Lanosterol 14 alpha-methyl demethylase. Isolation and characterization of the third metabolically generated oxidative demethylation intermediate

CYP5122A1 encodes an essential sterol C4-methyl oxidase in Leishmania donovani and determines the antileishmanial activity of antifungal azoles

Visceral leishmaniasis is a life-threatening parasitic disease, but current antileishmanial drugs have severe drawbacks. Antifungal azoles inhibit the activity of cytochrome P450 (CYP) 51 enzymes which are responsible for removing the C14α-methyl group of lanosterol, a key step in ergosterol biosynthesis in Leishmania. However, they exhibit varying degrees of antileishmanial activities in culture, suggesting the existence of unrecognized molecular targets. Our previous study reveals that, in Leishmania, lanosterol undergoes parallel C4- and C14-demethylation to form 4α,14α-dimethylzymosterol and T-MAS, respectively. In the current study, CYP5122A1 is identified as a sterol C4-methyl oxidase that catalyzes the sequential oxidation of lanosterol to form C4-oxidation metabolites. CYP5122A1 is essential for both L. donovani promastigotes in culture and intracellular amastigotes in infected mice. CYP5122A1 overexpression results in growth delay, increased tolerance to stress, and altered expression of lipophosphoglycan and proteophosphoglycan. CYP5122A1 also helps to determine the antileishmanial effect of antifungal azoles in vitro. Dual inhibitors of CYP51 and CYP5122A1 possess superior antileishmanial activity against L. donovani promastigotes whereas CYP51-selective inhibitors have little effect on promastigote growth. Our findings uncover the critical biochemical and biological role of CYP5122A1 in L. donovani and provide an important foundation for developing new antileishmanial drugs by targeting both CYP enzymes.

Updates on Mechanisms of Cytochrome P450 Catalysis of Complex Steroid Oxidations

Cytochrome P450 (P450) enzymes dominate steroid metabolism. In general, the simple C-hydroxylation reactions are mechanistically straightforward and are generally agreed to involve a perferryl oxygen species (formally FeO3+). Several of the steroid transformations are more complex and involve C-C bond scission. We initiated mechanistic studies with several of these (i.e., 11A1, 17A1, 19A1, and 51A1) and have now established that the dominant modes of catalysis for P450s 19A1 and 51A1 involve a ferric peroxide anion (i.e., Fe3+O2¯) instead of a perferryl ion complex (FeO3+), as demonstrated with 18O incorporation studies. P450 17A1 is less clear. The indicated P450 reactions all involve sequential oxidations, and we have explored the processivity of these multi-step reactions. P450 19A1 is distributive, i.e., intermediate products dissociate and reassociate, but P450s 11A1 and 51A1 are highly processive. P450 17A1 shows intermediate processivity, as expected from the release of 17-hydroxysteroids for the biosynthesis of key molecules, and P450 19A1 is very distributive. P450 11B2 catalyzes a processive multi-step oxidation process with the complexity of a chemical closure of an intermediate to a locked lactol form.

Roles of Ferric Peroxide Anion Intermediates (Fe3+O2 -, Compound 0) in Cytochrome P450 19A1 Steroid Aromatization and a Cytochrome P450 2B4 Secosteroid Oxidation Model

Cytochrome P450 (P450, CYP) 19A1 is the steroid aromatase, the enzyme responsible for the 3-step conversion of androgens (androstenedione or testosterone) to estrogens. The final step is C-C bond scission (removing the 19-oxo group as formic acid) that proceeds via a historically controversial reaction mechanism. The two competing mechanistic possibilities involve a ferric peroxide anion (Fe3+O2 -, Compound 0) and a perferryl oxy species (FeO3+, Compound I). One approach to discern the role of each species in the reaction is with the use of oxygen-18 labeling, i.e., from 18O2 and H2 18O of the reaction product formic acid. We applied this approach, using several technical improvements, to study the deformylation of 19-oxo-androstenedione by human P450 19A1 and of a model secosteroid, 3-oxodecaline-4-ene-10-carboxaldehyde (ODEC), by rabbit P450 2B4. Both aldehyde substrates were sensitive to non-enzymatic acid-catalyzed deformylation, yielding 19-norsteroids, and conditions were established to avoid issues with artifactual generation of formic acid. The Compound 0 reaction pathway predominated (i.e., Fe3+O2 -) in both P450 19A1 oxidation of 19-oxo-androstenedione and P450 2B4 oxidation of ODEC. The P450 19A1 results contrast with our prior conclusions (J. Am. Chem. Soc. 2014, 136, 15016-16025), attributed to several technical modifications.

Oxygen-18 Labeling Reveals a Mixed Fe-O Mechanism in the Last Step of Cytochrome P450 51 Sterol 14α-Demethylation

The 14α-demethylation step is critical in eukaryotic sterol biosynthesis, catalyzed by cytochrome P450 (P450) Family 51 enzymes, for example, with lanosterol in mammals. This conserved three-step reaction terminates in a C-C cleavage step that generates formic acid, the nature of which has been controversial. Proposed mechanisms involve roles of P450 Compound 0 (ferric peroxide anion, FeO2 - ) or Compound I (perferryl oxygen, FeO3+ ) reacting with either the aldehyde or its hydrate, respectively. Analysis of 18 O incorporation into formic acid from 18 O2 provides a means of distinguishing the two mechanisms. Human P450 51A1 incorporated 88 % 18 O (one atom) into formic acid, consistent with a major but not exclusive FeO2 - mechanism. Two P450 51 orthologs from amoeba and yeast showed similar results, while two orthologs from pathogenic trypanosomes showed roughly equal contributions of both mechanisms. An X-ray crystal structure of the human enzyme showed the aldehyde oxygen atom 3.5 Å away from the heme iron atom. Experiments with human P450 51A1 and H2 18 O yielded primarily one 18 O atom but 14 % of the formic acid product with two 18 O atoms, indicative of a minor contribution of a Compound I mechanism. LC-MS evidence for a Compound 0-derived Baeyer-Villiger reaction product (a 14α-formyl ester) was also found.

