Biophysical characterization of the sterol demethylase P450 from Mycobacterium tuberculosis, its cognate ferredoxin, and their interactions

Characterization of the Orphan Cytochrome P450 CYP135B1 from Mycobacterium tuberculosis: Involvement in Metabolism but Not in the Antibacterial Activity of the Antitubercular Drug SQ109

The rise of multidrug-resistant tuberculosis (TB) has increased the need for new antitubercular (anti-TB) drugs and the identification of novel drug targets. One promising target is Mycobacterium tuberculosis (Mtb) cytochrome P450 enzymes (P450s). This study focuses on the characterization of CYP135B1, a prevalent Mtb P450. Using a combination of microbiology, genomics, bioinformatics, docking, spectroscopy, and mass spectrometry, researchers successfully expressed, purified, and characterized CYP135B1. A 3D model was built with AlphaFold 3. The enzyme displayed typical features of P450 proteins and showed strong binding to imidazole derivatives. Notably, CYP135B1 metabolized the anti-TB drug SQ109 by inserting oxygen into its geranyl moiety in a manner distinct from CYP124A1. However, genetic studies using a ΔCYP135B1 mutant strain revealed that CYP135B1 is not required for SQ109's antibacterial activity, as its deletion did not affect drug efficacy despite CYP135B1 metabolizes SQ109.

Fragment-based development of small molecule inhibitors targeting Mycobacterium tuberculosis cholesterol metabolism

Mycobacterium tuberculosis (Mtb) is the world's most deadly infectious pathogen and new drugs are urgently required to combat the emergence of multi- (MDR) and extensively- (XDR) drug resistant strains. The bacterium specifically upregulates sterol uptake pathways in infected macrophages and the metabolism of host-derived cholesterol is essential for Mtb's long-term survival in vivo. Here, we report the development of antitubercular small molecules that inhibit the Mtb cholesterol oxidases CYP125 and CYP142, which catalyze the initial step of cholesterol metabolism. An efficient biophysical fragment screen was used to characterize the structure-activity relationships of CYP125 and CYP142, and identify a non-azole small molecule 1a that can bind to the heme cofactor of both enzymes. A structure-guided fragment-linking strategy was used to optimize the binding affinity of 1a, yielding a potent dual CYP125/142 inhibitor 5m (KD CYP125/CYP142 = 0.04/0.16 μM). Compound 5m potently inhibits the catalytic activity of CYP125 and CYP142 in vitro (KI values < 0.1 μM), and rapidly depletes Mtb intracellular ATP (IC50 = 0.15 μM). The compound has antimicrobial activity against both drug susceptible and MDR Mtb (MIC99 values 0.4 - 1.5 μM) in extracellular assays, and inhibits the growth of Mtb in human macrophages (MIC = 1.7 μM) with good selectivity over mammalian cytotoxicity (LD50 ≥ 50 μM). The combination of small molecule inhibitors and structural data reported here provide useful tools to study the role of cholesterol metabolism in Mtb and are a promising step towards novel antibiotics targeting bioenergetic pathways, which could be used to help combat MDR-TB.

Melatonin Activation by Human Cytochrome P450 Enzymes: A Comparison between Different Isozymes

Cytochrome P450 enzymes in the human body play a pivotal role in both the biosynthesis and the degradation of the hormone melatonin. Melatonin plays a key role in circadian rhythms in the body, but its concentration is also linked to mood fluctuations as well as emotional well-being. In the present study, we present a computational analysis of the binding and activation of melatonin by various P450 isozymes that are known to yield different products and product distributions. In particular, the P450 isozymes 1A1, 1A2, and 1B1 generally react with melatonin to provide dominant aromatic hydroxylation at the C6-position, whereas the P450 2C19 isozyme mostly provides O-demethylation products. To gain insight into the origin of these product distributions of the P450 isozymes, we performed a comprehensive computational study of P450 2C19 isozymes and compared our work with previous studies on alternative isozymes. The work covers molecular mechanics, molecular dynamics and quantum mechanics approaches. Our work highlights major differences in the size and shape of the substrate binding pocket amongst the different P450 isozymes. Consequently, substrate binding and positioning in the active site varies substantially within the P450 isozymes. Thus, in P450 2C19, the substrate is oriented with its methoxy group pointing towards the heme, and therefore reacts favorably through hydrogen atom abstraction, leading to the production of O-demethylation products. On the other hand, the substrate-binding pockets in P450 1A1, 1A2, and 1B1 are tighter, direct the methoxy group away from the heme, and consequently activate an alternative site and lead to aromatic hydroxylation instead.

The binding of nitrogen-donor ligands to the ferric and ferrous forms of cytochrome P450 enzymes

The cytochrome P450 superfamily of heme-thiolate monooxygenase enzymes can catalyse various oxidation reactions. The addition of a substrate or an inhibitor ligand induces changes in the absorption spectrum of these enzymes and UV-visible (UV-vis) absorbance spectroscopy is the most common and readily available technique used to interrogate their heme and active site environment. Nitrogen-containing ligands can inhibit the catalytic cycle of heme enzymes by interacting with the heme. Here we evaluate the binding of imidazole and pyridine-based ligands to the ferric and ferrous forms of a selection of bacterial cytochrome P450 enzymes using UV-visible absorbance spectroscopy. The majority of these ligands interact with the heme as one would expect for type II nitrogen directly coordinated to a ferric heme-thiolate species. However, the spectroscopic changes observed in the ligand-bound ferrous forms indicated differences in the heme environment across these P450 enzyme/ligand combinations. Multiple species were observed in the UV-vis spectra of the ferrous ligand-bound P450s. None of the enzymes gave rise to the isolation of a single species with a Soret band at ∼442-447 nm, indicative of a 6-coordinate ferrous thiolate species with a nitrogen-donor ligand. A ferrous species with Soret band at ∼427 nm coupled with an α-band of increased intensity was observed with the imidazole ligands. With some enzyme-ligand combinations reduction resulted in breaking of the iron‑nitrogen bond yielding a 5-coordinate high-spin ferrous species. In other instances, the ferrous form was readily oxidised back to the ferric form on addition of the ligand.

Structural insights into 3Fe-4S ferredoxins diversity in M. tuberculosis highlighted by a first redox complex with P450

Ferredoxins are small iron-sulfur proteins and key players in essential metabolic pathways. Among all types, 3Fe-4S ferredoxins are less studied mostly due to anaerobic requirements. Their complexes with cytochrome P450 redox partners have not been structurally characterized. In the present work, we solved the structures of both 3Fe-4S ferredoxins from M. tuberculosis-Fdx alone and the fusion FdxE-CYP143. Our SPR analysis demonstrated a high-affinity binding of FdxE to CYP143. According to SAXS data, the same complex is present in solution. The structure reveals extended multipoint interactions and the shape/charge complementarity of redox partners. Furthermore, FdxE binding induced conformational changes in CYP143 as evident from the solved CYP143 structure alone. The comparison of FdxE-CYP143 and modeled Fdx-CYP51 complexes further revealed the specificity of ferredoxins. Our results illuminate the diversity of electron transfer complexes for the production of different secondary metabolites.

