species. This is the result of longer electron relaxation times in P450s than in peroxidases and metmyoglobin. This property, as well as the electron paramagnetic resonance (EPR) spectrum of the substrate-free form, are discussed in terms of the presence of the cysteine as the fifth ligand of the iron ion instead of a histidine as it occurs in peroxidases and myoglobin.

Site-directed mutagenesis of mouse steroid 7 alpha-hydroxylase (cytochrome P-450(7) alpha): role of residue-209 in determining steroid-cytochrome P-450 interaction

We have cloned a cDNA encoding mouse steroid 7 alpha-hydroxylase P450(7) alpha (cytochrome P-450(7) alpha) and expressed it in Saccharomyces cerevisiae. Mouse P450(7) alpha is 70% identical in its amino acid sequence with the mouse steroid 15 alpha-hydroxylase P450(15) alpha (2A4). The Leu at position 209 of P450(15) alpha is the most important residue to determine the steroid hydroxylase activity of the P450 [Lindberg and Negishi (1989) Nature (London) 339, 632-634]. The P450(7) alpha contains Asn at the position corresponding to the Leu-209 of P450(15) alpha, although both P450s hydroxylate testosterone. The CO-reduced P450(7) alpha complex is unstable, so that it is quickly converted into the inactive P420, whereas the P450(15) alpha is very stable. The P450(7) alpha, however, is stabilized either by addition of testosterone or by a mutation of Asn-209 to Leu. The mutant P450(7) alpha displays a 17-fold lower Vmax. value than the wild-type enzyme. Unexpectedly, it also has 3-fold lower Km and Kd values. Residue 209 in P450(7) alpha, therefore, appears to be located at a critical site of the haem-substrate-binding pocket. Corticosterone inhibits the testosterone 7 alpha-hydroxylase activity of the wild-type P450(7) alpha, whereas it does not inhibit the mutant P450(7) alpha. Conversely, the P450(15) alpha activity becomes inhibited by corticosterone upon the replacement of Leu-209 by Asn. In addition, this mutation increases the corticosterone 15 alpha-hydroxylase activity of P450(15) alpha at least 20-fold. Whereas the inhibition by corticosterone depends on the presence of Asn at position 209, deoxycorticosterone inhibits the activities of the P450s regardless of the type of residue at 209. The results indicate, therefore, that the identity of residue 209 determines the affinity as well as specificity of steroid binding to both P450(7) alpha and P450(15) alpha.

Purification and characterization of cytochrome P450RR1 from Rhodococcus rhodochrous

A soluble cytochrome P450 whose synthesis is induced by and that binds 2-ethoxyphenol was purified to apparent homogeneity from Rhodococcus rhodochrous strain 116. The enzyme had a subunit molecular mass of 44.5 kDa as determined by SDS/PAGE and a pI of 5.2. The electronic absorption spectrum indicates that the native cytochrome in the absence of substrate is predominantly in the low-spin state (13% high-spin state in 50 mM Mops, pH 7.0 25 degrees C). 2-Methoxyphenol binds to the cytochrome with a macroscopic dissociation constant of 0.53 +/- 0.03 microM (50 mM Mops, pH 7.0, 25 degrees C) and induces a 99.7% transition of the heme iron to the pentacoordinate high-spin form. Using a reconstituted in-vitro activity assay, it was demonstrated that P450RR1 catalyzed the O-dealkylation of 2-ethoxyphenol and 2-methoxyphenol to produce catechol. The cytochrome binds other ortho-substituted phenols, including 2-ethoxyphenol, 2-methylphenol (o-cresol) and 2-chlorophenol. The affinity of P450RR1 for these compounds is lower than that of 2-methoxyphenol and they are less effective than 2-methoxyphenol at inducing a transition in the heme iron to the high-spin state. Para-substituted and meta-substituted ether phenols did not induce a spin transition.