Oxygen-18 Labeling Defines a Ferric Peroxide (Compound 0) Mechanism in the Oxidative Deformylation of Aldehydes by Cytochrome P450 2B4

Most cytochrome P450 (P450) oxidations are considered to occur with the active oxidant being a perferryl oxygen (FeO3+, Compound I). However, a ferric peroxide (FeO2®, Compound 0) mechanism has been proposed, as well, particularly for aldehyde substrates. We investigated three of these systems, the oxidative deformylation of the model substrates citronellal, 2-phenylpropionaldehyde, and 2-methyl-2-phenylpropionaldehyde by rabbit P450 2B4, using 18O labeling. The formic acid product contained one 18O derived from 18O2, which is indicative of a dominant Compound 0 mechanism. The formic acid also contained only one 18O derived from H218O, which ruled out a Compound I mechanism. The possibility of a Baeyer-Villiger reaction was examined by using synthesized possible intermediates, but our data do not support its presence. Overall, these findings unambiguously demonstrate the role of the Compound 0 pathway in these aldehyde oxidative deformylation reactions.

Conversion and synthesis of chemicals catalyzed by fungal cytochrome P450 monooxygenases: A review

Cytochrome P450s (also called CYPs or P450s) are a superfamily of heme-containing monooxygenases. They are distributed in all biological kingdoms. Most fungi have at least two P450-encoding genes, CYP51 and CYP61, which are housekeeping genes that play important roles in the synthesis of sterols. However, the kingdom fungi is an interesting source of numerous P450s. Here, we review reports on fungal P450s and their applications in the bioconversion and biosynthesis of chemicals. We highlight their history, availability, and versatility. We describe their involvement in hydroxylation, dealkylation, oxygenation, C═C epoxidation, C-C cleavage, C-C ring formation and expansion, C-C ring contraction, and uncommon reactions in bioconversion and/or biosynthesis pathways. The ability of P450s to catalyze these reactions makes them promising enzymes for many applications. Thus, we also discuss future prospects in this field. We hope that this review will stimulate further study and exploitation of fungal P450s for specific reactions and applications.

Engineering C-C Bond Cleavage Activity into a P450 Monooxygenase Enzyme

The cytochrome P450 (CYP) superfamily of heme monooxygenases has demonstrated ability to facilitate hydroxylation, desaturation, sulfoxidation, epoxidation, heteroatom dealkylation, and carbon-carbon bond formation and cleavage (lyase) reactions. Seeking to study the carbon-carbon cleavage reaction of α-hydroxy ketones in mechanistic detail using a microbial P450, we synthesized α-hydroxy ketone probes based on the physiological substrate for a well-characterized benzoic acid metabolizing P450, CYP199A4. After observing low activity with wild-type CYP199A4, subsequent assays with an F182L mutant demonstrated enzyme-dependent C-C bond cleavage toward one of the α-hydroxy ketones. This C-C cleavage reaction was subject to an inverse kinetic solvent isotope effect analogous to that observed in the lyase activity of the human P450 CYP17A1, suggesting the involvement of a species earlier than Compound I in the catalytic cycle. Co-crystallization of F182L-CYP199A4 with this α-hydroxy ketone showed that the substrate bound in the active site with a preference for the (S)-enantiomer in a position which could mimic the topology of the lyase reaction in CYP17A1. Molecular dynamics simulations with an oxy-ferrous model of CYP199A4 revealed a displacement of the substrate to allow for oxygen binding and the formation of the lyase transition state proposed for CYP17A1. This demonstration that a correctly positioned α-hydroxy ketone substrate can realize lyase activity with an unusual inverse solvent isotope effect in an engineered microbial system opens the door for further detailed biophysical and structural characterization of CYP catalytic intermediates.

Metabolic and Pharmaceutical Aspects of Fluorinated Compounds

The applications of fluorine in drug design continue to expand, facilitated by an improved understanding of its effects on physicochemical properties and the development of synthetic methodologies that are providing access to new fluorinated motifs. In turn, studies of fluorinated molecules are providing deeper insights into the effects of fluorine on metabolic pathways, distribution, and disposition. Despite the high strength of the C-F bond, the departure of fluoride from metabolic intermediates can be facile. This reactivity has been leveraged in the design of mechanism-based enzyme inhibitors and has influenced the metabolic fate of fluorinated compounds. In this Perspective, we summarize the literature associated with the metabolism of fluorinated molecules, focusing on examples where the presence of fluorine influences the metabolic profile. These studies have revealed potentially problematic outcomes with some fluorinated motifs and are enhancing our understanding of how fluorine should be deployed.

Human cytochrome P450 enzymes 5-51 as targets of drugs and natural and environmental compounds: mechanisms, induction, and inhibition - toxic effects and benefits

Cytochrome P450 (P450, CYP) enzymes have long been of interest due to their roles in the metabolism of drugs, pesticides, pro-carcinogens, and other xenobiotic chemicals. They have also been of interest due to their very critical roles in the biosynthesis and metabolism of steroids, vitamins, and certain eicosanoids. This review covers the 22 (of the total of 57) human P450s in Families 5-51 and their substrate selectivity. Furthermore, included is information and references regarding inducibility, inhibition, and (in some cases) stimulation by chemicals. We update and discuss important aspects of each of these 22 P450s and questions that remain open.

Formation and Cleavage of C-C Bonds by Enzymatic Oxidation-Reduction Reactions

Many oxidation-reduction (redox) enzymes, particularly oxygenases, have roles in reactions that make and break C-C bonds. The list includes cytochrome P450 and other heme-based monooxygenases, heme-based dioxygenases, nonheme iron mono- and dioxygenases, flavoproteins, radical S-adenosylmethionine enzymes, copper enzymes, and peroxidases. Reactions involve steroids, intermediary metabolism, secondary natural products, drugs, and industrial and agricultural chemicals. Many C-C bonds are formed via either (i) coupling of diradicals or (ii) generation of unstable products that rearrange. C-C cleavage reactions involve several themes: (i) rearrangement of unstable oxidized products produced by the enzymes, (ii) oxidation and collapse of radicals or cations via rearrangement, (iii) oxygenation to yield products that are readily hydrolyzed by other enzymes, and (iv) activation of O₂ in systems in which the binding of a substrate facilitates O₂ activation. Many of the enzymes involve metals, but of these, iron is clearly predominant.