A comparison of the bacterial CYP51 cytochrome P450 enzymes from Mycobacterium marinum and Mycobacterium tuberculosis

Members of the CYP51 family of cytochrome P450 enzymes are classified as sterol demethylases involved in the metabolic formation of cholesterol and related derivatives. The CYP51 enzyme from Mycobacterium marinum was studied and compared to its counterpart from Mycobacterium tuberculosis to determine the degree of functional conservation between them. Spectroscopic analyses of substrate and inhibitor binding of the purified CYP51 enzymes from M. marinum and M. tuberculosis were performed. The catalytic oxidation of lanosterol and related steroids was investigated. M. marinum CYP51 was structurally characterized by X-ray crystallography. The CYP51 enzyme of M. marinum is sequentially closely related to CYP51B1 from M. tuberculosis. However, differences in the heme spin state of each enzyme were observed upon the addition of steroids and other ligands. Both enzymes displayed different binding properties to those reported for the CYP51-Fdx fusion protein from the bacterium Methylococcus capsulatus. The enzymes were able to oxidatively demethylate lanosterol to generate 14-demethylanosterol, but no products were detected for the related species dihydrolanosterol and eburicol. The crystal structure of CYP51 from M. marinum in the absence of added substrate but with a Bis-Tris molecule within the active site was resolved. The CYP51 enzyme of M. marinum displays differences in how steroids and other ligands bind compared to the M. tuberculosis enzyme. This was related to structural differences between the two enzymes. Overall, both of these CYP51 enzymes from mycobacterial species displayed significant differences to the CYP51 enzymes of eukaryotic species and the bacterial CYP51-Fdx enzyme of Me. capsulatus.

Contrasting Health Effects of Bacteroidetes and Firmicutes Lies in Their Genomes: Analysis of P450s, Ferredoxins, and Secondary Metabolite Clusters

Species belonging to the bacterial phyla Bacteroidetes and Firmicutes represent over 90% of the gastrointestinal microbiota. Changes in the ratio of these two bacterial groups were found to have contrasting health effects, including obesity and inflammatory diseases. Despite the availability of many bacterial genomes, comparative genomic studies on the gene pools of these two bacterial groups concerning cytochrome P450 monooxygenases (P450s), ferredoxins, and secondary metabolite biosynthetic gene clusters (smBGCs) are not reported. This study is aimed to address this research gap. The study revealed the presence of diverse sets of P450s, ferredoxins, and smBGCs in their genomes. Bacteroidetes species have the highest number of P450 families, ferredoxin cluster-types, and smBGCs compared to Firmicutes species. Only four P450 families, three ferredoxin cluster types, and five smBGCs are commonly shared between these two bacterial groups. Considering the above facts, we propose that the contrasting effects of these two bacterial groups on the host are partly due to the distinct nature of secondary metabolites produced by these organisms. Thus, the cause of the contrasting health effects of these two bacterial groups lies in their gene pools.

Mycolicibacterium cell factory for the production of steroid-based drug intermediates

Steroid-based drugs have been developed as the second largest medical category in pharmaceutics. The well-established route of steroid industry includes two steps: the conversion of natural products with a steroid framework to steroid-based drug intermediates and the synthesis of varied steroid-based drugs from steroid-based drug intermediates. The biosynthesis of steroid-based drug intermediates from phytosterols by Mycolicibacterium cell factories bypasses the potential undersupply of diosgenin in the traditional steroid chemical industry. Moreover, the biosynthesis route shows advantages on multiple steroid-based drug intermediate products, more ecofriendly processes, and consecutive reactions carried out in one operation step and in one pot. Androsta-4-ene-3,17-dione (AD), androsta-1,4-diene-3,17-dione (ADD) and 9-hydroxyandrostra-4-ene-3,17-dione (9-OH-AD) are the representative steroid-based drug intermediates synthesized by mycolicibacteria. Other steroid metabolites of mycolicibacteria, like 4-androstene-17β-ol-3-one (TS), 22-hydroxy-23,24-bisnorchol-4-ene-3-one (4-HBC), 22-hydroxy-23,24-bisnorchol-1,4-diene-3-one (1,4-HBC), 9,22-dihydroxy-23,24-bisnorchol-4-ene-3-one (9-OH-HBC), 3aα-H-4α-(3'-propionic acid)-7aβ-methylhexahydro-1,5-indanedione (HIP) and 3aα-H-4α-(3'-propionic acid)-5α-hydroxy-7aβ-methylhexahydro-1-indanone-δ-lactone (HIL), also show values as steroid-based drug intermediates. To improve the bio-production efficiency of the steroid-based drug intermediates, mycolicibacterial strains and biotransformation processes have been continuously studied in the past decades. Many mycolicibacteria that accumulate steroid drug intermediates have been isolated, and subsequently optimized by conventional mutagenesis and genetic engineering. Especially, with the clarification of the mycolicibacterial steroid metabolic pathway and the developments on gene editing technologies, rational design is becoming an important measure for the construction and optimization of engineered mycolicibacteria strains that produce steroid-based drug intermediates. Hence, by reviewing researches in the past two decades, this article updates the overall process of steroid metabolism in mycolicibacteria and provides comprehensive schemes for the rational construction of mycolicibacterial strains that accumulate steroid-based drug intermediates. In addition, the special strategies for the bioconversion of highly hydrophobic steroid in aqueous media are discussed as well.

Diversification of Ferredoxins across Living Organisms

Ferredoxins, iron-sulfur (Fe-S) cluster proteins, play a key role in oxidoreduction reactions. To date, evolutionary analysis of these proteins across the domains of life have been confined to observing the abundance of Fe-S cluster types (2Fe-2S, 3Fe-4S, 4Fe-4S, 7Fe-8S (3Fe-4s and 4Fe-4S) and 2[4Fe-4S]) and the diversity of ferredoxins within these cluster types was not studied. To address this research gap, here we propose a subtype classification and nomenclature for ferredoxins based on the characteristic spacing between the cysteine amino acids of the Fe-S binding motif as a subtype signature to assess the diversity of ferredoxins across the living organisms. To test this hypothesis, comparative analysis of ferredoxins between bacterial groups, Alphaproteobacteria and Firmicutes and ferredoxins collected from species of different domains of life that are reported in the literature has been carried out. Ferredoxins were found to be highly diverse within their types. Large numbers of alphaproteobacterial species ferredoxin subtypes were found in Firmicutes species and the same ferredoxin subtypes across the species of Bacteria, Archaea, and Eukarya, suggesting shared common ancestral origin of ferredoxins between Archaea and Bacteria and lateral gene transfer of ferredoxins from prokaryotes (Archaea/Bacteria) to eukaryotes. This study opened new vistas for further analysis of diversity of ferredoxins in living organisms.

A comparison of steroid and lipid binding cytochrome P450s from Mycobacterium marinum and Mycobacterium tuberculosis

The steroid lipid binding cytochrome P450 (CYP) enzymes of Mycobacterium tuberculosis are essential for organism survival through metabolism of cholesterol and its derivatives. The counterparts to these enzymes from Mycobacterium marinum were studied to determine the degree of functional conservation between them. Spectroscopic analyses of substrate and inhibitor binding for the four M. marinum enzymes CYP125A6, CYP125A7, CYP142A3 and CYP124A1 were performed and compared to the equivalent enzymes of M. tuberculosis. The sequence of CYP125A7 from M. marinum was more similar to CYP125A1 from M. tuberculosis than CYP125A6 but both showed differences in the resting heme spin state and in the binding modes and affinities of certain azole inhibitors. CYP125A7 did not show a significant Type II inhibitor-like shift with any of the azoles tested. CYP142A3 bound a similar range of steroids and inhibitors to CYP142A1. However, there were some differences in the extent of the Type I shifts to the high-spin form with steroids and a higher affinity for the azole inhibitors compared to CYP142A1. The two CYP124 enzymes had similar substrate binding properties. M. marinum CYP124 was characterised by X-ray crystallography and displayed strong conservation of active site residues, except near the region where the carboxylate terminus of the phytanic acid substrate would be bound. As these enzymes in M. tuberculosis have been identified as candidates for inhibition the data here demonstrates that alternative strategies for inhibitor design may be required to target CYP family members from distinct pathogenic Mycobacterium species or other bacteria.

