Fusarium oxysporum fatty-acid subterminal hydroxylase (CYP505) is a membrane-bound eukaryotic counterpart of Bacillus megaterium cytochrome P450BM3

Cytochrome P450 monooxygenase systems: Diversity and plasticity for adaptive stress response

Superfamily of cytochromes P450 (CYPs) is composed of heme-thiolate-containing monooxygenase enzymes, which play crucial roles in the biosynthesis, bioactivation, and detoxification of a variety of organic compounds, both endogenic and exogenic. Majority of CYP monooxygenase systems are multi-component and contain various redox partners, cofactors and auxiliary proteins, which contribute to their diversity in both prokaryotes and eukaryotes. Recent progress in bioinformatics and computational biology approaches make it possible to undertake whole-genome and phylogenetic analyses of CYPomes of a variety of organisms. Considerable variations in sequences within and between CYP families and high similarity in secondary and tertiary structures between all CYPs along with dramatic conformational changes in secondary structure elements of a substrate binding site during catalysis have been reported. This provides structural plasticity and substrate promiscuity, which underlie functional diversity of CYPs. Gene duplication and mutation events underlie CYP evolutionary diversity and emergence of novel selectable functions, which provide the involvement of CYPs in high adaptability to changing environmental conditions and dietary restrictions. In our review, we discuss the recent advancements and challenges in the elucidating the evolutionary origin and mechanisms underlying the CYP monooxygenase system diversity and plasticity. Our review is in the view of hypothesis that diversity of CYP monooxygenase systems is translated into the broad metabolic profiles, and this has been acquired during the long evolutionary time to provide structural plasticity leading to high adaptative capabilities to environmental stress conditions.

Evolutionary history of the cytochrome P450s from Colletotrichum species and prediction of their putative functional roles during host-pathogen interactions

The genomes of species belonging to the genus Colletotrichum harbor a substantial number of cytochrome P450 monooxygenases (CYPs) encoded by a broad diversity of gene families. However, the biological role of their CYP complement (CYPome) has not been elucidated. Here, we investigated the putative evolutionary scenarios that occurred during the evolution of the CYPome belonging to the Colletotrichum Graminicola species complex (s.c.) and their biological implications. The study revealed that most of the CYPome gene families belonging to the Graminicola s.c. experienced gene contractions. The reductive evolution resulted in species restricted CYPs are predominant in each CYPome of members from the Graminicola s.c., whereas only 18 families are absolutely conserved among these species. However, members of CYP families displayed a notably different phylogenetic relationship at the tertiary structure level, suggesting a putative convergent evolution scenario. Most of the CYP enzymes of the Graminicola s.c. share redundant functions in secondary metabolite biosynthesis and xenobiotic metabolism. Hence, this current work suggests that the presence of a broad CYPome in the genus Colletotrichum plays a critical role in the optimization of the colonization capability and virulence.

Delineation of the CYP505E subfamily of fungal self-sufficient in-chain hydroxylating cytochrome P450 monooxygenases

Cytochrome P450 monooxygenases (CYP450s) are abundant in eukaryotes, specifically in plants and fungi where they play important roles in the synthesis and degradation of secondary metabolites. In eukaryotes, the best studied "self-sufficient" CYP450s, with a fused redox partner, belong to the CYP505 family. Members of the CYP505 family are generally considered sub-terminal fatty acid hydroxylases. CYP505E3 from Aspergillus terreus, however, gives remarkable in-chain hydroxylation at the ω-7 position of C10 to C16 alkanes and C12 and C14 fatty alcohols. Because CYP505E3 is a promising catalyst for the synthesis of δ-dodecalactone, we set out to delineate the unique ω-7 hydroxylase activity of CYP505E3. CYP505E3 and six additional CYP505Es as well as four closely related CYP505s from four different subfamilies were expressed in Pichia pastoris. Only the CYP505Es, sharing more than 70% amino acid identity, displayed significant ω-7 hydroxylase activity toward 1-dodecanol, dodecanoic acid, and tetradecanoic acid giving products that can readily be converted to δ-dodecalactone. Concentrations of δ-dodecalactone, directly extracted from dodecanoic acid biotransformations, were higher than previously obtained with E. coli. Searches of the UniProt and NCBI databases yielded a total of only 23 unique CYP505Es, all from the Aspergillaceae. Given that CYP505Es with this remarkable activity occur in only a few Aspergillus and Penicillium spp., we further explored the genetic environments in which they occur. These were found to be very distinct environments which include a specific ABC transporter but could not be linked to apparent secondary metabolite gene clusters. KEY POINTS: • Identified CYP505Es share > 70% amino acid identity. • CYP505Es hydroxylate 1-dodecanol, dodecanoic, and tetradecanoic acid at ω-7 position. • CYP505E genes occur in Aspergillus and Penicillium spp. near an ABC transporter.

Cytochrome P450 168A1 from Pseudomonas aeruginosa is involved in the hydroxylation of biologically relevant fatty acids

The cytochrome P450 CYP168A1 from Pseudomonas aeruginosa was cloned and expressed in Escherichia coli followed by purification and characterization of function. CYP168A1 is a fatty acid hydroxylase that hydroxylates saturated fatty acids, including myristic (0.30 min-1), palmitic (1.61 min-1) and stearic acids (1.24 min-1), at both the ω-1- and ω-2-positions. However, CYP168A1 only hydroxylates unsaturated fatty acids, including palmitoleic (0.38 min-1), oleic (1.28 min-1) and linoleic acids (0.35 min-1), at the ω-1-position. CYP168A1 exhibited a catalytic preference for palmitic, oleic and stearic acids as substrates in keeping with the phosphatidylcholine-rich environment deep in the lung that is colonized by P. aeruginosa.