Inherent steroid 17α,20-lyase activity in defunct cytochrome P450 17A enzymes

Cytochrome P450 (P450) 17A1 catalyzes the oxidations of progesterone and pregnenolone and is the major source of androgens. The enzyme catalyzes both 17α-hydroxylation and a subsequent 17α,20-lyase reaction, and several mechanisms have been proposed for the latter step. Zebrafish P450 17A2 catalyzes only the 17α-hydroxylations. We previously reported high similarity of the crystal structures of zebrafish P450 17A1 and 17A2 and human P450 17A1. Five residues near the heme, which differed, were changed. We also crystallized this five-residue zebrafish P450 17A1 mutant, and the active site still resembled the structure in the other proteins, with some important differences. These P450 17A1 and 17A2 mutants had catalytic profiles more similar to each other than did the wildtype proteins. Docking with these structures can explain several minor products, which require multiple enzyme conformations. The 17α-hydroperoxy (OOH) derivatives of the steroids were used as oxygen surrogates. Human P450 17A1 and zebrafish P450s 17A1 and P450 17A2 readily converted these to the lyase products in the absence of other proteins or cofactors (with catalytically competent kinetics) plus hydroxylated 17α-hydroxysteroids. The 17α-OOH results indicate that a "Compound I" (FeO³⁺) intermediate is capable of formation and can be used to rationalize the products. We conclude that zebrafish P450 17A2 is capable of lyase activity with the 17α-OOH steroids because it can achieve an appropriate conformation for lyase catalysis in this system that is precluded in the conventional reaction.

CYP51 is an essential drug target for the treatment of primary amoebic meningoencephalitis (PAM)

Primary Amoebic Meningoencephalitis (PAM) is caused by Naegleria fowleri, a free-living amoeba that occasionally infects humans. While considered "rare" (but likely underreported) the high mortality rate and lack of established success in treatment makes PAM a particularly devastating infection. In the absence of economic inducements to invest in development of anti-PAM drugs by the pharmaceutical industry, anti-PAM drug discovery largely relies on drug 'repurposing'-a cost effective strategy to apply known drugs for treatment of rare or neglected diseases. Similar to fungi, N. fowleri has an essential requirement for ergosterol, a building block of plasma and cell membranes. Disruption of sterol biosynthesis by small-molecule inhibitors is a validated interventional strategy against fungal pathogens of medical and agricultural importance. The N. fowleri genome encodes the sterol 14-demethylase (CYP51) target sharing ~35% sequence identity to fungal orthologues. The similarity of targets raises the possibility of repurposing anti-mycotic drugs and optimization of their usage for the treatment of PAM. In this work, we (i) systematically assessed the impact of anti-fungal azole drugs, known as conazoles, on sterol biosynthesis and viability of cultured N. fowleri trophozotes, (ii) identified the endogenous CYP51 substrate by mass spectrometry analysis of N. fowleri lipids, and (iii) analyzed the interactions between the recombinant CYP51 target and conazoles by UV-vis spectroscopy and x-ray crystallography. Collectively, the target-based and parasite-based data obtained in these studies validated CYP51 as a potentially 'druggable' target in N. fowleri, and conazole drugs as the candidates for assessment in the animal model of PAM.

Isotope-Labeling Studies Support the Electrophilic Compound I Iron Active Species, FeO(3+), for the Carbon-Carbon Bond Cleavage Reaction of the Cholesterol Side-Chain Cleavage Enzyme, Cytochrome P450 11A1

The enzyme cytochrome P450 11A1 cleaves the C20-C22 carbon-carbon bond of cholesterol to form pregnenolone, the first 21-carbon precursor of all steroid hormones. Various reaction mechanisms are possible for the carbon-carbon bond cleavage step of P450 11A1, and most current proposals involve the oxoferryl active species, Compound I (FeO(3+)). Compound I can either (i) abstract an O-H hydrogen atom or (ii) be attacked by a nucleophilic hydroxy group of its substrate, 20R,22R-dihydroxycholesterol. The mechanism of this carbon-carbon bond cleavage step was tested using (18)O-labeled molecular oxygen and purified P450 11A1. P450 11A1 was incubated with 20R,22R-dihydroxycholesterol in the presence of molecular oxygen ((18)O2), and coupled assays were used to trap the labile (18)O atoms in the enzymatic products (i.e., isocaproaldehyde and pregnenolone). The resulting products were derivatized and the (18)O content was analyzed by high-resolution mass spectrometry. P450 11A1 showed no incorporation of an (18)O atom into either of its carbon-carbon bond cleavage products, pregnenolone and isocaproaldehyde . The positive control experiments established retention of the carbonyl oxygens in the enzymatic products during the trapping and derivatization processes. These results reveal a mechanism involving an electrophilic Compound I species that reacts with nucleophilic hydroxy groups in the 20R,22R-dihydroxycholesterol intermediate of the P450 11A1 reaction to produce the key steroid pregnenolone.

Polymorphisms of CYP51A1 from cholesterol synthesis: associations with birth weight and maternal lipid levels and impact on CYP51 protein structure

We investigated the housekeeping cytochrome P450 CYP51A1 encoding lanosterol 14α-demethylase from cholesterol synthesis that was so far not directly linked to human disorders. By direct sequencing of CYP51A1 in 188 women with spontaneous preterm delivery and 188 unrelated preterm infants (gestational age <37 weeks) we identified 22 variants where 10 are novel and rare. In infants there were two novel CYP51A1 variants where damaging effects of p.Tyr145Asp from the substrate recognition region, but not p.Asn193Asp, were predicted by PolyPhen2 and SIFT. This was confirmed by molecular modeling showing that Tyr145Asp substitution results in changed electrostatic potential of the CYP51 protein surface and lengthened distance to the heme which prevents hydrogen bonding. The CYP51 Tyr145Asp mutation is rare and thus very interesting for further structure/function relationship studies. From the 12 identified known variants rs6465348 was chosen for family based association studies due to its high minor allele frequency. Interestingly, this CYP51A1 common variant associates with small for gestational age weight in newborns (p = 0.028) and lower blood total cholesterol and low density lipoprotein cholesterol levels in mothers in 2nd trimester of pregnancy (p = 0.042 and p = 0.046 respectively). Our results indicate a new link between a cholesterol synthesis gene CYP51A1 and pregnancy pathologies.