Multiple drug binding modes in Mycobacterium tuberculosis CYP51B1

The Mycobacterium tuberculosis (Mtb) genome encodes 20 different cytochrome P450 enzymes (CYPs), many of which serve essential biosynthetic roles. CYP51B1, the Mtb version of eukaryotic sterol demethylase, remains a potential therapeutic target. The binding of three drug fragments containing nitrogen heterocycles to CYP51B1 is studied here by continuous wave (CW) and pulsed electron paramagnetic resonance (EPR) techniques to determine how each drug fragment binds to the heme active-site. All three drug fragments form a mixture of complexes, some of which retain the axial water ligand from the resting state. Hyperfine sublevel correlation spectroscopy (HYSCORE) and electron-nuclear double resonance spectroscopy (ENDOR) observe protons of the axial water and on the drug fragments that reveal drug binding modes. Binding in CYP51B1 is complicated by the presence of multiple binding modes that coexist in the same solution. These results aid our understanding of CYP-inhibitor interactions and will help guide future inhibitor design.

Structure-Activity Relationships of cyclo(l-Tyrosyl-l-tyrosine) Derivatives Binding to Mycobacterium tuberculosis CYP121: Iodinated Analogues Promote Shift to High-Spin Adduct

A series of analogues of cyclo(l-tyrosyl-l-tyrosine), the substrate of the Mycobacterium tuberculosis enzyme CYP121, have been synthesized and analyzed by UV-vis and electron paramagnetic resonance spectroscopy and by X-ray crystallography. The introduction of iodine substituents onto cyclo(l-tyrosyl-l-tyrosine) results in sub-μM binding affinity for the CYP121 enzyme and a complete shift to the high-spin state of the heme Fe^III. The introduction of halogens that are able to interact with heme groups is thus a feasible approach to the development of next-generation, tight binding inhibitors of the CYP121 enzyme, in the search for novel antitubercular compounds.

Cytochrome P450 Monooxygenase CYP139 Family Involved in the Synthesis of Secondary Metabolites in 824 Mycobacterial Species

Tuberculosis (TB) is one of the top infectious diseases causing numerous human deaths in the world. Despite enormous efforts, the physiology of the causative agent, Mycobacterium tuberculosis, is poorly understood. To contribute to better understanding the physiological capacity of these microbes, we have carried out extensive in silico analyses of the 1111 mycobacterial species genomes focusing on revealing the role of the orphan cytochrome P450 monooxygenase (CYP) CYP139 family. We have found that CYP139 members are present in 894 species belonging to three mycobacterial groups: M. tuberculosis complex (850-species), Mycobacterium avium complex (34-species), and non-tuberculosis mycobacteria (10-species), with all CYP139 members belonging to the subfamily “A”. CYP139 members have unique amino acid patterns at the CXG motif. Amino acid conservation analysis placed this family in the 8th among CYP families belonging to different biological domains and kingdoms. Biosynthetic gene cluster analyses have revealed that 92% of CYP139As might be associated with producing different secondary metabolites. Such enhanced secondary metabolic potentials with the involvement of CYP139A members might have provided mycobacterial species with advantageous traits in diverse niches competing with other microbial or viral agents, and might help these microbes infect hosts by interfering with the hosts’ metabolism and immune system.

The characterisation of two members of the cytochrome P450 CYP150 family: CYP150A5 and CYP150A6 from Mycobacterium marinum

BACKGROUND

Actinobacteria, including the Mycobacteria, have a large component of cytochrome P450 family monooxygenases. This includes Mycobacterium tuberculosis, M. ulcerans and M. marinum, and M. vanbaalenii. These enzymes can abstract CH bonds and have important roles in natural product biosynthesis.

METHODS

Two members of the bacterial CYP150 family, CYP150A5 and CYP150A6 from M. marinum, were produced, purified and characterised. The potential substrate ranges of both enzymes were analysed and the monooxygenase activity of CYP150A5 was reconstituted using a physiological electron transfer partner system. CYP150A6 was structurally characterised by X-ray crystallography.

RESULTS

CYP150A5 was shown to bind various norisoprenoids and terpenoids. It could regioselectively hydroxylate β-ionol. The X-ray crystal structure of substrate-free CYP150A6 was solved to 1.5 Å. This displayed an open conformation with short F and G helices, an unresolved F-G loop region and exposed active site pocket. The active site residues could be identified and important variations were found among the CYP150A enzymes. Haem-binding azole inhibitors were identified for both enzymes.

CONCLUSIONS

The structure of CYP150A6 will facilitate the identification of physiological substrates and the design of better inhibitors for members of this P450 family. Based on the observed differences in substrate binding preference and sequence variations among the active site residues, their roles are predicted to be different.

GENERAL SIGNIFICANCE

Multiple CYP150 family members were found in many bacteria and are prevalent in the Mycobacteria including several human pathogens. Inhibition and structural data are reported here for these enzymes for the first time.

Function, essentiality, and expression of cytochrome P450 enzymes and their cognate redox partners in Mycobacterium tuberculosis: are they drug targets?

This review covers the current knowledge of the cytochrome P450 enzymes (CYPs) of the human pathogen Mycobacterium tuberculosis (Mtb) and their endogenous redox partners, focusing on their biological function, expression, regulation, involvement in antibiotic resistance, and suitability for exploitation as antitubercular targets. The Mtb genome encodes twenty  CYPs and nine associated redox partners required for CYP catalytic activity. Transposon insertion mutagenesis studies have established the (conditional) essentiality of several of these enzymes for in vitro growth and host infection. Biochemical characterization of a handful of Mtb CYPs has revealed that they have specific physiological functions in bacterial virulence and persistence in the host. Analysis of the transcriptional response of Mtb CYPs and redox partners to external insults and to first-line antibiotics used to treat tuberculosis showed a diverse expression landscape, suggesting for some enzymes a potential role in drug resistance. Combining the knowledge about the physiological roles and expression profiles indicates that, at least five Mtb CYPs, CYP121A1, CYP125A1, CYP139A1, CYP142A1, and CYP143A1, as well as two ferredoxins, FdxA and FdxC, can be considered promising novel therapeutic targets.

Electron transfer ferredoxins with unusual cluster binding motifs support secondary metabolism in many bacteria

The proteins responsible for controlling electron transfer in bacterial secondary metabolism are not always known or characterised. Here we demonstrate that many bacteria contain a set of unfamiliar ferredoxin encoding genes which are associated with those of cytochrome P450 (CYP) monooxygenases and as such are involved in anabolic and catabolic metabolism. The model organism Mycobacterium marinum M contains eleven of these genes which encode [3Fe-4S] or [4Fe-4S] single cluster containing ferredoxins but which have unusual iron-sulfur cluster binding motif sequences, CXX?XXC(X) n CP, where '?' indicates a variable amino acid residue. Rather than a cysteine residue, which is highly conserved in [4Fe-4S] clusters, or alanine or glycine residues, which are common in [3Fe-4S] ferredoxins, these genes encode at this position histidine, asparagine, tyrosine, serine, threonine or phenylalanine. We have purified, characterised and reconstituted the activity of several of these CYP/electron transfer partner systems and show that all those examined contain a [3Fe-4S] cluster. Furthermore, the ferredoxin used and the identity of the variable motif residue in these proteins affects the functionality of the monooxygenase system and has a significant influence on the redox properties of the ferredoxins. Similar ferredoxin encoding genes were identified across Mycobacterium species, including in the pathogenic M. tuberculosis and M. ulcerans, as well as in a wide range of other bacteria such as Rhodococcus and Streptomyces. In the majority of instances these are associated with CYP genes. These ferredoxin systems are important in controlling electron transfer across bacterial secondary metabolite production processes which include antibiotic and pigment formation among others.