Transcriptome analysis implicates secondary metabolite production, redox reactions, and programmed cell death during allorecognition in Cryphonectria parasitica

The underlying molecular mechanisms of programmed cell death associated with fungal allorecognition, a form of innate immunity, remain largely unknown. In this study, transcriptome analysis was used to infer mechanisms activated during barrage formation in vic3-incompatible strains of Cryphonectria parasitica, the chestnut blight fungus. Pronounced differential expression occurred in barraging strains of genes involved in mating pheromone (mf2-1, mf2-2), secondary metabolite production, detoxification (including oxidative stress), apoptosis-related, RNA interference, and HET-domain genes. Evidence for secondary metabolite production and reactive oxygen species (ROS) accumulation is supported through UPLC-HRMS analysis and cytological staining, respectively. Differential expression of mating-related genes and HET-domain genes was further examined by RT-qPCR of incompatible interactions involving each of the six vegetative incompatibility (vic) loci in C. parasitica and revealed distinct recognition process networks. We infer that vegetative incompatibility in C. parasitica activates defence reactions that involve secondary metabolism, resulting in increased toxicity of the extra- and intracellular environment. Accumulation of ROS (and other potential toxins) may result in detoxification failure and activation of apoptosis, sporulation, and the expression of associated pheromone genes. The incompatible reaction leaves abundant traces of a process-specific metabolome as conidiation is initiated.

Structure of the fungal hydroxylase, CYP505A30, and rational transfer of mutation data from CYP102A1 to alter regioselectivity

Regioselective oxyfunctionalisation of n-alkanes and production of non-vicinal diols by evolved CYP505A30 through rational transfer of knowledge between protein scaffolds.

Taxonomic Distribution of Cytochrome P450 Monooxygenases (CYPs) among the Budding Yeasts (Sub-Phylum Saccharomycotina)

Cytochrome P450 monooxygenases (CYPs) are ubiquitous throughout the tree of life and play diverse roles in metabolism including the synthesis of secondary metabolites as well as the degradation of recalcitrant organic substrates. The genomes of budding yeasts (phylum Ascomycota, sub-phylum Saccharomycotina) typically contain fewer families of CYPs than filamentous fungi. There are currently five CYP families among budding yeasts with known function while at least another six CYP families with unknown function (“orphan CYPs”) have been described. The current study surveyed the genomes of 372 species of budding yeasts for CYP-encoding genes in order to determine the taxonomic distribution of individual CYP families across the sub-phylum as well as to identify novel CYP families. Families CYP51 and CYP61 (represented by the ergosterol biosynthetic genes ERG11 and ERG5, respectively) were essentially ubiquitous among the budding yeasts while families CYP52 (alkane/fatty acid hydroxylases), CYP56 (N-formyl-l-tyrosine oxidase) displayed several instances of gene loss at the genus or family level. Phylogenetic analysis suggested that the three orphan families CYP5217, CYP5223 and CYP5252 diverged from a common ancestor gene following the origin of the budding yeast sub-phylum. The genomic survey also identified eight CYP families that had not previously been reported in budding yeasts.

Biochemical Characterization of CYP505D6, a Self-Sufficient Cytochrome P450 from the White-Rot Fungus Phanerochaete chrysosporium

The activity of a self-sufficient cytochrome P450 enzyme, CYP505D6, from the lignin-degrading basidiomycete Phanerochaete chrysosporium was characterized. Recombinant CYP505D6 was produced in Escherichia coli and purified. In the presence of NADPH, CYP505D6 used a series of saturated fatty alcohols with C_9-18 carbon chain lengths as the substrates. Hydroxylation occurred at the ω-1 to ω-6 positions of such substrates with C_9-15 carbon chain lengths, except for 1-dodecanol, which was hydroxylated at the ω-1 to ω-7 positions. Fatty acids were also substrates of CYP505D6. Based on the sequence alignment, the corresponding amino acid of Tyr51, which is located at the entrance to the active-site pocket in CYP102A1, was Val51 in CYP505D6. To understand the diverse hydroxylation mechanism, wild-type CYP505D6 and its V51Y variant and wild-type CYP102A1 and its Y51V variant were generated, and the products of their reaction with dodecanoic acid were analyzed. Compared with wild-type CYP505D6, its V51Y variant generated few products hydroxylated at the ω-4 to ω-6 positions. The products generated by wild-type CYP102A1 were hydroxylated at the ω-1 to ω-4 positions, whereas its Y51V variant generated ω-1 to ω-7 hydroxydodecanoic acids. These observations indicated that Val51 plays an important role in determining the regiospecificity of fatty acid hydroxylation, at least that at the ω-4 to ω-6 positions. Aromatic compounds, such as naphthalene and 1-naphthol, were also hydroxylated by CYP505D6. These findings highlight a unique broad substrate spectrum of CYP505D6, rendering it an attractive candidate enzyme for the biotechnological industry.IMPORTANCE Phanerochaete chrysosporium is a white-rot fungus whose metabolism of lignin, aromatic pollutants, and lipids has been most extensively studied. This fungus harbors 154 cytochrome P450-encoding genes in the genome. As evidenced in this study, P. chrysosporium CYP505D6, a fused protein of P450 and its reductase, hydroxylates fatty alcohols (C_9-15) and fatty acids (C_9-15) at the ω-1 to ω-7 or ω-1 to ω-6 positions, respectively. Naphthalene and 1-naphthol were also hydroxylated, indicating that the substrate specificity of CYP505D6 is broader than those of the known fused proteins CYP102A1 and CYP505A1. The substrate versatility of CYP505D6 makes this enzyme an attractive candidate for biotechnological applications.

Genome-wide analysis of cytochrome P450s of Trichoderma spp.: annotation and evolutionary relationships

Background

Cytochrome P450s form an important group of enzymes involved in xenobiotics degradation and metabolism, both primary and secondary. These enzymes are also useful in industry as biotechnological tools for bioconversion and a few are reported to be involved in pathogenicity. Trichoderma spp. are widely used in industry and agriculture and are known for their biosynthetic potential of a large number of secondary metabolites. For realising the full biosynthetic potential of an organism, it is important to do a genome-wide annotation and cataloguing of these enzymes.

Results

Here, we have studied the genomes of seven species (T. asperellum, T. atroviride, T. citrinoviride, T. longibrachiatum, T. reesei , T. harzianum and T. virens) and identified a total of 477 cytochrome P450s. We present here the classification, evolution and structure as well as predicted function of these proteins. This study would pave the way for functional characterization of these groups of enzymes and will also help in realization of their full economic potential.