Cytochrome P450(cin) (CYP176A1) D241N: investigating the role of the conserved acid in the active site of cytochrome P450s

P450(cin) (CYP176A) is a rare bacterial P450 in that contains an asparagine (Asn242) instead of the conserved threonine that almost all other P450s possess that directs oxygen activation by the heme prosthetic group. However, P450(cin) does have the neighbouring, conserved acid (Asp241) that is thought to be involved indirectly in the protonation of the dioxygen and affect the lifetime of the ferric-peroxo species produced during oxygen activation. In this study, the P450(cin) D241N mutant has been produced and found to be analogous to the P450(cam) D251N mutant. P450(cin) catalyses the hydroxylation of cineole to give only (1R)-6β-hydroxycineole and is well coupled (NADPH consumed: product produced). The P450(cin) D241N mutant also hydroxylated cineole to produce only (1R)-6β-hydroxycineole, was moderately well coupled (31±3%) but a significant reduction in the rate of the reaction (2% as compared to wild type) was observed. Catalytic oxidation of a variety of substrates by D241N P450(cin) were used to examine if typical reactions ascribed to the ferric-peroxo species increased as this intermediate is known to be more persistent in the P450(cam) D251N mutant. However, little change was observed in the product profiles of each of these substrates between wild type and mutant enzymes and no products consistent with chemistry of the ferric-peroxo species were observed to increase.

Proximal ligand electron donation and reactivity of the cytochrome P450 ferric-peroxo anion

CYP125 from Mycobacterium tuberculosis catalyzes sequential oxidation of the cholesterol side-chain terminal methyl group to the alcohol, aldehyde, and finally acid. Here, we demonstrate that CYP125 simultaneously catalyzes the formation of five other products, all of which result from deformylation of the sterol side chain. The aldehyde intermediate is shown to be the precursor of both the conventional acid metabolite and the five deformylation products. The acid arises by protonation of the ferric-peroxo anion species and formation of the ferryl-oxene species, also known as Compound I, followed by hydrogen abstraction and oxygen transfer. The deformylation products arise by addition of the same ferric-peroxo anion to the aldehyde intermediate to give a peroxyhemiacetal that leads to C-C bond cleavage. This bifurcation of the catalytic sequence has allowed us to examine the effect of electron donation by the proximal ligand on the properties of the ferric-peroxo anion. Replacement of the cysteine thiolate iron ligand by a selenocysteine results in UV-vis, EPR, and resonance Raman spectral changes indicative of an increased electron donation from the proximal selenolate ligand to the iron. Analysis of the product distribution in the reaction of the selenocysteine substituted enzyme reveals a gain in the formation of the acid (Compound I pathway) at the expense of deformylation products. These observations are consistent with an increase in the pK(a) of the ferric-peroxo anion, which favors its protonation and, therefore, Compound I formation.

Steroid-transforming enzymes in fungi

Fungal species are a very important source of many different enzymes, and the ability of fungi to transform steroids has been used for several decades in the production of compounds with a sterane skeleton. Here, we review the characterised and/or purified enzymes for steroid transformations, dividing them into two groups: (i) enzymes of the ergosterol biosynthetic pathway, including data for, e.g. ERG11 (14α-demethylase), ERG6 (C-24 methyltransferase), ERG5 (C-22 desaturase) and ERG4 (C-24 reductase); and (ii) the other steroid-transforming enzymes, including different hydroxylases (7α-, 11α-, 11β-, 14α-hydroxylase), oxidoreductases (5α-reductase, 3β-hydroxysteroid dehydrogenase/isomerase, 17β-hydroxysteroid dehydrogenase, C-1/C-2 dehydrogenase) and C-17-C-20 lyase. The substrate specificities of these enzymes, their cellular localisation, their association with protein super-families, and their potential applications are discussed. Article from a special issue on steroids and microorganisms.

Rearrangement reactions catalyzed by cytochrome P450s

Cytochrome P450s promote a variety of rearrangement reactions both as a consequence of the nature of the radical and other intermediates generated during catalysis, and of the neighboring structures in the substrate that can interact either with the initial radical intermediates or with further downstream products of the reactions. This article will review several kinds of previously published cytochrome P450-catalyzed rearrangement reactions, including changes in stereochemistry, radical clock reactions, allylic rearrangements, "NIH" and related shifts, ring contractions and expansions, and cyclizations that result from neighboring group interactions. Although most of these reactions can be carried out by many members of the cytochrome P450 superfamily, some have only been observed with select P450s, including some reactions that are catalyzed by specific endoperoxidases and cytochrome P450s found in plants.

A nonazole CYP51 inhibitor cures Chagas' disease in a mouse model of acute infection

Chagas' disease, the leading cause of heart failure in Latin America, is caused by the kinetoplastid protozoan Trypanosoma cruzi. The sterols of T. cruzi resemble those of fungi, both in composition and in biosynthesis. Azole inhibitors of sterol 14alpha-demethylase (CYP51) successfully treat fungal infections in humans, and efforts to adapt the success of antifungal azoles posaconazole and ravuconazole as second-use agents for Chagas' disease are under way. However, to address concerns about the use of azoles for Chagas' disease, including drug resistance and cost, the rational design of nonazole CYP51 inhibitors can provide promising alternative drug chemotypes. We report the curative effect of the nonazole CYP51 inhibitor LP10 in an acute mouse model of T. cruzi infection. Mice treated with an oral dose of 40 mg LP10/kg of body weight twice a day (BID) for 30 days, initiated 24 h postinfection, showed no signs of acute disease and had histologically normal tissues after 6 months. A very stringent test of cure showed that 4/5 mice had negative PCR results for T. cruzi, and parasites were amplified by hemoculture in only two treated mice. These results compare favorably with those reported for posaconazole. Electron microscopy and gas chromatography-mass spectrometry (GC-MS) analysis of sterol composition confirmed that treatment with LP10 blocked the 14alpha-demethylation step and induced breakdown of parasite cell membranes, culminating in severe ultrastructural and morphological alterations and death of the clinically relevant amastigote stage of the parasite.