Structural and catalytic properties of the peroxygenase P450 enzyme CYP152K6 from Bacillus methanolicus

The CYP152 family of cytochrome P450 enzymes (P450s or CYPs) are bacterial peroxygenases that use hydrogen peroxide to drive hydroxylation and decarboxylation of fatty acid substrates. We have expressed and purified a novel CYP152 family member - CYP152K6 from the methylotroph Bacillus methanolicus MGA3. CYP152K6 was characterized using spectroscopic, analytical and structural methods. CYP152K6, like its peroxygenase counterpart P450SPα (CYP152B1) from Sphingomonas paucimobilis, does not undergo significant fatty acid-induced perturbation to the heme spectrum, with the exception of a minor Soret shift observed on binding dodecanoic acid. However, CYP152K6 purified from an E. coli expression system was crystallized and its structure was determined to 1.3 Å with tetradecanoic acid bound. No lipids were present in conditions used for crystallogenesis, and thus CYP152K6 must form a complex by incorporating the fatty acid from E. coli cells. Turnover studies with dodecanoic acid revealed several products, with 2-hydroxydodecanoic acid as the major product and much smaller quantities of 3-hydroxydodecanoic acid. Secondary turnover products were undec-1-en-1-ol, 2-hydroxydodec-2-enoic acid and 2,3-dihydroxydodecanoic acid. This is the first report of a 2,3-hydroxylated fatty acid product made by a peroxygenase P450, with the dihydroxylated product formed by CYP152K6-catalyzed 3-hydroxylation of 2-hydroxydodecanoic acid, but not by 2-hydroxylation of 3-hydroxydodecanoic acid.

Linking cytochrome P450 enzymes from Mycobacterium tuberculosis to their cognate ferredoxin partners

Mycobacterium tuberculosis (Mtb) codes for 20 cytochrome P450 enzymes (CYPs), considered potential drug-targets due to their essential roles in bacterial viability and host infection. Catalytic activity of mycobacterial CYPs is dependent on electron transfer from a NAD (P)H-ferredoxin-reductase (FNR) and a ferredoxin (Fd). Two FNRs (FdrA and FprA) and five ferredoxins (Fdx, FdxA, FdxC, FdxD, and Rv1786) have been found in the Mtb genome. However, as of yet, the cognate redox partnerships have not been fully established. This is confounded by the fact that heterologous redox partners are routinely used to reconstitute Mtb CYP metabolism. To this end, this study aimed to biochemically characterize and identify cognate redox partnerships for Mtb CYPs. Interestingly, all combinations of FNRs and ferredoxins were active in the reduction of oxidized cytochrome c, but steady-state kinetic assays revealed FdxD as the most efficient redox partner for FdrA, whereas Fdx coupled preferably with FprA. CYP121A1, CYP124A1, CYP125A1, and CYP142A1 metabolism with the cognate redox partners was reconstituted in vitro showing an unanticipated selectivity in the requirement for electron transfer partnership, which did not necessarily correlate with proximity in the genome. This is the first description of microbial P450 metabolism in which multiple ferredoxins are functionally linked to multiple CYPs.

Probing the specificity of CYP112 in bacterial gibberellin biosynthesis

Biosynthesis of the gibberellin A (GA) plant hormones evolved independently in plant-associated fungi and bacteria. While the relevant enzymes have distinct evolutionary origins, the pathways proceed via highly similar reactions. One particularly complex transformation involves combined demethylation and γ-lactone ring formation, catalyzed in bacteria by the cytochrome P450 CYP112 in three individual steps, which involves large structural changes in the transition from substrate to product, with further divergence in the recently demonstrated use of two separate mechanistic routes. Here, the substrate specificity of the isozyme from Erwinia tracheiphila, EtCYP112, was probed via UV-Vis spectral binding studies and activity assays with alternate substrates from the GA biosynthetic pathway. EtCYP112 tightly binds its native substrate GA₁₂ and reaction intermediates GA₁₅ and GA₂₄, as well as the methylated derivatives of GA₁₂ and GA₁₅ It, however, only poorly binds methylated GA₂₄, its GA₉ final product and the C-20 carboxylate side product GA₂₅ These distinct affinities are consistent with the known reactivity of EtCYP112. However, while it binds to the immediately preceding pathway metabolite GA₁₂-aldehyde and even earlier oxygenated ent-kaurene precursors, EtCYP112 only reacts with GA₁₂-aldehyde and not the earlier ent-kaurene-derived metabolites. Even with GA₁₂-aldehyde conversion is limited to the first two steps, and the full combined demethylation and γ-lactone ring-forming transformation is not catalyzed. Thus, CYP112 has evolved specificity at the catalytic rather than substrate-binding level to enable its role in GA biosynthesis.

Structure and function of the cytochrome P450 peroxygenase enzymes

The cytochromes P450 (P450s or CYPs) constitute a large heme enzyme superfamily, members of which catalyze the oxidative transformation of a wide range of organic substrates, and whose functions are crucial to xenobiotic metabolism and steroid transformation in humans and other organisms. The P450 peroxygenases are a subgroup of the P450s that have evolved in microbes to catalyze the oxidative metabolism of fatty acids, using hydrogen peroxide as an oxidant rather than NAD(P)H-driven redox partner systems typical of the vast majority of other characterized P450 enzymes. Early members of the peroxygenase (CYP152) family were shown to catalyze hydroxylation at the α and β carbons of medium-to-long-chain fatty acids. However, more recent studies on other CYP152 family P450s revealed the ability to oxidatively decarboxylate fatty acids, generating terminal alkenes with potential applications as drop-in biofuels. Other research has revealed their capacity to decarboxylate and to desaturate hydroxylated fatty acids to form novel products. Structural data have revealed a common active site motif for the binding of the substrate carboxylate group in the peroxygenases, and mechanistic and transient kinetic analyses have demonstrated the formation of reactive iron-oxo species (compounds I and II) that are ultimately responsible for hydroxylation and decarboxylation of fatty acids, respectively. This short review will focus on the biochemical properties of the P450 peroxygenases and on their biotechnological applications with respect to production of volatile alkenes as biofuels, as well as other fine chemicals.

Potential drug targets in the Mycobacterium tuberculosis cytochrome P450 system

The Mycobacterium tuberculosis genome encodes twenty cytochrome P450 enzymes, most or all of which appear to have specific physiological functions rather than being devoted to the removal of xenobiotics. However, in many cases their specific functions remain obscure. Considerable spectroscopic, biophysical, crystallographic, and catalytic information is available on nine of these cytochrome P450 enzymes, although gaps exist in our knowledge of even these enzymes. The available evidence indicates that at least three of the better-characterized enzymes are promising targets for antituberculosis drug discovery. This review summarizes the information on the nine relatively well-characterized cytochrome P450 enzymes, with a particular emphasis on CYP121, CYP125, and CYP142 from Mycobacterium tuberculosis and Mycobacterium smegmatis.

Spectroscopic studies of the cytochrome P450 reaction mechanisms

The cytochrome P450 monooxygenases (P450s) are thiolate heme proteins that can, often under physiological conditions, catalyze many distinct oxidative transformations on a wide variety of molecules, including relatively simple alkanes or fatty acids, as well as more complex compounds such as steroids and exogenous pollutants. They perform such impressive chemistry utilizing a sophisticated catalytic cycle that involves a series of consecutive chemical transformations of heme prosthetic group. Each of these steps provides a unique spectral signature that reflects changes in oxidation or spin states, deformation of the porphyrin ring or alteration of dioxygen moieties. For a long time, the focus of cytochrome P450 research was to understand the underlying reaction mechanism of each enzymatic step, with the biggest challenge being identification and characterization of the powerful oxidizing intermediates. Spectroscopic methods, such as electronic absorption (UV-Vis), electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), electron nuclear double resonance (ENDOR), Mössbauer, X-ray absorption (XAS), and resonance Raman (rR), have been useful tools in providing multifaceted and detailed mechanistic insights into the biophysics and biochemistry of these fascinating enzymes. The combination of spectroscopic techniques with novel approaches, such as cryoreduction and Nanodisc technology, allowed for generation, trapping and characterizing long sought transient intermediates, a task that has been difficult to achieve using other methods. Results obtained from the UV-Vis, rR and EPR spectroscopies are the main focus of this review, while the remaining spectroscopic techniques are briefly summarized. This article is part of a Special Issue entitled: Cytochrome P450 biodiversity and biotechnology, edited by Erika Plettner, Gianfranco Gilardi, Luet Wong, Vlada Urlacher, Jared Goldstone.