Conclusion

Our CYPome annotation and evolutionary studies of the seven Trichoderma species now provides opportunities for exploration of research-driven strategies to select Trichoderma species for various applications especially in relation to secondary metabolism and degradation of environmental pollutants.

In silico analysis of cytochrome P450 monooxygenases in chronic granulomatous infectious fungus Sporothrix schenckii: Special focus on CYP51

Sporotrichosis is an emerging chronic, granulomatous, subcutaneous, mycotic infection caused by Sporothrix species. Sporotrichosis is treated with the azole drug itraconazole as ketoconazole is ineffective. It is a well-known fact that azole drugs act by inhibiting cytochrome P450 monooxygenases (P450s), heme-thiolate proteins. To date, nothing is known about P450s in Sporothrix schenckii and the molecular basis of its resistance to ketoconazole. Here we present genome-wide identification, annotation, phylogenetic analysis and comprehensive P450 family-level comparative analysis of S. schenckii P450s with pathogenic fungi P450s, along with a rationale for ketoconazole resistance by S. schenckii based on in silico structural analysis of CYP51. Genome data-mining of S. schenckii revealed 40 P450s in its genome that can be grouped into 32 P450 families and 39 P450 subfamilies. Comprehensive comparative analysis of P450s revealed that S. schenckii shares 11 P450 families with plant pathogenic fungi and has three unique P450 families: CYP5077, CYP5386 and CYP5696 (novel family). Among P450s, CYP51, the main target of azole drugs was also found in S. schenckii. 3D modeling of S. schenckii CYP51 revealed the presence of characteristic P450 motifs with exceptionally large reductase interaction site 2. In silico analysis revealed number of mutations that can be associated with ketoconazole resistance, especially at the channel entrance to the active site. One of possible reason for better stabilization of itraconazole, compared to ketoconazole, is that the more extended molecule of itraconazole may form a hydrogen bond with ASN-230. This in turn may explain its effectiveness against S. schenckii vis-a-vis resistant to ketoconazole. 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.

A genomic comparison of putative pathogenicity-related gene families in five members of the Ophiostomatales with different lifestyles

Ophiostomatoid fungi are vectored by their bark-beetle associates and colonize different host tree species. To survive and proliferate in the host, they have evolved mechanisms for detoxification and elimination of host defence compounds, efficient nutrient sequestration, and, in pathogenic species, virulence towards plants. Here, we assembled a draft genome of the spruce pathogen Ophiostoma bicolor. For our comparative and phylogenetic analyses, we mined the genomes of closely related species (Ophiostoma piceae, Ophiostoma ulmi, Ophiostoma novo-ulmi, and Grosmannia clavigera). Our aim was to acquire a genomic and evolutionary perspective of gene families important in host colonization. Genome comparisons showed that both the nuclear and mitochondrial genomes in our assembly were largely complete. Our O. bicolor 25.3 Mbp draft genome had 10 018 predicted genes, 6041 proteins with gene ontology (GO) annotation, 269 carbohydrate-active enzymes (CAZymes), 559 peptidases and inhibitors, and 1373 genes likely involved in pathogen-host interactions. Phylogenetic analyses of selected protein families revealed core sets of cytochrome P450 genes, ABC transporters and backbone genes involved in secondary metabolite (SM) biosynthesis (polyketide synthases (PKS) and non-ribosomal synthases), and species-specific gene losses and duplications. Phylogenetic analyses of protein families of interest provided insight into evolutionary adaptations to host biochemistry in ophiostomatoid fungi.

Divergent and Convergent Evolution of Fungal Pathogenicity

Fungal pathogens of plants and animals have multifarious effects; they cause devastating damages to agricultures, lead to life-threatening diseases in humans, or induce beneficial effects by reducing insect pest populations. Many virulence factors have been determined in different fungal pathogens; however, the molecular determinants contributing to fungal host selection and adaptation are largely unknown. In this study, we sequenced the genomes of seven ascomycete insect pathogens and performed the genome-wide analyses of 33 species of filamentous ascomycete pathogenic fungi that infect insects (12 species), plants (12), and humans (9). Our results revealed that the genomes of plant pathogens encode more proteins and protein families than the insect and human pathogens. Unexpectedly, more common orthologous protein groups are shared between the insect and plant pathogens than between the two animal group pathogens. We also found that the pathogenicity of host-adapted fungi evolved multiple times, and that both divergent and convergent evolutions occurred during pathogen–host cospeciation thus resulting in protein families with similar features in each fungal group. However, the role of phylogenetic relatedness on the evolution of protein families and therefore pathotype formation could not be ruled out due to the effect of common ancestry. The evolutionary correlation analyses led to the identification of different protein families that correlated with alternate pathotypes. Particularly, the effector-like proteins identified in plant and animal pathogens were strongly linked to fungal host adaptation, suggesting the existence of similar gene-for-gene relationships in fungus–animal interactions that has not been established before. These results well advance our understanding of the evolution of fungal pathogenicity and the factors that contribute to fungal pathotype formation.

Diversity and evolution of cytochrome P450 monooxygenases in Oomycetes

Cytochrome P450 monooxygenases (P450s) are heme-thiolate proteins whose role as drug targets against pathogens, as well as in valuable chemical production and bioremediation, has been explored. In this study we performed comprehensive comparative analysis of P450s in 13 newly explored oomycete pathogens. Three hundred and fifty-six P450s were found in oomycetes. These P450s were grouped into 15 P450 families and 84 P450 subfamilies. Among these, nine P450 families and 31 P450 subfamilies were newly found in oomycetes. Research revealed that oomycetes belonging to different orders contain distinct P450 families and subfamilies in their genomes. Evolutionary analysis and sequence homology data revealed P450 family blooms in oomycetes. Tandem arrangement of a large number of P450s belonging to the same family indicated that P450 family blooming is possibly due to its members’ duplications. A unique combination of amino acid patterns was observed at EXXR and CXG motifs for the P450 families CYP5014, CYP5015 and CYP5017. A novel P450 fusion protein (CYP5619 family) with an N-terminal P450 domain fused to a heme peroxidase/dioxygenase domain was discovered in Saprolegnia declina. Oomycete P450 patterns suggested host influence in shaping their P450 content. This manuscript serves as reference for future P450 annotations in newly explored oomycetes.