Substrate specificity of 6-deoxyerythronolide B hydroxylase, a bacterial cytochrome P450 of erythromycin A biosynthesis

The 6-deoxyerythronolide B hydroxylase (EryF) is a soluble cytochrome P450 responsible for the stereospecific C-6 hydroxylation of the erythromycin precursor, 6-deoxyerythronolide B. Using the expression of the eryF gene in Escherichia coli [Andersen, J. F., & Hutchinson, C. R. (1992) J. Bacteriol. 174, 725-735] as the enzyme source, we examined the catalytic activity of the EryF protein toward several macrolide substrates related to 6-deoxyerythronolide B. The results of these studies were compared with measurements of the apparent dissociation constants for various substrates and with information from molecular modeling studies of the substrates and the enzyme-substrate complex. Only minor changes in the structure of 6-deoxyerythronolide B resulted in substrates with catalytic rates less than 1% of those seen with the natural substrate. Although the 9S epimer of 9-deoxo-9-hydroxy-6-deoxyerythronolide B was hydroxylated at a rate approximately equal to the natural substrate, the 9R epimer was hydroxylated at a 2-fold lower rate. Examination of molecular models revealed that the position of the 9-hydroxyl oxygen in the 9S epimer resembles that of the 9-oxo oxygen in the natural substrate more closely than in the 9R epimer. 8,8a-Deoxyoleandolide, which is identical to 6-deoxyerythronolide B except for the presence of a C-13 methyl group, and its (9S)-9-deoxo-9-hydroxy derivative were C-6 hydroxylated at a 4-fold lower rate than the natural substrate, and the 9-oxo form showed a substantially larger apparent dissociation constant.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular modeling of the 3-D structure of cytochrome P-450scc

Sequence-alignment studies of the bovine mitochondrial cholesterol side-chain cleavage enzyme cytochrome P-450scc with the bacterial cytochrome P-450cam (camphor hydroxylating enzyme) have been undertaken. Our novel alignment of the sequences revealed 69 identical residues and many highly conserved regions. The results of the sequence alignment studies were used to model the 3-D structure of P-450scc based on the available crystal structure of P-450cam. The major insertions in the sequence are found mainly on four external-loop regions of the molecule, while the core structure of P-450cam is retained with subtle internal modifications. The most hydrophobic of these four external loops is proposed as a candidate for membrane attachment.

Crystallization and preliminary x-ray diffraction analysis of P450terp and the hemoprotein domain of P450BM-3, enzymes belonging to two distinct classes of the cytochrome P450 superfamily

Cytochromes P450 are members of a superfamily of hemoproteins that are involved in the metabolism of various physiologic and xenobiotic organic compounds. This superfamily of proteins can be divided into two classes based on the electron donor proximal to the P450: an iron-sulfur protein for class I P450s or a flavoprotein for class II. The only known tertiary structure of any of the cytochromes P450 is that of P450cam, a class I soluble enzyme isolated from Pseudomonas putida (product of the CYP101 gene). To understand the details of the structure-function relationships within and between the two classes, structural studies on additional cytochromes P450 are crucial. We report here characterization of the crystal forms of two soluble, bacterial enzymes: cytochrome P450terp [class I enzyme from a Pseudomonas species (product of CYP108 gene)] and the hemoprotein domain of cytochrome P450BM-3 [class II enzyme from Bacillus megaterium (product of the CYP102 gene)]. The crystals of cytochrome P450terp are hexagonal and belong to the space group P6(1)22 (or its enantiomorph, P6(5)22) with unit cell dimensions a = b = 68.9 A and c = 458.7 A. The crystals of the hemoprotein domain of cytochrome P450BM-3 are monoclinic and belong to the space group P2(1) with unit cell dimensions a = 59.4 A, b = 154.0 A, c = 62.2 A, and beta = 94.7 degrees. Diffraction data for the crystals of these two proteins were obtained to a resolution better than 2.2 A. Assuming the presence of two molecules in the asymmetric unit for the hemoprotein domain of P450BM-3 and one molecule for P450terp, the calculated values of Vm are 2.6 and 3.3 A3/Da, respectively.

Mapping determinants of the substrate selectivities of P450 enzymes by site-directed mutagenesis

Point-mutation studies in cytochrome P450s by site-directed mutagenesis have identified key residues that can confer the catalytic properties of one cytochrome P450 onto another. Most of these key residues cluster at sites that map to amino acids forming the substrate-binding site of P450cam, a distantly related enzyme. These sites are found on topological elements of P450cam, which by their surface location and lack of extensive secondary structure are likely to permit genetic variation without extensive disruption of the overall topology of the enzyme. If these topological features of P450cam are conserved in the mammalian enzymes, they are likely to accommodate the structural diversity seen for mammalian P450s in a manner that conserves a basic structure for P450 enzymes but that leads to the catalytic diversity seen for the mammalian enzymes.