Engineering a self-sufficient Mycobacterium tuberculosis CYP130 by gene fusion with the reductase-domain of CYP102A1 from Bacillus megaterium

CYP130 belongs to the subset of cytochrome P450s from Mycobacterium tuberculosis (Mtb) that have been structurally characterized. Despite several efforts for its functional characterization, CYP130 is still considered an orphan enzyme for which no endogenous or exogenous substrate has been identified. In addition, functional redox-partners for CYP130 have not been clearly established yet, hampering the elucidation of its physiological role. In the present study, a catalytically active fusion protein involving CYP130 and the NADPH reductase-domain of CYP102A1 from Bacillus megaterium was created. By screening a panel of known substrates of human P450s, dextromethorphan N-demethylation was identified as a reaction catalyzed by CYP130. The fusion enzyme showed higher catalytic activity, when compared to CYP130 reconstituted with a selection of non-native redox-partners. Molecular dynamics simulation studies based on the crystal structure of CYP130 revealed two primary docking poses of dextromethorphan within the active site consistent with the experimentally observed N-demethylation reaction during the entire molecular dynamics simulation. The dextromethorphan N-demethylation reaction was strongly inhibited by azole-drugs and maybe applied to identify mechanism-based inhibitors of CYP130. Furthermore, the present active CYP130-fusion protein may facilitate the identification of endogenous substrates from Mtb.

Mycobacterium tuberculosis Proteome Response to Antituberculosis Compounds Reveals Metabolic "Escape" Pathways That Prolong Bacterial Survival

Tuberculosis (TB) continues to be one of the most common bacterial infectious diseases and is the leading cause of death in many parts of the world. A major limitation of TB therapy is slow killing of the infecting organism, increasing the risk for the development of a tolerance phenotype and drug resistance. Studies indicate that Mycobacterium tuberculosis takes several days to be killed upon treatment with lethal concentrations of antibiotics both in vitro and in vivo To investigate how metabolic remodeling can enable transient bacterial survival during exposure to bactericidal concentrations of compounds, M. tuberculosis strain H37Rv was exposed to twice the MIC of isoniazid, rifampin, moxifloxacin, mefloquine, or bedaquiline for 24 h, 48 h, 4 days, and 6 days, and the bacterial proteomic response was analyzed using quantitative shotgun mass spectrometry. Numerous sets of de novo bacterial proteins were identified over the 6-day treatment. Network analysis and comparisons between the drug treatment groups revealed several shared sets of predominant proteins and enzymes simultaneously belonging to a number of diverse pathways. Overexpression of some of these proteins in the nonpathogenic Mycobacterium smegmatis extended bacterial survival upon exposure to bactericidal concentrations of antimicrobials, and inactivation of some proteins in M. tuberculosis prevented the pathogen from escaping the fast killing in vitro and in macrophages, as well. Our biology-driven approach identified promising bacterial metabolic pathways and enzymes that might be targeted by novel drugs to reduce the length of tuberculosis therapy.

Cholesterol metabolism: a potential therapeutic target in Mycobacteria

Tuberculosis (TB), although a curable disease, is still one of the most difficult infections to treat. Mycobacterium tuberculosis infects 10 million people worldwide and kills 1.5 million people each year. Reactivation of a latent infection is the major cause of TB. Cholesterol is a critical carbon source during latent infection. Catabolism of cholesterol contributes to the pool of propionyl-CoA, a precursor that is incorporated into lipid virulence factors. The M. tuberculosis genome contains a large regulon of cholesterol catabolic genes suggesting that the microorganism can utilize host sterol for infection and persistence. The protein products of these genes present ideal targets for rational drug discovery programmes. This review summarizes the development of enzyme inhibitors targeting the cholesterol pathway in M. tuberculosis. This knowledge is essential for the discovery of novel agents to treat M. tuberculosis infection.

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This article is part of a themed section on Drug Metabolism and Antibiotic Resistance in Micro-organisms. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.14/issuetoc.

Catalytic Determinants of Alkene Production by the Cytochrome P450 Peroxygenase OleTJE

The Jeotgalicoccus sp. peroxygenase cytochrome P450 OleT_JE (CYP152L1) is a hydrogen peroxide-driven oxidase that catalyzes oxidative decarboxylation of fatty acids, producing terminal alkenes with applications as fine chemicals and biofuels. Understanding mechanisms that favor decarboxylation over fatty acid hydroxylation in OleT_JE could enable protein engineering to improve catalysis or to introduce decarboxylation activity into P450s with different substrate preferences. In this manuscript, we have focused on OleT_JE active site residues Phe⁷⁹, His⁸⁵, and Arg²⁴⁵ to interrogate their roles in substrate binding and catalytic activity. His⁸⁵ is a potential proton donor to reactive iron-oxo species during substrate decarboxylation. The H85Q mutant substitutes a glutamine found in several peroxygenases that favor fatty acid hydroxylation. H85Q OleT_JE still favors alkene production, suggesting alternative protonation mechanisms. However, the mutant undergoes only minor substrate binding-induced heme iron spin state shift toward high spin by comparison with WT OleT_JE, indicating the key role of His⁸⁵ in this process. Phe⁷⁹ interacts with His⁸⁵, and Phe⁷⁹ mutants showed diminished affinity for shorter chain (C10–C16) fatty acids and weak substrate-induced high spin conversion. F79A OleT_JE is least affected in substrate oxidation, whereas the F79W/Y mutants exhibit lower stability and cysteine thiolate protonation on reduction. Finally, Arg²⁴⁵ is crucial for binding the substrate carboxylate, and R245E/L mutations severely compromise activity and heme content, although alkene products are formed from some substrates, including stearic acid (C18:0). The results identify crucial roles for the active site amino acid trio in determining OleT_JE catalytic efficiency in alkene production and in regulating protein stability, heme iron coordination, and spin state.

Structural Characterization and Ligand/Inhibitor Identification Provide Functional Insights into the Mycobacterium tuberculosis Cytochrome P450 CYP126A1

The Mycobacterium tuberculosis H37Rv genome encodes 20 cytochromes P450, including P450s crucial to infection and bacterial viability. Many M. tuberculosis P450s remain uncharacterized, suggesting that their further analysis may provide new insights into M. tuberculosis metabolic processes and new targets for drug discovery. CYP126A1 is representative of a P450 family widely distributed in mycobacteria and other bacteria. Here we explore the biochemical and structural properties of CYP126A1, including its interactions with new chemical ligands. A survey of azole antifungal drugs showed that CYP126A1 is inhibited strongly by azoles containing an imidazole ring but not by those tested containing a triazole ring. To further explore the molecular preferences of CYP126A1 and search for probes of enzyme function, we conducted a high throughput screen. Compounds containing three or more ring structures dominated the screening hits, including nitroaromatic compounds that induce substrate-like shifts in the heme spectrum of CYP126A1. Spectroelectrochemical measurements revealed a 155-mV increase in heme iron potential when bound to one of the newly identified nitroaromatic drugs. CYP126A1 dimers were observed in crystal structures of ligand-free CYP126A1 and for CYP126A1 bound to compounds discovered in the screen. However, ketoconazole binds in an orientation that disrupts the BC-loop regions at the P450 dimer interface and results in a CYP126A1 monomeric crystal form. Structural data also reveal that nitroaromatic ligands "moonlight" as substrates by displacing the CYP126A1 distal water but inhibit enzyme activity. The relatively polar active site of CYP126A1 distinguishes it from its most closely related sterol-binding P450s in M. tuberculosis, suggesting that further investigations will reveal its diverse substrate selectivity.