Mechanistic basis of electron transfer to cytochromes p450 by natural redox partners and artificial donor constructs

Cytochromes P450 (P450s) are hemoproteins catalyzing oxidative biotransformation of a vast array of natural and xenobiotic compounds. Reducing equivalents required for dioxygen cleavage and substrate hydroxylation originate from different redox partners including diflavin reductases, flavodoxins, ferredoxins and phthalate dioxygenase reductase (PDR)-type proteins. Accordingly, circumstantial analysis of structural and physicochemical features governing donor-acceptor recognition and electron transfer poses an intriguing challenge. Thus, conformational flexibility reflected by togging between closed and open states of solvent exposed patches on the redox components was shown to be instrumental to steered electron transmission. Here, the membrane-interactive tails of the P450 enzymes and donor proteins were recognized to be crucial to proper orientation toward each other of surface sites on the redox modules steering functional coupling. Also, mobile electron shuttling may come into play. While charge-pairing mechanisms are of primary importance in attraction and complexation of the redox partners, hydrophobic and van der Waals cohesion forces play a minor role in docking events. Due to catalytic plasticity of P450 enzymes, there is considerable promise in biotechnological applications. Here, deeper insight into the mechanistic basis of the redox machinery will permit optimization of redox processes via directed evolution and DNA shuffling. Thus, creation of hybrid systems by fusion of the modified heme domain of P450s with proteinaceous electron carriers helps obviate the tedious reconstitution procedure and induces novel activities. Also, P450-based amperometric biosensors may open new vistas in pharmaceutical and clinical implementation and environmental monitoring.

CYP116B5: a new class VII catalytically self-sufficient cytochrome P450 from Acinetobacter radioresistens that enables growth on alkanes

A gene coding for a class VII cytochrome P450 monooxygenase (CYP116B5) was identified from Acinetobacter radioresistens S13 growing on media with medium (C14, C16) and long (C24, C36) chain alkanes as the sole energy source. Phylogenetic analysis of its N- and C-terminal domains suggests an evolutionary model involving a plasmid-mediated horizontal gene transfer from the donor Rhodococcus jostii RHA1 to the receiving A. radioresistens S13. This event was followed by fusion and integration of the new gene in A. radioresistens chromosome. Heterologous expression of CYP116B5 in Escherichia coli BL21, together with the A. radioresistens Baeyer-Villiger monooxygenase, allowed the recombinant bacteria to grow on long- and medium-chain alkanes, showing that CYP116B5 is involved in the first step of terminal oxidation of medium-chain alkanes overlapping AlkB and in the first step of sub-terminal oxidation of long-chain alkanes. It was also demonstrated that CYP116B5 is a self-sufficient cytochrome P450 consisting of a heme domain (aa 1-392) involved in the oxidation step of n-alkanes degradation, and its reductase domain (aa 444-758) comprising the NADPH-, FMN- and [2Fe2S]-binding sites. To our knowledge, CYP116B5 is the first member of this class to have its natural substrate and function identified.

Elucidating how the saprophytic fungus Aspergillus nidulans uses the plant polyester suberin as carbon source

BACKGROUND

Lipid polymers in plant cell walls, such as cutin and suberin, build recalcitrant hydrophobic protective barriers. Their degradation is of foremost importance for both plant pathogenic and saprophytic fungi. Regardless of numerous reports on fungal degradation of emulsified fatty acids or cutin, and on fungi-plant interactions, the pathways involved in the degradation and utilisation of suberin remain largely overlooked. As a structural component of the plant cell wall, suberin isolation, in general, uses harsh depolymerisation methods that destroy its macromolecular structure. We recently overcame this limitation isolating suberin macromolecules in a near-native state.

RESULTS

Suberin macromolecules were used here to analyse the pathways involved in suberin degradation and utilisation by Aspergillus nidulans. Whole-genome profiling data revealed the complex degrading enzymatic machinery used by this saprophytic fungus. Initial suberin modification involved ester hydrolysis and ω-hydroxy fatty acid oxidation that released long chain fatty acids. These fatty acids were processed through peroxisomal β-oxidation, leading to up-regulation of genes encoding the major enzymes of these pathways (e.g. faaB and aoxA). The obtained transcriptome data was further complemented by secretome, microscopic and spectroscopic analyses.

CONCLUSIONS

Data support that during fungal growth on suberin, cutinase 1 and some lipases (e.g. AN8046) acted as the major suberin degrading enzymes (regulated by FarA and possibly by some unknown regulatory elements). Suberin also induced the onset of sexual development and the boost of secondary metabolism.

Cytochromes P450 as promising catalysts for biotechnological application: chances and limitations

Cytochromes P450 (CYPs) belong to the superfamily of heme b containing monooxygenases with currently more than 21,000 members. These enzymes accept a vast range of organic molecules and catalyze diverse reactions. These extraordinary capabilities of CYP systems that are unmet by other enzymes make them attractive for biotechnology. However, the complexity of these systems due to the need of electron transfer from nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) via redox partner proteins for the initial hydroxylation step limits a broader technical implementation of CYP enzymes. There have been several reviews during the past years tackling the potential CYPs for synthetic application. The aim of this review is to give a critical overview about possibilities and chances for application of these interesting catalysts as well as to discuss drawbacks and problems related to their use. Solutions to overcome these limitations will be demonstrated, and several selected examples of successful CYP applications under industrial conditions will be reviewed.

Unusual cytochrome p450 enzymes and reactions

Cytochrome P450 enzymes primarily catalyze mixed-function oxidation reactions, plus some reductions and rearrangements of oxygenated species, e.g. prostaglandins. Most of these reactions can be rationalized in a paradigm involving Compound I, a high-valent iron-oxygen complex (FeO(3+)), to explain seemingly unusual reactions, including ring couplings, ring expansion and contraction, and fusion of substrates. Most P450s interact with flavoenzymes or iron-sulfur proteins to receive electrons from NAD(P)H. In some cases, P450s are fused to protein partners. Other P450s catalyze non-redox isomerization reactions. A number of permutations on the P450 theme reveal the diversity of cytochrome P450 form and function.