Cloning and characterization of the lanosterol 14alpha-demethylase gene from Antrodia cinnamomea

Sterol 14alpha-demethylase (CYP51) is one of the key enzymes for sterol biosynthesis in fungi; it is widely distributed in all members of the cytochrome P450 superfamily. In this study, AcCyp51, encoding a cytochrome P450 sterol 14alpha-demethylase, was obtained from the sequences of EST libraries of Antrodia cinnamomea by using 5' RACE and genome walking methods. The open reading frame of AcCyp51 is 1635 bp and encodes 544 amino acids. The recombinant protein of AcCYP51 fused with glutathione-S-transferase from Escherichia coli revealed the demethylating activity by using lanosterol as substrate and GC-MS analysis. Gene expression levels of AcCYP51 were higher in natural basidiomes than in other cell types. Transcription of AcCYP51 increased in various culture conditions including adding squalene, lanosterol, itroconazole, and oleic acid as inducers. These reveal the important functions of AcCYP51 in basidiomatal formation and suggest that it might participate in other biological processes.

Raman evidence for specific substrate-induced structural changes in the heme pocket of human cytochrome P450 aromatase during the three consecutive oxygen activation steps

Specific substrate-induced structural changes in the heme pocket are proposed for human cytochrome P450 aromatase (P450arom) which undergoes three consecutive oxygen activation steps. We have experimentally investigated this heme environment by resonance Raman spectra of both substrate-free and substrate-bound forms of the purified enzyme. The Fe-CO stretching mode (nu(Fe)(-)(CO)) of the CO complex and Fe(3+)-S stretching mode (nu(Fe)(-)(S)) of the oxidized form were monitored as a structural marker of the distal and proximal sides of the heme, respectively. The nu(Fe)(-)(CO) mode was upshifted from 477 to 485 and to 490 cm(-)(1) by the binding of androstenedione and 19-aldehyde-androstenedione, substrates for the first and third steps, respectively, whereas nu(Fe)(-)(CO) was not observed for P450arom with 19-hydroxyandrostenedione, a substrate for the second step, indicating that the heme distal site is very flexible and changes its structure depending on the substrate. The 19-aldehyde-androstenedione binding could reduce the electron donation from the axial thiolate, which was evident from the low-frequency shift of nu(Fe)(-)(S) by 5 cm(-)(1) compared to that of androstenedione-bound P450arom. Changes in the environment in the heme distal site and the reduced electron donation from the axial thiolate upon 19-aldehyde-androstenedione binding might stabilize the ferric peroxo species, an active intermediate for the third step, with the suppression of the formation of compound I (Fe(4+)=O porphyrin(+)(*)) that is the active species for the first and second steps. We, therefore, propose that the substrates can regulate the formation of alternative reaction intermediates by modulating the structure on both the heme distal and proximal sites in P450arom.

Cloning and characterization of CYP51 from Mycobacterium avium

Mycobacterium avium complex (MAC) causes chronic lung disease in immunocompetent people and disseminated infection in patients with AIDS. MAC is intrinsically resistant to many conventional antimycobacterial agents, it develops drug resistance rapidly to macrolide antibiotics, and patients with MAC infection experience frequent relapses or the inability to completely eradicate the infection with current treatment. Treatment regimens are prolonged and complicated by drug toxicity or intolerances. We sought to identify biochemical pathways in MAC that can serve as targets for novel antimycobacterial treatment. The cytochrome P450 enzyme, CYP51, catalyzes an essential early step in sterol metabolism, removing a methyl group from lanosterol in animals and fungi, or from obtusifoliol in plants. Azoles inhibit CYP51 function, leading to an accumulation of methylated sterol precursors. This perturbation of normal sterol metabolism compromises cell membrane integrity, resulting in growth inhibition or cell death. We have cloned and characterized a CYP51 from MAC that functions as a lanosterol 14alpha-demethylase. We show the direct interactions of azoles with purified MAC-CYP51 by absorbance and electron paramagnetic resonance spectroscopy, and determine the minimum inhibitory concentrations (MICs) of econazole, ketoconazole, itraconazole, fluconazole, and voriconazole against MAC. Furthermore, we demonstrate that econazole has a MIC of 4 mug/ml and a minimum bacteriocidal concentration of 4 mug/ml, whereas ketoconazole has a MIC of 8 mug/ml and a minimum bacteriocidal concentration of 16 mug/ml. Itraconazole, voriconazole, and fluconazole did not inhibit MAC growth to any significant extent.

Oxidative aldehyde deformylation catalyzed by NADPH-cytochrome P450 reductase and the flavoprotein domain of neuronal nitric oxide synthase

We report here the unexpected finding that recombinant or hepatic microsomal NADPH-cytochrome P450 reductase catalyzes the oxidative deformylation of a model xenobiotic aldehyde, 2-phenylpropionaldehyde, to the n-1 alcohol, 1-phenylethanol, in the absence of cytochrome P450. The flavoprotein and NADPH are absolute requirements, and the reaction displays a dependence on time and on NADPH and reductase concentration. Not surprisingly, the hydrophobic tail of the flavoprotein is not required for catalytic competence. The reductase domain of neuronal nitric oxide synthase is about 30% more active than P450 reductase, and neither flavoprotein catalyzes conversion of the aldehyde to the carboxylic acid, by far the predominant metabolite with P450s in a reconstituted system. Reductase-catalyzed deformylation is unaffected by metal ion chelators and oxygen radical scavengers, but is strongly inhibited by catalase, and the catalase-mediated inhibition is prevented by azide. These results, together with observed parallel increases in 1-phenylethanol and H(2)O(2) formation as a function of NADPH concentration, are evidence that free H(2)O(2) is rate-limiting in aldehyde deformylation by the flavoprotein reductases. This contrasts sharply with the P450-catalyzed reaction, which is brought about by iron-bound peroxide that is inaccessible to catalase.

Novel pathways of bile acid metabolism involving CYP3A4

The hepatic predominating cytochrome P450, CYP3A4, plays an essential role in the detoxification of bile acids and is important in pathological conditions such as cholestasis where CYP3A4 is adaptively up-regulated. However, the mechanism that triggers the up-regulation of CYP3A4 is still not clear. In this study, using recombinant CYP3A4 and human liver microsomes, we demonstrate that CYP3A4 can metabolise lithocholic acid into 3-dehydrolithocholic acid, a potent activator of the nuclear receptors, pregnane X receptor and 1,25-dihydroxy vitamin D3 receptor, which are known to regulate the expression of CYP3A4. This process thus provides a feed-forward metabolism of toxic bile acid that may be of importance in maintaining bile acid homeostasis. We also provide evidence for a novel CYP3A4-mediated metabolic pathway of the secondary bile acid deoxycholic acid. Patients treated with the antiepileptic drug carbamazepine, a CYP3A4 inducer, had markedly elevated urinary excretion of 1beta-hydroxydeoxycholic acid compared to healthy controls. The importance of CYP3A4 in this process was verified by incubations with recombinant CYP3A4 and human liver microsomes, both of which efficiently converted deoxycholic acid into 1beta-hydroxydeoxycholic acid. Interestingly, CYP3A4 was also found to be active against the secondary bile acid ursodeoxycholic acid.