Fragment-Based Approaches to the Development of Mycobacterium tuberculosis CYP121 Inhibitors

The essential enzyme CYP121 is a target for drug development against antibiotic resistant strains of Mycobacterium tuberculosis. A triazol-1-yl phenol fragment 1 was identified to bind to CYP121 using a cascade of biophysical assays. Synthetic merging and optimization of 1 produced a 100-fold improvement in binding affinity, yielding lead compound 2 (KD = 15 μM). Deconstruction of 2 into its component retrofragments allowed the group efficiency of structural motifs to be assessed, the identification of more LE scaffolds for optimization and highlighted binding affinity hotspots. Structure-guided addition of a metal-binding pharmacophore onto LE retrofragment scaffolds produced low nanomolar (KD = 15 nM) CYP121 ligands. Elaboration of these compounds to target binding hotspots in the distal active site afforded compounds with excellent selectivity against human drug-metabolizing P450s. Analysis of the factors governing ligand potency and selectivity using X-ray crystallography, UV-vis spectroscopy, and native mass spectrometry provides insight for subsequent drug development.

The structure of a novel electron-transfer ferredoxin from Rhodopseudomonas palustris HaA2 which contains a histidine residue in its iron-sulfur cluster-binding motif

Rhodopseudomonas palustris HaA2 contains a gene, RPB3630, encoding a ferredoxin, HaPuxC, with an atypical CXXHXXC(X)nCP iron-sulfur cluster-binding motif. The ferredoxin gene is associated with a cytochrome P450 (CYP) monooxygenase-encoding gene, CYP194A3, an arrangement which is conserved in several strains of bacteria. Similar ferredoxin genes are found in other bacteria, such as Mycobacterium tuberculosis, where they are also associated with CYP genes. The crystal structure of HaPuxC has been solved at 2.3 Å resolution. The overall fold of this [3Fe-4S] cluster-containing ferredoxin is similar to other [3Fe-4S] and [4Fe-4S] species, with the loop around the iron-sulfur cluster more closely resembling those of [3Fe-4S] ferredoxins. The side chain of His17 from the cluster-binding motif in HaPuxC points away from the vacant site of the cluster and interacts with Glu61 and one of the sulfide ions of the cluster. This is the first cytochrome P450 electron-transfer partner of this type to be structurally characterized and will provide a better understanding of the electron-transfer processes between these ferredoxins and their CYP enzymes.

Spin equilibrium and O₂-binding kinetics of Mycobacterium tuberculosis CYP51 with mutations in the histidine-threonine dyad

The acidic residues of the "acid-alcohol pair" in CYP51 enzymes are uniformly replaced with histidine. Herein, we adopt the Mycobacterium tuberculosis (mt) enzyme as a model system to investigate these residues' roles in finely tuning the heme conformation, iron spin state, and formation and decay of the oxyferrous enzyme. Properties of the mtCYP51 and the T260A, T260V, and H259A mutants were interrogated using UV-Vis and resonance Raman spectroscopies. Evidence supports that these mutations induce comprehensive changes in the heme environment. The heme iron spin states are differentially sensitive to the binding of the substrate, dihydrolanosterol (DHL). DHL and clotrimazole perturb the local environments of the heme vinyl and propionate substituents. Molecular dynamics (MD) simulations of the DHL-enzyme complexes support that the observed perturbations are attributable to changes in the DHL binding mode. Furthermore, the rates of the oxyferrous formation were measured using stopped-flow methods. These studies demonstrate that both HT mutations and DHL modulate the rates of oxyferrous formation. Paradoxically, the binding rate to the H259A mutant-DHL complex was approximately four-fold that of mtCYP51, a phenomenon that is predicted to result from the creation of an additional diffusion channel from loss of the H259-E173 ion pair in the mutant. Oxyferrous enzyme auto-oxidation rates were relatively constant, with the exception of the T260V-DHL complex. MD simulations lead us to speculate that this behavior may be attributed to the distortion of the heme macrocycle by the substrate.

Strength of axial water ligation in substrate-free cytochrome P450s is isoform dependent

The heme-containing cytochrome P450s exhibit isoform-dependent ferric spin equilibria in the resting state and differential substrate-dependent spin equilibria. The basis for these differences is not well understood. Here, magnetic circular dichroism (MCD) reveals significant differences in the resting low spin ligand field of CYPs 3A4, 2E1, 2C9, 125A1, and 51B1, which indicates differences in the strength of axial water ligation to the heme. The near-infrared bands that specifically correspond to charge-transfer porphyrin-to-metal transitions span a range of energies of nearly 2 kcal/mol. In addition, the experimentally determined MCD bands are not entirely in agreement with the expected MCD energies calculated from electron paramagnetic resonance parameters, thus emphasizing the need for the experimental data. MCD marker bands of the high spin heme between 500 and 680 nm were also measured and suggest only a narrow range of energies for this ensemble of high spin Cys(S(-)) → Fe(3+) transitions among these isoforms. The differences in axial ligand energies between CYP isoforms of the low spin states likely contribute to the energetics of substrate-dependent spin state perturbation. However, these ligand field energies do not correlate with the fraction of high spin vs low spin in the resting state enzyme, suggestive of differences in water access to the heme or isoform-dependent differences in the substrate-free high spin states as well.

Structure and biochemical properties of the alkene producing cytochrome P450 OleTJE (CYP152L1) from the Jeotgalicoccus sp. 8456 bacterium

The production of hydrocarbons in nature has been documented for only a limited set of organisms, with many of the molecular components underpinning these processes only recently identified. There is an obvious scope for application of these catalysts and engineered variants thereof in the future production of biofuels. Here we present biochemical characterization and crystal structures of a cytochrome P450 fatty acid peroxygenase: the terminal alkene forming OleTJE (CYP152L1) from Jeotgalicoccus sp. 8456. OleTJE is stabilized at high ionic strength, but aggregation and precipitation of OleTJE in low salt buffer can be turned to advantage for purification, because resolubilized OleTJE is fully active and extensively dissociated from lipids. OleTJE binds avidly to a range of long chain fatty acids, and structures of both ligand-free and arachidic acid-bound OleTJE reveal that the P450 active site is preformed for fatty acid binding. OleTJE heme iron has an unusually positive redox potential (-103 mV versus normal hydrogen electrode), which is not significantly affected by substrate binding, despite extensive conversion of the heme iron to a high spin ferric state. Terminal alkenes are produced from a range of saturated fatty acids (C12-C20), and stopped-flow spectroscopy indicates a rapid reaction between peroxide and fatty acid-bound OleTJE (167 s(-1) at 200 μm H2O2). Surprisingly, the active site is highly similar in structure to the related P450BSβ, which catalyzes hydroxylation of fatty acids as opposed to decarboxylation. Our data provide new insights into structural and mechanistic properties of a robust P450 with potential industrial applications.

Investigations of heme ligation and ligand switching in cytochromes p450 and p420

It is generally accepted that the inactive P420 form of cytochrome P450 (CYP) involves the protonation of the native cysteine thiolate to form a neutral thiol heme ligand. On the other hand, it has also been suggested that recruitment of a histidine to replace the native cysteine thiolate ligand might underlie the P450 → P420 transition. Here, we discuss resonance Raman investigations of the H93G myoglobin (Mb) mutant in the presence of tetrahydrothiophene (THT) or cyclopentathiol (CPSH), and on pressure-induced cytochrome P420cam (CYP101), that show a histidine becomes the heme ligand upon CO binding. The Raman mode near 220 cm⁻¹, normally associated with the Fe-histidine vibration in heme proteins, is not observed in either reduced P420cam or the reduced H93G Mb samples, indicating that histidine is not the ligand in the reduced state. The absence of a mode near 220 cm⁻¹ is also inconsistent with a generalization of the suggestion that the 221 cm⁻¹ Raman mode, observed in the P420-CO photoproduct of inducible nitric oxide synthase (iNOS), arises from a thiol-bound ferrous heme. This leads us to assign the 218 cm⁻¹ mode observed in the 10 ns P420cam-CO photoproduct Raman spectrum to a Fe-histidine vibration, in analogy to many other histidine-bound heme systems. Additionally, the inverse correlation plots of the νFe-His and νCO frequencies for the CO adducts of P420cam and the H93G analogs provide supporting evidence that histidine is the heme ligand in the P420-CO-bound state. We conclude that, when CO binds to the ferrous P420 state, a histidine ligand is recruited as the heme ligand. The common existence of an HXC-Fe motif in many CYP systems allows the C → H ligand switch to occur with only minor conformational changes. One suggested conformation of P420-CO involves the addition of another turn in the proximal L helix so that, when the protonated Cys ligand is dissociated from the heme, it can become part of the helix, and the heme is ligated by the His residue from the adjoining loop region. In other systems, such as iNOS and CYP3A4 (where the HXC-Fe motif is not found), a somewhat larger conformational change would be necessary to recuit a nearby histidine.