The cytochrome P450 complement (CYPome) of Mycosphaerella graminicola

Mycosphaerella graminicola is a key fungal pathogen of wheat and a major target for azole fungicides, many of whose central mode of action is through inhibition of cytochrome P450 51 (lanosterol 14α-demethylase) in the ergosterol biosynthetic pathway. The range of activities of other fungal CYPs is thought to be a reflection of the differences between different organisms and their range of secondary metabolic pathways as a response to their niche environments, for example, in the production of mycotoxins. The present study collates information from a range of databases, to classify the CYPs found in M. graminicola and assign them an internationally recognized nomenclature, which, when referenced to the recent publication of the JGI version 2.0 genome model, creates a current, robust model for the CYP complement (CYPome) of M. graminicola. These CYPome data, which examined 82 CYPs and one pseudo-gene, may be utilized not only to further characterize and describe the physiology of the organism but also to enhance our understanding of CYP function and diversity.

Microbial cytochromes P450: biodiversity and biotechnology. Where do cytochromes P450 come from, what do they do and what can they do for us?

The first eukaryote genome revealed three yeast cytochromes P450 (CYPs), hence the subsequent realization that some microbial fungal genomes encode these proteins in 1 per cent or more of all genes (greater than 100) has been surprising. They are unique biocatalysts undertaking a wide array of stereo- and regio-specific reactions and so hold promise in many applications. Based on ancestral activities that included 14α-demethylation during sterol biosynthesis, it is now seen that CYPs are part of the genes and metabolism of most eukaryotes. In contrast, Archaea and Eubacteria often do not contain CYPs, while those that do are frequently interesting as producers of natural products undertaking their oxidative tailoring. Apart from roles in primary and secondary metabolism, microbial CYPs are actual/potential targets of drugs/agrochemicals and CYP51 in sterol biosynthesis is exhibiting evolution to resistance in the clinic and the field. Other CYP applications include the first industrial biotransformation for corticosteroid production in the 1950s, the diversion into penicillin synthesis in early mutations in fungal strain improvement and bioremediation using bacteria and fungi. The vast untapped resource of orphan CYPs in numerous genomes is being probed and new methods for discovering function and for discovering desired activities are being investigated.

P450 monooxygenases (P450ome) of the model white rot fungus Phanerochaete chrysosporium

Phanerochaete chrysosporium, the model white rot fungus, has been the focus of research for the past about four decades for understanding the mechanisms and processes of biodegradation of the natural aromatic polymer lignin and a broad range of environmental toxic chemicals. The ability to degrade this vast array of xenobiotic compounds was originally attributed to its lignin-degrading enzyme system, mainly the extracellular peroxidases. However, subsequent physiological, biochemical, and/or genetic studies by us and others identified the involvement of a peroxidase-independent oxidoreductase system, the cytochrome P450 monooxygenase system. The whole genome sequence revealed an extraordinarily large P450 contingent (P450ome) with an estimated 149 P450s in this organism. This review focuses on the current status of understanding on the P450 monooxygenase system of P. chrysosproium in terms of pre-genomic and post-genomic identification, structural and evolutionary analysis, transcriptional regulation, redox partners, and functional characterization for its biodegradative potential. Future research on this catalytically diverse oxidoreductase enzyme system and its major role as a newly emerged player in xenobiotic metabolism/degradation is discussed.

The cytochromes P450 of Grosmannia clavigera: Genome organization, phylogeny, and expression in response to pine host chemicals

Grosmannia clavigera is a fungal associate of the mountain pine beetle (Dendroctonus ponderosae) and a pathogen of lodgepole pine (Pinus contorta) that must overcome terpenoid oleoresin and phenolic defenses of host trees. G. clavigera responds to monoterpene influx with complementary mechanisms that include export and the use of these compounds as a carbon source. Cytochromes P450 (CYPs) may also be involved in the metabolism of host defense compounds. We have identified and phylogenetically classified G. clavigera CYPs (CYPome). We show that although the G. clavigera CYPome has contracted in evolution, certain CYP families have expanded by duplication. We analyzed RNA-seq data for CYP expression following treatment with terpenes and pine phloem extracts to identify CYPs potentially involved in detoxification of these pine defense compounds. We also used transcriptome analysis of G. clavigera grown on monoterpenes, triglycerides or oleic acid as a carbon source to identify up-regulated CYPs that may be involved in the utilization of these compounds to support fungal growth. Finally, we identify secondary metabolite biosynthetic gene clusters that contain CYPs, and CYPs in clusters that may be involved in conversion of host chemicals.

Systematic and searchable classification of cytochrome P450 proteins encoded by fungal and oomycete genomes

BACKGROUND

Cytochrome P450 proteins (CYPs) play diverse and pivotal roles in fungal metabolism and adaptation to specific ecological niches. Fungal genomes encode extremely variable "CYPomes" ranging from one to more than 300 CYPs. Despite the rapid growth of sequenced fungal and oomycete genomes and the resulting influx of predicted CYPs, the vast majority of CYPs remain functionally uncharacterized. To facilitate the curation and functional and evolutionary studies of CYPs, we previously developed Fungal Cytochrome P450 Database (FCPD), which included CYPs from 70 fungal and oomycete species. Here we present a new version of FCPD (1.2) with more data and an improved classification scheme.

RESULTS

The new database contains 22,940 CYPs from 213 species divided into 2,579 clusters and 115 clans. By optimizing the clustering pipeline, we were able to uncover 36 novel clans and to assign 153 orphan CYP families to specific clans. To augment their functional annotation, CYP clusters were mapped to David Nelson's P450 databases, which archive a total of 12,500 manually curated CYPs. Additionally, over 150 clusters were functionally classified based on sequence similarity to experimentally characterized CYPs. Comparative analysis of fungal and oomycete CYPomes revealed cases of both extreme expansion and contraction. The most dramatic expansions in fungi were observed in clans CYP58 and CYP68 (Pezizomycotina), clans CYP5150 and CYP63 (Agaricomycotina), and family CYP509 (Mucoromycotina). Although much of the extraordinary diversity of the pan-fungal CYPome can be attributed to gene duplication and adaptive divergence, our analysis also suggests a few potential horizontal gene transfer events. Updated families and clans can be accessed through the new version of the FCPD database.