Substrate modulation of the properties and reactivity of the oxy-ferrous and hydroperoxo-ferric intermediates of cytochrome P450cam as shown by cryoreduction-EPR/ENDOR spectroscopy

EPR/ENDOR studies have been carried out on oxyferrous cytochrome P450cam one-electron cryoreduced by gamma-irradiation at 77 K in the absence of substrate and in the presence of a variety of substrates including its native hydroxylation substrate, camphor (a), and the alternate substrates, 5-methylenyl-camphor (b), 5,5-difluorocamphor (c), norcamphor (d), and adamantanone (e); the equivalent experiments have been performed on the T252A mutant complexed with a and b. The present study shows that the properties and reactivity of the oxyheme and of both the primary and the annealed intermediates are modulated by a bound substrate. This includes alterations in the properties of the heme center itself (g tensor; (14)N, (1)H, hyperfine couplings). It also includes dramatic changes in reactivity: the presence of any substrate increases the lifetime of hydroperoxoferri-P450cam (2) no less than ca. 20-fold. Among the substrates, b stands out as having an exceptionally strong influence on the properties and reactivity of the P450cam intermediates, especially in the T252A mutant. The intermediate, 2(T252A)-b, does not lose H(2)O(2), as occurs with 2(T252A)-a, but decays with formation of the epoxide of b. Thus, these observations show that substrate can modulate the properties of both the monoxygenase active-oxygen intermediates and the proton-delivery network that encompasses them.

The last reaction producing brassinolide is catalyzed by cytochrome P-450s, CYP85A3 in tomato and CYP85A2 in Arabidopsis

Brassinosteroids are steroidal hormones essential for the growth and development of plants. Brassinolide, the most biologically active brassinosteroid, has a seven-membered lactone ring that is formed by a Baeyer-Villiger oxidation of its immediate precursor castasterone. Despite its potential key role in controlling plant development, brassinolide synthase has not been identified. Previous work has shown that the formation of castasterone from 6-deoxocastasterone is catalyzed by members of the CYP85A family of cytochrome P-450 monooxygenases. A null mutation in the tomato Dwarf (CYP85A1) gene, extreme dwarf (d(x)), causes severe dwarfism due to brassinosteroid deficiency, but the d(x) mutant still produces fruits. Here, we show that d(x) fruits contain brassinolide at a higher level than wild-type fruits and that a new CYP85A gene, CYP85A3, is preferentially expressed in tomato fruits. Tomato CYP85A3 catalyzed the Baeyer-Villiger oxidation to produce brassinolide from castasterone in yeast, in addition to the conversion of 6-deoxocastasterone to castasterone. We also show that Arabidopsis CYP85A2, which was initially characterized as castasterone synthase, also has brassinolide synthase activity. Exogenous application of castasterone and brassinolide to the Arabidopsis cyp85a1/cyp85a2 double mutant suggests that castasterone can function as an active brassinosteroid but that its conversion into brassinolide is necessary for normal vegetative development in Arabidopsis. We postulate that castasterone is the major active brassinosteroid during vegetative growth in tomato, whereas brassinolide may play an organ-specific role in fruit development in this species.

Abolition of oxygenase function, retention of NADPH oxidase activity, and emergence of peroxidase activity upon replacement of the axial cysteine-436 ligand by histidine in cytochrome P450 2B4

A fundamental aspect of cytochrome P450 function is the role of the strictly conserved axial cysteine ligand, replacement of which by histidine has invariably resulted in mammalian and bacterial preparations devoid of heme. Isolation of the His-436 variant of NH2-truncated P450 2B4 partly as the holoenzyme was achieved in the present study by mutagenesis of the I-helix Ala-298 residue to Glu and subsequent conversion of the axial Cys-436 to His. The expressed A298E/C436H double mutant, cloned with a hexahistidine tag, had a molecular mass equivalent to that of the primary structure of His-tagged truncated 2B4 and the sum of the two mutated residues, and contained a heme group which, when released on HPLC, showed a retention time and spectrum identical to those of iron protoporphyrin IX. The absolute spectra of A298E/C436H indicate a change in heme coordination structure from low- to high-spin, and, as expected for a His-ligated hemeprotein, the Soret maximum of the ferrous CO complex is at 422 nm. The double mutant has no oxygenase activity with representative substrates known to undergo transformation by the oxene [(FeO)3+] or peroxo activated oxygen species, but catalyzes significant H2O2 formation that is NADPH- and time-dependent, and directly proportional to the concentration of A298E/C436H in the presence of saturating reductase. Moreover, the catalytic efficiency of A298E/C436H in the H2O2-supported peroxidation of pyrogallol is more than two orders of magnitude greater than that of wild-type 2B4 or the A298E variant. The results unambiguously demonstrate that the proximal thiolate ligand is essential for substrate oxygenation by P450.

Cryoreduction EPR and 13C, 19F ENDOR study of substrate-bound substates and solvent kinetic isotope effects in the catalytic cycle of cytochrome P450cam and its T252A mutant

We recently used cryoreduction EPR/ENDOR techniques to show that a substrate can modulate the properties of both the monooxygenase active-oxygen intermediates and of the proton-delivery network which encompasses them. In the present report we use Q-band pulsed 19F ENDOR (Mims 3-pulse sequence) to examine the substrate binding geometries of camphor, through use of the 5,5'--difluorocamphor, and 13C ENDOR to examine the binding of 5-methylenyl camphor labeled with 13C at C11. These probes are examined in multiple states of the catalytic cycle of P450cam and its T252A mutant. As part of this investigation we further report a new cryoreduction reaction, the reduction of a ferroheme to the EPR-visible Fe(I) state, and use it to probe the substrate binding to the EPR-silent ferroheme state. Finally we report the solvent kinetic isotope effect on the decay of the camphor complex of the hydroperoxo-ferric intermediate, the first such measurement on an individual step within the P450cam reaction cycle. Following reduction of oxyferrous-P450cam, this step is the rate-limiting step in camphor hydroxylation, and its solv-KIE of 1.8 at 190 K establishes that it involves activation of the hydroperoxo moiety by transfer of the 'second' proton of catalysis. We suggest that the finding that the heme pocket can exist in multiple substates, including multiple substrate binding locations, even in P450cam, along with the established possibility that the hydroperoxo-ferriheme intermediate can react with substrate, may explain the formation of multiple products by P450s.