Unusual spectroscopic and ligand binding properties of the cytochrome P450-flavodoxin fusion enzyme XplA

The Rhodococcus rhodochrous strain 11Y XplA enzyme is an unusual cytochrome P450-flavodoxin fusion enzyme that catalyzes reductive denitration of the explosive hexahydro-1,3,5-trinitro-1,3,5-triazene (RDX). We show by light scattering that XplA is a monomeric enzyme. XplA has high affinity for imidazole (K(d) = 1.6 μM), explaining previous reports of a red-shifted XplA Soret band in pure enzyme. The true Soret maximum of XplA is at 417 nm. Similarly, unusually weak XplA flavodoxin FMN binding (K(d) = 1.09 μM) necessitates its purification in the presence of the cofactor to produce hallmark flavin contributions absent in previously reported spectra. Structural and ligand-binding data reveal a constricted active site able to accommodate RDX and small inhibitory ligands (e.g. 4-phenylimidazole and morpholine) while discriminating against larger azole drugs. The crystal structure also identifies a high affinity imidazole binding site, consistent with its low K(d), and shows active site penetration by PEG, perhaps indicative of an evolutionary lipid-metabolizing function for XplA. EPR studies indicate heterogeneity in binding mode for RDX and other ligands. The substrate analog trinitrobenzene does not induce a substrate-like type I optical shift but creates a unique low spin EPR spectrum due to influence on structure around the distal water heme ligand. The substrate-free heme iron potential (-268 mV versus NHE) is positive for a low spin P450, and the elevated potential of the FMN semiquinone/hydroquinone couple (-172 mV) is also an adaptation that may reflect (along with the absence of a key Thr/Ser residue conserved in oxygen-activating P450s) the evolution of XplA as a specialized RDX reductase catalyst.

Mycobacterium tuberculosis cytochrome P450 enzymes: a cohort of novel TB drug targets

TB (tuberculosis) disease remains responsible for the death of over 1.5 million people each year. The alarming emergence of drug-resistant TB has sparked a critical need for new front-line TB drugs with a novel mode of action. In the present paper, we review recent genomic and biochemical evidence implicating Mycobacterium tuberculosis CYP (cytochrome P450) enzymes as exciting potential targets for new classes of anti-tuberculars. We also discuss HTS (high-throughput screening) and fragment-based drug-discovery campaigns that are being used to probe their potential druggability.

Characterization of Cupriavidus metallidurans CYP116B1--a thiocarbamate herbicide oxygenating P450-phthalate dioxygenase reductase fusion protein

The novel cytochrome P450/redox partner fusion enzyme CYP116B1 from Cupriavidus metallidurans was expressed in and purified from Escherichia coli. Isolated CYP116B1 exhibited a characteristic Fe(II)CO complex with Soret maximum at 449 nm. EPR and resonance Raman analyses indicated low-spin, cysteinate-coordinated ferric haem iron at both 10 K and ambient temperature, respectively, for oxidized CYP116B1. The EPR of reduced CYP116B1 demonstrated stoichiometric binding of a 2Fe-2S cluster in the reductase domain. FMN binding in the reductase domain was confirmed by flavin fluorescence studies. Steady-state reduction of cytochrome c and ferricyanide were supported by both NADPH/NADH, with NADPH used more efficiently (K(m[NADPH]) = 0.9 ± 0.5 μM and K(m[NADH]) = 399.1 ± 52.1 μM). Stopped-flow studies of NAD(P)H-dependent electron transfer to the reductase confirmed the preference for NADPH. The reduction potential of the P450 haem iron was -301 ± 7 mV, with retention of haem thiolate ligation in the ferrous enzyme. Redox potentials for the 2Fe-2S and FMN cofactors were more positive than that of the haem iron. Multi-angle laser light scattering demonstrated CYP116B1 to be monomeric. Type I (substrate-like) binding of selected unsaturated fatty acids (myristoleic, palmitoleic and arachidonic acids) was shown, but these substrates were not oxidized by CYP116B1. However, CYP116B1 catalysed hydroxylation (on propyl chains) of the herbicides S-ethyl dipropylthiocarbamate (EPTC) and S-propyl dipropylthiocarbamate (vernolate), and the subsequent N-dealkylation of vernolate. CYP116B1 thus has similar thiocarbamate-oxidizing catalytic properties to Rhodoccocus erythropolis CYP116A1, a P450 involved in the oxidative degradation of EPTC.

Structure and function of CYP108D1 from Novosphingobium aromaticivorans DSM12444: an aromatic hydrocarbon-binding P450 enzyme

CYP108D1 from Novosphingobium aromaticivorans DSM12444 binds a range of aromatic hydrocarbons such as phenanthrene, biphenyl and phenylcyclohexane. Its structure, which is reported here at 2.2 Å resolution, is closely related to that of CYP108A1 (P450terp), an α-terpineol-oxidizing enzyme. The compositions and structures of the active sites of these two enzymes are very similar; the most significant changes are the replacement of Glu77 and Thr103 in CYP108A1 by Thr79 and Val105 in CYP108D1. Other residue differences lead to a larger and more hydrophobic access channel in CYP108D1. These structural features are likely to account for the weaker α-terpineol binding by CYP108D1 and, when combined with the presence of three hydrophobic phenylalanine residues in the active site, promote the binding of aromatic hydrocarbons. The haem-proximal surface of CYP108D1 shows a different charge distribution and topology to those of CYP101D1, CYP101A1 and CYP108A1, including a pronounced kink in the proximal loop of CYP108D1, which may result in poor complementarity with the [2Fe-2S] ferredoxins Arx, putidaredoxin and terpredoxin that are the respective redox partners of these three P450 enzymes. The unexpectedly low reduction potential of phenylcyclohexane-bound CYP108D1 (-401 mV) may also contribute to the low activity observed with these ferredoxins. CYP108D1 appears to function as an aromatic hydrocarbon hydroxylase that requires a different electron-transfer cofactor protein.

Carbon monoxide poisoning is prevented by the energy costs of conformational changes in gas-binding haemproteins

Carbon monoxide (CO) is a product of haem metabolism and organisms must evolve strategies to prevent endogenous CO poisoning of haemoproteins. We show that energy costs associated with conformational changes play a key role in preventing irreversible CO binding. AxCYTcp is a member of a family of haem proteins that form stable 5c-NO and 6c-CO complexes but do not form O(2) complexes. Structure of the AxCYTcp-CO complex at 1.25 Å resolution shows that CO binds in two conformations moderated by the extent of displacement of the distal residue Leu16 toward the haem 7-propionate. The presence of two CO conformations is confirmed by cryogenic resonance Raman data. The preferred linear Fe-C-O arrangement (170 ± 8°) is accompanied by a flip of the propionate from the distal to proximal face of the haem. In the second conformation, the Fe-C-O unit is bent (158 ± 8°) with no flip of propionate. The energetic cost of the CO-induced Leu-propionate movements is reflected in a 600 mV (57.9 kJ mol(-1)) decrease in haem potential, a value in good agreement with density functional theory calculations. Substitution of Leu by Ala or Gly (structures determined at 1.03 and 1.04 Å resolutions) resulted in a haem site that binds CO in the linear mode only and where no significant change in redox potential is observed. Remarkably, these variants were isolated as ferrous 6c-CO complexes, attributable to the observed eight orders of magnitude increase in affinity for CO, including an approximately 10,000-fold decrease in the rate of dissociation. These new findings have wide implications for preventing CO poisoning of gas-binding haem proteins.