CONCLUSIONS

FCPD version 1.2 provides a systematic and searchable catalogue of 9,550 fungal CYP sequences (292 families) encoded by 108 fungal species and 147 CYP sequences (9 families) encoded by five oomycete species. In comparison to the first version, it offers a more comprehensive clan classification, is fully compatible with Nelson's P450 databases, and has expanded functional categorization. These features will facilitate functional annotation and classification of CYPs encoded by newly sequenced fungal and oomycete genomes. Additionally, the classification system will aid in studying the roles of CYPs in the evolution of fungal adaptation to specific ecological niches.

Involvement of a natural fusion of a cytochrome P450 and a hydrolase in mycophenolic acid biosynthesis

Mycophenolic acid (MPA) is a fungal secondary metabolite and the active component in several immunosuppressive pharmaceuticals. The gene cluster coding for the MPA biosynthetic pathway has recently been discovered in Penicillium brevicompactum, demonstrating that the first step is catalyzed by MpaC, a polyketide synthase producing 5-methylorsellinic acid (5-MOA). However, the biochemical role of the enzymes encoded by the remaining genes in the MPA gene cluster is still unknown. Based on bioinformatic analysis of the MPA gene cluster, we hypothesized that the step following 5-MOA production in the pathway is carried out by a natural fusion enzyme MpaDE, consisting of a cytochrome P450 (MpaD) in the N-terminal region and a hydrolase (MpaE) in the C-terminal region. We verified that the fusion gene is indeed expressed in P. brevicompactum by obtaining full-length sequence of the mpaDE cDNA prepared from the extracted RNA. Heterologous coexpression of mpaC and the fusion gene mpaDE in the MPA-nonproducer Aspergillus nidulans resulted in the production of 5,7-dihydroxy-4-methylphthalide (DHMP), the second intermediate in MPA biosynthesis. Analysis of the strain coexpressing mpaC and the mpaD part of mpaDE shows that the P450 catalyzes hydroxylation of 5-MOA to 4,6-dihydroxy-2-(hydroxymethyl)-3-methylbenzoic acid (DHMB). DHMB is then converted to DHMP, and our results suggest that the hydrolase domain aids this second step by acting as a lactone synthase that catalyzes the ring closure. Overall, the chimeric enzyme MpaDE provides insight into the genetic organization of the MPA biosynthesis pathway.

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.

FMN binding site of yeast NADPH-cytochrome P450 reductase exposed at the surface is highly specific

NADPH-cytochrome P450 reductase (CPR) transfers two reducing equivalents derived from NADPH via FAD and FMN to microsomal P450 monooxygenases in one-electron transfer steps. The crystal structure of yeast CPR (yCPR) contains a surface-exposed FMN binding site (FMN2 site) at the interface of the FMN binding and connecting domains, in addition to the single buried site that has been observed in rat CPR. This finding provides a testable hypothesis of how intramolecular (between FAD and FMN) and intermolecular (between FMN and P450) electron transfer may occur in CPR. To verify that occupancy of the FMN2 site is not an artifact of crystallization, a surface plasmon resonance (SPR) biosensor technique has been applied to probe the selectivity of this site under functional conditions. A series of kinetic and equilibrium binding experiments involving yCPR immobilized on different sensor chip surfaces was performed using FMN and FAD, as well as FMN-derived compounds, including riboflavin, dimethylalloxazine, and alloxazine, and other molecules that resemble the planar isoalloxazine ring structure. Only FMN and FAD showed stoichiometric binding responses. Binding affinity for FMN was in the submicromolar range, 30 times higher than that for FAD. Association kinetic rates for the yCPR/FMN complex were up to 60-fold higher than for the yCPR/FAD complex. Taken together, these data indicate that (i) the surface-exposed site in yCPR is highly selective toward binding flavins, (ii) binding of FMN in this site is notably favored, and finally, (iii) both the phosphate group and the isoalloxazine ring of FMN are essential for binding.

Cytochrome P450 enzymes in the fungal kingdom

Cytochrome P450 monooxygenases of fungi are involved in many essential cellular processes and play diverse roles. The enzymes catalyze the conversion of hydrophobic intermediates of primary and secondary metabolic pathways, detoxify natural and environmental pollutants and allow fungi to grow under different conditions. Fungal genome sequencing projects have enabled the annotation of several thousand novel cytochromes P450, many of which constitute new families. This review presents the characteristics of fungal cytochrome P450 systems and updates information on the functions of characterized fungal P450 monooxygenases as well as outlines the currently used strategies for determining the function of the many putative P450 enzymes.

Streptomyces coelicolor A3(2) CYP102 protein, a novel fatty acid hydroxylase encoded as a heme domain without an N-terminal redox partner

The gene from Streptomyces coelicolor A3(2) encoding CYP102B1, a recently discovered CYP102 subfamily which exists solely as a single P450 heme domain, has been cloned, expressed in Escherichia coli, purified, characterized, and compared to its fusion protein family members. Purified reconstitution metabolism experiments with spinach ferredoxin, ferredoxin reductase, and NADPH revealed differences in the regio- and stereoselective metabolism of arachidonic acid compared to that of CYP102A1, exclusively producing 11,12-epoxyeicosa-5,8,14-trienoic acid in addition to the shared metabolites 18-hydroxy arachidonic acid and 14,15-epoxyeicosa-5,8,11-trienoic acid. Consequently, in order to elucidate the physiological function of CYP102B1, transposon mutagenesis was used to generate an S. coelicolor A3(2) strain lacking CYP102B1 activity and the phenotype was assessed.