Structural diversities of active site in clinical azole-bound forms between sterol 14alpha-demethylases (CYP51s) from human and Mycobacterium tuberculosis

To gain insights into the molecular basis of the design for the selective azole anti-fungals, we compared the binding properties of azole-based inhibitors for cytochrome P450 sterol 14alpha-demethylase (CYP51) from human (HuCYP51) and Mycobacterium tuberculosis (MtCYP51). Spectroscopic titration of azoles to the CYP51s revealed that HuCYP51 has higher affinity for ketoconazole (KET), an azole derivative that has long lipophilic groups, than MtCYP51, but the affinity for fluconazole (FLU), which is a member of the anti-fungal armamentarium, was lower in HuCYP51. The affinity for 4-phenylimidazole (4-PhIm) to MtCYP51 was quite low compared with that to HuCYP51. In the resonance Raman spectra for HuCYP51, the FLU binding induced only minor spectral changes, whereas the prominent high frequency shift of the bending mode of the heme vinyl group was detected in the KET- or 4-PhIm-bound forms. On the other hand, the bending mode of the heme propionate group for the FLU-bound form of MtCYP51 was shifted to high frequency as found for the KET-bound form, but that for 4-PhIm was shifted to low frequency. The EPR spectra for 4-PhIm-bound MtCYP51 and FLU-bound HuCYP51 gave multiple g values, showing heterogeneous binding of the azoles, whereas the single gx and gz values were observed for other azole-bound forms. Together with the alignment of the amino acid sequence, these spectroscopic differences suggest that the region between the B' and C helices, particularly the hydrophobicity of the C helix, in CYP51s plays primary roles in determining strength of interactions with azoles; this differentiates the binding specificity of azoles to CYP51s.

Models and mechanisms of O-O bond activation by cytochrome P450. A critical assessment of the potential role of multiple active intermediates in oxidative catalysis

Cytochrome P450 enzymes promote a number of oxidative biotransformations including the hydroxylation of unactivated hydrocarbons. Whereas the long-standing consensus view of the P450 mechanism implicates a high-valent iron-oxene species as the predominant oxidant in the radicalar hydrogen abstraction/oxygen rebound pathway, more recent studies on isotope partitioning, product rearrangements with 'radical clocks', and the impact of threonine mutagenesis in P450s on hydroxylation rates support the notion of the nucleophilic and/or electrophilic (hydro)peroxo-iron intermediate(s) to be operative in P450 catalysis in addition to the electrophilic oxenoid-iron entity; this may contribute to the remarkable versatility of P450s in substrate modification. Precedent to this mechanistic concept is given by studies with natural and synthetic P450 biomimics. While the concept of an alternative electrophilic oxidant necessitates C-H hydroxylation to be brought about by a cationic insertion process, recent calculations employing density functional theory favour a 'two-state reactivity' scenario, implicating the usual ferryl-dependent oxygen rebound pathway to proceed via two spin states (doublet and quartet); state crossing is thought to be associated with either an insertion or a radicalar mechanism. Hence, challenge to future strategies should be to fold the disparate and sometimes contradictory data into a harmonized overall picture.

The cellular biochemistry of cholesterol and statins: insights into the pathophysiology and therapy of Alzheimer's disease

The causes of late onset Alzheimer disease (AD) are poorly understood. Although beta-amyloid (Abeta) is thought to play a critical role in the pathophysiology of AD, no genetic evidence directly ties Abeta to late onset AD. This suggests that the accumulation of Abeta and neurodegeneration associated with AD might result from an abnormality that indirectly affects Abeta production or accumulation. Increasing evidence suggests that abnormalities in the metabolism of cholesterol and related molecules, such as cholseterol esters and 24(S) hydroxycholesterol might contribute to the pathophysiology of late onset AD by increasing production of Abeta. 24(S) Hydroxycholesterol is a member of a family of oxidized cholesterol catabolites, termed oxysterols, which function to regulate export of cholesterol from the cell and transcription of genes related to cholesterol metabolism. Cholesterol esters are cholesterol derivatives used for cholesterol storage. Levels of 24(S) hydroxycholesterol increase with AD. Polymorphisms in several different genes important for cholesterol physiology are associated with an increased load or level of Abeta in AD. These genes include apolipoprotein E, cholesterol 24 hydroxylase (Cyp46), acyl-CoA:cholesterol acetyltransferase (ACAT), and the cholesterol transporter ABCA1. Other studies show that levels of cholesterol, or its precursors, are elevated in subjects early in the course of AD. Finally, studies of the processing of amyloid precursor protein show that cholesterol and its catabolites modulate amyloid precursor protein processing and Abeta production. These lines of evidence raise the possibility that genetic abnormalities in cholesterol metabolism might contribute to the pathophysiology of AD.

CYP51 from Trypanosoma brucei Is Obtusifoliol-Specific

New isoforms of CYP51 (sterol 14alpha-demethylase), an essential enzyme in sterol biosynthesis and primary target of azole antimycotic drugs, are found in pathogenic protists, Trypanosoma brucei(TB), T. vivax, T. cruzi, and Leishmania major. The sequences share approximately 80% amino acid identity and are approximately 25% identical to sterol 14alpha-demethylases from other biological kingdoms. Differences of residues conserved throughout the rest of the CYP51 family that align with the BC-loop and helices F and G of CYP51 from Mycobacterium tuberculosis (MT)) imply possible alterations in the topology of the active site cavity of the protozoan enzymes. CYP51 and cytochrome P450 reductase (CPR) from TB were cloned, expressed in Escherichia coli, and purified. The P450 has normal spectral features (including absolute absorbance, carbon monoxide, and ligand binding spectra), is efficiently reduced by TB and rat CPR but demonstrates altered specificity in comparison with human CYP51 toward three tested azole inhibitors, and contrary to the human, Candida albicans, and MT isoforms, reveals profound substrate preference toward obtusifoliol (turnover 5.6 min(-1)). It weakly interacts with the other known CYP51 substrates; slow lanosterol conversion predominantly produces the 14alpha-carboxyaldehyde intermediate. Although obtusifoliol specificity is typical for plant isoforms of CYP51, the set of sterol biosynthetic enzymes in the protozoan genomes together with available information about sterol composition of kinetoplastid cells suggest that the substrate preference of TBCYP51 may reflect a novel sterol biosynthetic pathway in Trypanosomatidae.