The Mycobacterium tuberculosis cytochromes P450: physiology, biochemistry & molecular intervention

The human pathogen Mycobacterium tuberculosis (Mtb) encodes 20 cytochrome P450 (P450) enzymes. Gene essentiality for viability or host infection was demonstrated for Mtb P450s CYP128, CYP121 and CYP125. Structure/function studies on Mtb P450s revealed key roles contributing to bacterial virulence and persistence in the host. Various azole-class drugs bind with high affinity to the Mtb P450 heme and are potent Mtb antibiotics. This paper reviews the current understanding of the biochemistry of Mtb P450s, their interactions with azoles and their potential as novel Mtb drug targets. Mtb multidrug resistance is widespread and novel therapeutics are desperately needed. Simultaneous drug targeting of several Mtb P450s crucial to bacterial viability/persistence could offer a new route to effective antibiotics and minimize the development of drug resistance.

Identification of N-terminal regions of wheat leaf ferredoxin NADP+ oxidoreductase important for interactions with ferredoxin

Wheat leaves contain two isoproteins of the photosynthetic ferredoxin:NADP(+) reductase (pFNRI and pFNRII). Truncated forms of both enzymes have been detected in vivo, but only pFNRII displays N-terminal length-dependent changes in activity. To investigate the impact of N-terminal truncation on interaction with ferredoxin (Fd), recombinant pFNRII proteins, differing by deletions of up to 25 amino acids, were generated. During purification of the isoproteins found in vivo, the longer forms of pFNRII bound more strongly to a Fd affinity column than did the shorter forms, pFNRII(ISKK) and pFNRII[N-2](KKQD). Further truncation of the N-termini resulted in a pFNRII protein which failed to bind to a Fd column. Similar k(cat) values (104-140 s(-1)) for cytochrome c reduction were measured for all but the most truncated pFNRII[N-5](DEGV), which had a k(cat) of 38 s(-1). Stopped-flow kinetic studies, examining the impact of truncation on electron flow between mutant pFNRII proteins and Fd, showed there was a variation in k(obs) from 76 to 265 s(-1) dependent on the pFNRII partner. To analyze the sites which contribute to Fd binding at the pFNRII N-terminal, three mutants were generated, in which a single or double lysine residue was changed to glutamine within the in vivo N-terminal truncation region. The mutations affected binding of pFNRII to the Fd column. Based on activity measurements, the double lysine residue change resulted in a pFNRII enzyme with decreased Fd affinity. The results highlight the importance of this flexible N-terminal region of the pFNRII protein in binding the Fd partner.

Structural and biochemical characterization of Mycobacterium tuberculosis CYP142: evidence for multiple cholesterol 27-hydroxylase activities in a human pathogen

The Mycobacterium tuberculosis cytochrome P450 enzyme CYP142 is encoded in a large gene cluster involved in metabolism of host cholesterol. CYP142 was expressed and purified as a soluble, low spin P450 hemoprotein. CYP142 binds tightly to cholesterol and its oxidized derivative cholest-4-en-3-one, with extensive shift of the heme iron to the high spin state. High affinity for azole antibiotics was demonstrated, highlighting their therapeutic potential. CYP142 catalyzes either 27-hydroxylation of cholesterol/cholest-4-en-3-one or generates 5-cholestenoic acid/cholest-4-en-3-one-27-oic acid from these substrates by successive sterol oxidations, with the catalytic outcome dependent on the redox partner system used. The CYP142 crystal structure was solved to 1.6 Å, revealing a similar active site organization to the cholesterol-metabolizing M. tuberculosis CYP125, but having a near-identical organization of distal pocket residues to the branched fatty acid oxidizing M. tuberculosis CYP124. The cholesterol oxidizing activity of CYP142 provides an explanation for previous findings that ΔCYP125 strains of Mycobacterium bovis and M. bovis BCG cannot grow on cholesterol, because these strains have a defective CYP142 gene. CYP142 is revealed as a cholesterol 27-oxidase with likely roles in host response modulation and cholesterol metabolism.

Expression and characterization of Mycobacterium tuberculosis CYP144: common themes and lessons learned in the M. tuberculosis P450 enzyme family

CYP144 from Mycobacterium tuberculosis was expressed and purified. CYP144 demonstrates heme thiolate coordination in its ferric form, but the cysteinate is protonated to thiol in both the carbon monoxide-bound and ligand-free ferrous forms (forming P420 in the former). Tight binding of various azole drugs was shown, with affinity for miconazole (K(d)=0.98 μM), clotrimazole (0.37 μM) and econazole (0.78 μM) being highest. These azoles are also the trio with the highest affinity for the essential CYP121 and for the cholesterol oxidase CYP125 (essential for host infection), and have high potency as anti-mycobacterial drugs. Construction of a Mtb gene knockout strain demonstrated that CYP144 is not essential for growth in vitro. However the deletion strain was more sensitive to azole inhibition in culture suggesting an important role for CYP144 in cell physiology and/or in mediating azole resistance. The biophysical and genetic features of CYP144 are compared to those of other characterized Mtb P450s, identifying both commonality in properties (including thiolate protonation in ferrous P450s) and intriguing differences in thermodynamic and spectroscopic features. Our developing knowledge of the Mtb P450s has revealed unusual biochemistry and gene essentiality, highlighting their potential as drug targets in this human pathogen.

The Structure of Mycobacterium tuberculosis CYP125: molecular basis for cholesterol binding in a P450 needed for host infection

We report characterization and the crystal structure of the Mycobacterium tuberculosis cytochrome P450 CYP125, a P450 implicated in metabolism of host cholesterol and essential for establishing infection in mice. CYP125 is purified in a high spin form and undergoes both type I and II spectral shifts with various azole drugs. The 1.4-A structure of ligand-free CYP125 reveals a "letterbox" active site cavity of dimensions appropriate for entry of a polycyclic sterol. A mixture of hexa-coordinate and penta-coordinate states could be discerned, with water binding as the 6th heme-ligand linked to conformation of the I-helix Val(267) residue. Structures in complex with androstenedione and the antitubercular drug econazole reveal that binding of hydrophobic ligands occurs within the active site cavity. Due to the funnel shape of the active site near the heme, neither approaches the heme iron. A model of the cholesterol CYP125 complex shows that the alkyl side chain extends toward the heme iron, predicting hydroxylation of cholesterol C27. The alkyl chain is in close contact to Val(267), suggesting a substrate binding-induced low- to high-spin transition coupled to reorientation of the latter residue. Reconstitution of CYP125 activity with a redox partner system revealed exclusively cholesterol 27-hydroxylation, consistent with structure and modeling. This activity may enable catabolism of host cholesterol or generation of immunomodulatory compounds that enable persistence in the host. This study reveals structural and catalytic properties of a potential M. tuberculosis drug target enzyme, and the likely mode by which the host-derived substrate is bound and hydroxylated.

The Mycobacterium tuberculosis cytochrome P450 system

Tuberculosis remains a leading cause of human mortality. The emergence of strains of Mycobacterium tuberculosis, the causative agent, that are resistant to the major frontline antitubercular drugs increases the urgency for the development of new therapeutic agents. Sequencing of the M. tuberculosis genome revealed the existence of 20 cytochrome P450 enzymes, some of which are potential candidates for drug targeting. The recent burst of studies reporting microarray-based gene essentiality and transcriptome analyses under in vitro, ex vivo and in vivo conditions highlight the importance of selected P450 isoforms for M. tuberculosis viability and pathogenicity. Current knowledge of the structural and biochemical properties of the M. tuberculosis P450 enzymes and their putative redox partners is reviewed, with an emphasis on findings related to their physiological function(s) as well as their potential as drug targets.