Structural diversity of cytochrome P450 enzyme system

Cytochrome P450 enzyme system consists of P450 and its NAD(P)H-linked reductase or reducing system, and catalyses monooxygenation reactions. The most prevalent type in eukaryotic organisms is 'microsomes type', which consists of membrane-bound P450 and NADPH-P450 reductase. The second type is 'mitochondria type', in which P450 is bound to the inner membrane while the reducing system consisting of an NADPH-linked flavoprotein and a ferredoxin-type iron-sulphur protein is soluble in the matrix space. The third type is 'bacteria type', in which both P450 and the reducing system are soluble in the cytoplasm. In addition to these three types, several forms of P450-reductase fusion proteins have been found in prokaryotic organisms. On the other hand, some P450s catalyse the re-arrangement of the oxygen atoms in the substrate molecules that does not require the supply of reducing equivalents for the reaction. A peculiar P450, P450nor, receives electrons directly from NADH for the reduction of nitric oxide.

Gene deletion of 7,8-linoleate diol synthase of the rice blast fungus: studies on pathogenicity, stereochemistry, and oxygenation mechanisms

Linoleate diol synthases (LDS) are heme enzymes, which oxygenate 18:2n-6 sequentially to (8R)-hydroperoxylinoleic acid ((8R)-HPODE) and to (5S,8R)-dihydroxy-, (7S,8S)-dihydroxy-, or (8R,11S)-dihydroxylinoleic acids (DiHODE). The genome of the rice blast fungus, Magnaporthe oryzae, contains two genes with homology to LDS. M. oryzae oxidized 18:2n-6 to (8R)-HPODE and to (7S,8S)-DiHODE, (6S,8R)-DiHODE, and (8R,11S)-HODE. Small amounts of 10-hydroxy-(8E,12Z)-octadecadienoic acid and traces of 5,8-DiHODE were also detected by liquid chromatography-mass spectrometry. The contribution of the 7,8-LDS gene to M. oryzae pathogenicity was evaluated by replacement of the catalytic domain with hygromycin and green fluorescent protein variant (SGFP) cassettes. This genetically modified strain Delta7,8-LDS infected rice leaves and roots and formed appressoria and conidia as the native fungus. The Delta7,8-LDS mutant had lost the capacity to biosynthesize all the metabolites except small amounts of 8-hydroxylinoleic acid. Studies with stereospecifically deuterated linoleic acids showed that (8R)-HPODE was formed by abstraction of the pro-S hydrogen at C-8 and antarafacial oxygenation, whereas (7S,8S)-DiHODE and (8R,11S)-DiHODE were formed from (8R)-HPODE by suprafacial hydrogen abstraction and oxygenation at C-7 and C-11, respectively. A mac1 suppressor mutant (Delta mac1 sum1-99) of M. oryzae, which shows cAMP-independent protein kinase A activity, oxygenated 18:2n-6 to increased amounts of (10R)-HPODE and (5S,8R)-DiHODE. Expression of the 7,8-LDS gene but not of the second homologue was detected in the suppressor mutant. This suggests that PKA-mediated signaling pathway regulates the dioxygenase and hydroperoxide isomerase activities of M. oryzae.

Possible involvement of G-proteins and cAMP in the induction of progesterone hydroxylating enzyme system in the vascular wilt fungus Fusarium oxysporum

Fungi present the ability to hydroxylate steroids. In some filamentous fungi, progesterone induces an enzyme system which converts the compound into a less toxic hydroxylated product. We investigated the progesterone response in the vascular wilt pathogen Fusarium oxysporum, using mass spectrometry and high performance liquid chromatography (HPLC). Progesterone was mainly transformed into 15alpha-hydroxyprogesterone, which was found predominantly in the extracellular medium. The role of two conserved fungal signaling cascades in the induction of the progesterone-transforming enzyme system was studied, using knockout mutants lacking the mitogen-activated protein kinase Fmk1 or the heterotrimeric G-protein beta subunit Fgb1 functioning upstream of the cyclic adenosine monophosphate (cAMP) pathway. No steroid hydroxylation was induced in the Deltafgb1 strain, suggesting a role for the G-protein beta subunit in progesterone signaling. Exogenous cAMP restored the induction of progesterone-transforming activity in the Deltafgb1 strain, suggesting that steroid signaling in F. oxysporum is mediated by the cAMP-PKA pathway.

Versatile capacity of shuffled cytochrome P450s for dye production

DNA family shuffling is a relatively new method of directed evolution used to create novel enzymes in order to improve their existing properties or to develop new features. This method of evolution in vitro has one basic requirement: a high similarity of initial parental sequences. Cytochrome P450 enzymes are relatively well conserved in their amino acid sequences. Members of the same family can have more than 40% of sequence identity at the protein level and are therefore good candidates for DNA family shuffling. These xenobiotic-metabolising enzymes have an ability to metabolise a wide range of chemicals and produce a variety of products including blue pigments such as indigo. By applying the specifically designed DNA family shuffling approach, catalytic properties of cytochrome P450 enzymes were further extended in the chimeric progeny to include a new range of blue colour formations. This mini-review evokes the possibility of exploiting directed evolution of cytochrome P450s and the novel enzymes created by DNA family shuffling for the production of new dyes.

Production of hydroxy-fatty acid derivatives from waste oil by Escherichia coli cells producing fungal cytochrome P450foxy

Cytochrome P450foxy (P450foxy) is a fatty acid (FA) monooxygenase that is characterized by self-sufficient catalysis and high turnover numbers due to the fused structure of cytochrome P450 and its reductase. Here we found that resting recombinant Escherichia coli cells producing P450foxy converted saturated FA with a chain length of 7-16 carbon atoms to their omega-1 to omega-3 hydroxy derivatives. Most products were recovered from the culture supernatant. Decanoic acid was most efficiently converted to omega-1 to omega-3 hydroxy decanoic acids in the order of omega-1>omega-2>omega-3, with a total product yield of 47%. We also found that P450foxy was more active against physiological fatty acyl esters such as monopalmitoyl glycerol, monopalmitoyl phospholipid, and palmitoyl CoA than free palmitic acid. The bacteria producing P450foxy were applicable as biocatalysts in the production of omega-1 hydroxy palmitic acid from lard, vegetable, and soy sauce oil wastes from the food industry.