CYP51--the omnipotent P450

Sterol 14 alpha-demethylase (CYP51) is the single cytochrome P450 (CYP) required for sterol biosynthesis in different phyla, and it is the most widely distributed P450 gene family being found in all biological kingdoms. It catalyzes the first step following cyclization in sterol biosynthesis such as removal of the 14 alpha-methyl group from lanosterol in the cholesterol biosynthetic pathway, leading to formation of the initial substrate in steroid hormone biosynthesis. CYP51 from different phyla have low sequence similarity across kingdoms and contain only about 40 conserved amino acid residues in the whole family. An attempt to predict the possible role of these conserved residues is being made by a combination of the results of site-directed mutagenesis and information from the known crystal structure of sterol 14 alpha-demethylase from Mycobacterium tuberculosis.

Conservation in the CYP51 family. Role of the B' helix/BC loop and helices F and G in enzymatic function

CYP51 (sterol 14 alpha-demethylase) is an essential enzyme in sterol biosynthetic pathways and the only P450 gene family having catalytically identical orthologues in different biological kingdoms. The proteins have low sequence similarity across phyla, and the whole family contains about 40 completely conserved amino acid residues. Fifteen of these residues lie in the secondary structural elements predicted to form potential substrate recognition sites within the P450 structural fold. The role of 10 of these residues, in the B' helix/BC loop, helices F and G, has been studied by site-directed mutagenesis using as a template the soluble sterol 14 alpha-demethylase of known structure, CYP51 from Mycobacterium tuberculosis (MT) and the human orthologue. Single amino acid substitutions of seven residues (Y76, F83, G84, D90, L172, G175, and R194) result in loss of the ability of the mutant MTCYP51 to metabolize lanosterol. Residual activity of D195A is very low, V87A is not expressed as a P450, and A197G has almost 1 order of magnitude increased activity. After purification, all of the mutants show normal spectral properties, heme incorporation, and the ability to be reduced enzymatically and to interact with azole inhibitors. Profound influence on the catalytic activity correlates well with the spectral response to substrate binding, effect of substrate stabilization on the reduced state of the P450, and substrate-enhanced efficiency of enzymatic reduction. Mutagenesis of corresponding residues in human CYP51 implies that the conserved amino acids might be essential for the evolutionary conservation of sterol 14 alpha-demethylation from bacteria to mammals.

Electron supply and catalytic oxidation of nitrogen by cytochrome P450 and nitric oxide synthase

Cytochrome P450 and nitric oxide synthase (NOS) oxidize nitrogen atoms, although the substrates and transformations are highly restricted for NOS. The first reaction catalyzed by NOS is mediated by a P450-like ferryl species, although it is generated by a distinct process in which a tetrahydrobiopterin molecule in NOS serves as a transient electron donor. The second NOS reaction appears to be mediated by an iron dioxygen precursor of the ferryl species. The transient tetrahydrobiopterin radical formed in these reactions is quenched by electron transfer from the NOS flavin domain. Electron transfer from the flavins is controlled by the binding of calmodulin, the presence of peptide inserts in the flavin domain, the substrate structure, and phosphorylation of the enzyme.

Biotransformation of 1,1,1,3,3-Pentafluoropropane (HFC-245fa)

1,1,1,3,3-Pentafluoropropane (HFC-245fa) is being developed as a CFC substitute. 1,1,1,3,3-Pentafluoropropane has a low potential for toxicity: the only remarkable toxic effect seen in rats after inhalation exposure to 1,1,1,3,3-pentafluoropropane in concentrations of up to 50,000 ppm for 90 days was an increased incidence of diffuse myocarditis. To elucidate the possible role of biotransformation in 1,1,1,3,3-pentafluoropropane-induced cardiotoxicity, the biotransformation of 1,1,1,3,3-pentafluoropropane was investigated in rats after inhalation exposure and in rat and human liver microsomes. Male and female rats were exposed by inhalation to 50 000, 10 000, and 2000 ppm 1,1,1,3,3-pentafluoropropane for 6 h, urine was collected for 72 h, and metabolites excreted were identified by 19F NMR spectroscopy and quantified by GC/MS. Trifluoroacetic acid and inorganic fluoride were identified as major urinary metabolites of 1,1,1,3,3-pentafluoropropane; 3,3,3-trifluoropropanoic acid and 1,1,1,3,3-pentafluoropropane-2-ol were minor metabolites. The extent of 1,1,1,3,3-pentafluoropropane biotransformation after inhalation was dependent on exposure concentrations. Neither 3,3,3-trifluoropropanoic acid nor 3,3,3-trifluoropyruvic acid were metabolized to trifluoroacetic acid in vitro or in rats. In rat and human liver microsomes, 1,1,1,3,3-pentafluoropropane was biotransformed by a cytochrome P450-dependent reaction to trifluoroacetic acid and 3,3,3-trifluoropropanoic acid. Rates of trifluoroacetic acid formation were 99.2 +/- 20.5 pmol (mg of protein)(-)(1) min(-)(1) and of 3,3,3-trifluoropropanoic acid formation were 17.5 +/- 4.0 pmol (mg of protein)(-)(1) min(-)(1) in liver microsomes from male rats. In human liver microsomes, rates of trifluoroacetic acid formation ranged from 0 to 11.6 pmol (mg of protein)(-)(1) min(-)(1), and rates of 3,3,3-trifluoropropanoic acid formation ranged from 0.7 to 7.6 pmol (mg of protein)(-)(1) min(-)(1). The results show that 1,1,1,3,3-pentafluoropropane is metabolized at low rates in vivo and in vitro. The toxic effects of 1,1,1,3,3-pentafluoropropane may be associated with the formation of the minor metabolite 3,3,3-trifluoropropanoic acid, which is highly toxic in rats.