Cytochrome P450 systems—biological variations of electron transport chains

Cytochromes P450 (P450) are hemoproteins encoded by a superfamily of genes nearly ubiquitously distributed in different organisms from all biological kingdoms. The reactions carried out by P450s are extremely diverse and contribute to the biotransformation of drugs, the bioconversion of xenobiotics, the bioactivation of chemical carcinogens, the biosynthesis of physiologically important compounds such as steroids, fatty acids, eicosanoids, fat-soluble vitamins and bile acids, the conversion of alkanes, terpenes and aromatic compounds as well as the degradation of herbicides and insecticides. Cytochromes P450 belong to the group of external monooxygenases and thus receive the necessary electrons for oxygen cleavage and substrate hydroxylation from different redox partners. The classical as well as the recently discovered P450 redox systems are compiled in this paper and classified according to their composition.

P450ome of the white rot fungus Phanerochaete chrysosporium: structure, evolution and regulation of expression of genomic P450 clusters

The model white rot fungus Phanerochaete chrysosporium has the extraordinary ability to degrade (to CO2) lignin and detoxify a variety of chemical pollutants. Whole genome sequencing of this fungus has revealed the presence of the largest P450ome in fungi comprising approx. 150 P450 genes, most of which have unknown function. On the basis of our genome-wide structural and evolutionary analysis, these P450 genes could be classified into 12 families and 23 subfamilies and under 11 fungal P450 clans. The analysis further revealed an extensive gene clustering with a total of 16 P450 clusters constituted of up to 11 members per cluster. In particular, evidence and role of gene duplications and horizontal gene transfer in the evolution of these P450 clusters have been discussed using two of the P450 families [CYP63 and CYP505 (where CYP is cytochrome P450)] as examples. In addition, the observed differential transcriptional induction of the clustered members of the CYP63 gene family, in response to different xenobiotic chemicals and carbon sources, indicated functional divergence within the P450 clusters, of this basidiomycete fungus.

Flavocytochrome P450 BM3 and the origin of CYP102 fusion species

Flavocytochrome P450 (cytochrome P450) BM3 is an intensively studied model system within the P450 enzyme superfamily, and is a natural fusion of a P450 to its P450 reductase redox partner. The fusion arrangement enables efficient electron transfer within the enzyme and a catalytic efficiency that cannot be matched in P450 systems from higher organisms. P450 BM3's potential for industrially relevant chemical transformations is now recognized, and variants with biotechnological applications have been constructed. Simultaneously, structural and mechanistic studies continue to reveal the intricate mechanistic details of this enzyme, including its dimeric organization and the relevance of this quaternary structure to catalysis. Homologues of BM3 have been found in several bacteria and fungi, indicating important physiological functions in these microbes and enabling first insights into evolution of the enzyme family. This short paper deals with recent developments in our understanding of structure, function, evolution and biotechnological applications of this important P450 system.

Cytochrome P450--redox partner fusion enzymes

The cytochromes P450 (P450s) are a broad class of heme b-containing mono-oxygenase enzymes. The vast majority of P450s catalyse reductive scission of molecular oxygen using electrons usually derived from coenzymes (NADH and NADPH) and delivered from redox partner proteins. Evolutionary advantages may be gained by fusion of one or more redox partners to the P450 enzyme in terms of e.g. catalytic efficiency. This route was taken by the well characterized flavocytochrome P450(BM3) system (CYP102A1) from Bacillus megaterium, in which soluble P450 and cytochrome P450 reductase enzymes are covalently linked to produce a highly efficient electron transport system for oxygenation of fatty acids and related molecules. However, genome analysis and ongoing enzyme characterization has revealed that there are a number of other novel classes of P450-redox partner fusion enzymes distributed widely in prokaryotes and eukaryotes. This review examines our current state of knowledge of the diversity of these fusion proteins and explores their structural composition and evolutionary origins.

Microbial degradation of nonylphenol and other alkylphenols—our evolving view

Because the endocrine disrupting effects of nonylphenol (NP) and octylphenol became evident, the degradation of long-chain alkylphenols (AP) by microorganisms was intensively studied. Most NP-degrading bacteria belong to the sphingomonads and closely related genera, while NP metabolism is not restricted to defined fungal taxa. Growth on NP and its mineralization was demonstrated for bacterial isolates, whereas ultimate degradation by fungi still remains unclear. While both bacterial and fungal degradation of short-chain AP, such as cresols, and the bacterial degradation of long-chain branched AP involves aromatic ring hydroxylation, alkyl chain oxidation and the formation of phenolic polymers seem to be preferential elimination pathways of long-chain branched AP in fungi, whereby both intracellular and extracellular oxidative enzymes may be involved. The degradation of NP by sphingomonads does not proceed via the common degradation mechanisms reported for short-chain AP, rather, via an unusual ipso-substitution mechanism. This fact underlies the peculiarity of long-chain AP such as NP isomers, which possess highly branched alkyl groups mostly containing a quaternary alpha-carbon. In addition to physicochemical parameters influencing degradation rates, this structural characteristic confers to branched isomers of NP a biodegradability different to that of the widely used linear isomer of NP. Potential biotechnological applications for the removal of AP from contaminated media and the difficulties of analysis and application inherent to the hydrophobic NP, in particular, are also discussed. The combination of bacteria and fungi, attacking NP at both the phenolic and alkylic moiety, represents a promising perspective.

A second FMN binding site in yeast NADPH-cytochrome P450 reductase suggests a mechanism of electron transfer by diflavin reductases

NADPH-cytochrome P450 reductase transfers two reducing equivalents derived from a hydride ion of NADPH via FAD and FMN to the large family of microsomal cytochrome P450 monooxygenases in one-electron transfer steps. The mechanism of electron transfer by diflavin reductases remains elusive and controversial. Here, we determined the crystal structure of truncated yeast NADPH-cytochrome P450 reductase, which is functionally active toward its physiological substrate cytochrome P450, and discovered a second FMN binding site at the interface of the connecting and FMN binding domains. The two FMN binding sites have different accessibilities to the bulk solvent and different amino acid environments, suggesting stabilization of different electronic structures of the reduced flavin. Since only one FMN cofactor is required for function, a hypothetical mechanism of electron transfer is discussed that proposes shuttling of a single FMN between these two sites coupled with the transition between two semiquinone forms, neutral (blue) and anionic (red).