A well-balanced preexisting equilibrium governs electron flux efficiency of a multidomain diflavin reductase

Structural insights into the semiquinone form of human Cytochrome P450 reductase by DEER distance measurements between a native flavin and a spin labelled non-canonical amino acid

The flavoprotein Cytochrome P450 reductase (CPR) is the unique electron pathway from NADPH to Cytochrome P450 (CYPs). The conformational dynamics of human CPR in solution, which involves transitions from a "locked/closed" to an "unlocked/open" state, is crucial for electron transfer. To date, however, the factors guiding these changes remain unknown. By Site-Directed Spin Labelling coupled to Electron Paramagnetic Resonance spectroscopy, we have incorporated a non-canonical amino acid onto the flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) domains of soluble human CPR, and labelled it with a specific nitroxide spin probe. Taking advantage of the endogenous FMN cofactor, we successfully measured for the first time, the distance distribution by DEER between the semiquinone state FMNH• and the nitroxide. The DEER data revealed a salt concentration-dependent distance distribution, evidence of an "open" CPR conformation at high salt concentrations exceeding previous reports. We also conducted molecular dynamics simulations which unveiled a diverse ensemble of conformations for the "open" semiquinone state of the CPR at high salt concentration. This study unravels the conformational landscape of the one electron reduced state of CPR, which had never been studied before.

Single Mutations in Cytochrome P450 Oxidoreductase Can Alter the Specificity of Human Cytochrome P450 1A2-Mediated Caffeine Metabolism

A unique cytochrome P450 (CYP) oxidoreductase (CPR) sustains activities of human microsomal CYPs. Its function requires toggling between a closed conformation enabling electron transfers from NADPH to FAD and then FMN cofactors and open conformations forming complexes and transferring electrons to CYPs. We previously demonstrated that distinct features of the hinge region linking the FAD and FMN domain (FD) modulate conformer poses and their interactions with CYPs. Specific FD residues contribute in a CYP isoform-dependent manner to the recognition and electron transfer mechanisms that are additionally modulated by the structure of CYP-bound substrate. To obtain insights into the underlying mechanisms, we analyzed how hinge region and FD mutations influence CYP1A2-mediated caffeine metabolism. Activities, metabolite profiles, regiospecificity and coupling efficiencies were evaluated in regard to the structural features and molecular dynamics of complexes bearing alternate substrate poses at the CYP active site. Studies reveal that FD variants not only modulate CYP activities but surprisingly the regiospecificity of reactions. Computational approaches evidenced that the considered mutations are generally in close contact with residues at the FD-CYP interface, exhibiting induced fits during complexation and modified dynamics depending on caffeine presence and orientation. It was concluded that dynamic coupling between FD mutations, the complex interface and CYP active site exist consistently with the observed regiospecific alterations.

Exploring optimal Taxol® CYP725A4 activity in Saccharomyces cerevisiae

Background

CYP725A4 catalyses the conversion of the first Taxol® precursor, taxadiene, to taxadiene-5α-ol (T5α-ol) and a range of other mono- and di-hydroxylated side products (oxygenated taxanes). Initially known to undergo a radical rebound mechanism, the recent studies have revealed that an intermediate epoxide mediates the formation of the main characterised products of the enzyme, being T5α-ol, 5(12)-oxa-3(11)-cyclotaxane (OCT) and its isomer, 5(11)-oxa-3(11)-cyclotaxane (iso-OCT) as well as taxadienediols. Besides the high side product: main product ratio and the low main product titre, CYP725A4 is also known for its slow enzymatic activity, massively hindering further progress in heterologous production of Taxol® precursors. Therefore, this study aimed to systematically explore the key parameters for improving the regioselectivity and activity of eukaryotic CYP725A4 enzyme in a whole-cell eukaryotic biocatalyst, Saccharomyces cerevisiae.

Results

Investigating the impact of CYP725A4 and reductase gene dosages along with construction of self-sufficient proteins with strong prokaryotic reductases showed that a potential uncoupling event accelerates the formation of oxygenated taxane products of this enzyme, particularly the side products OCT and iso-OCT. Due to the harmful effect of uncoupling products and the reactive metabolites on the enzyme, the impact of flavins and irons, existing as prosthetic groups in CYP725A4 and reductase, were examined in both their precursor and ready forms, and to investigate the changes in product distribution. We observed that the flavin adenine dinucleotide improved the diterpenoids titres and biomass accumulation. Hemin was found to decrease the titre of iso-OCT and T5α-ol, without impacting the side product OCT, suggesting the latter being the major product of CYP725A4. The interaction between this iron and the iron precursor, δ-Aminolevulinic acid, seemed to improve the production of these diterpenoids, further denoting that iso-OCT and T5α-ol were the later products. While no direct correlation between cellular-level oxidative stress and oxygenated taxanes was observed, investigating the impact of salt and antioxidant on CYP725A4 further showed the significant drop in OCT titre, highlighting the possibility of enzymatic-level uncoupling event and reactivity as the major mechanism behind the enzyme activity. To characterise the product spectrum and production capacity of CYP725A4 in the absence of cell growth, resting cell assays with optimal neutral pH revealed an array of novel diterpenoids along with higher quantities of characterised diterpenoids and independence of the oxygenated product spectra from the acidity effect. Besides reporting on the full product ranges of CYP725A4 in yeast for the first time, the highest total taxanes of around 361.4 ± 52.4 mg/L including 38.1 ± 8.4 mg/L of T5α-ol was produced herein at a small, 10-mL scale by resting cell assay, where the formation of some novel diterpenoids relied on the prior existence of other diterpenes/diterpenoids as shown by statistical analyses.

Conclusions

This study shows how rational strain engineering combined with an efficient design of experiment approach systematically uncovered the promoting effect of uncoupling for optimising the formation of the early oxygenated taxane precursors of Taxol®. The provided strategies can effectively accelerate the design of more efficient Taxol®-producing yeast strains.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01922-1.

Metabolic Division in an Escherichia coli Coculture System for Efficient Production of Kaempferide

Kaempferide, a plant-derived natural flavonoid, exhibits excellent pharmacological activities with nutraceutical and medicinal applications in human healthcare. Efficient microbial production of complex flavonoids suffers from metabolic crosstalk and burden, which is a big challenge for synthetic biology. Herein, we identified 4'-O-methyltransferases and divided the artificial biosynthetic pathway of kaempferide into upstream, midstream, and downstream modules. By combining heterologous genes from different sources and fine-tuning the expression, we optimized each module for the production of kaempferide. Furthermore, we designed and evaluated four division patterns of synthetic labor in coculture systems by plug-and-play modularity. The linear division of three modules in a three-strain coculture showed higher productivity of kaempferide than that in two-strain cocultures. The U-shaped division by co-distributing the upstream and downstream modules in one strain led to the best performance of the coculture system, which produced 116.0 ± 3.9 mg/L kaempferide, which was 510, 140, and 50% higher than that produced by the monoculture, two-strain coculture, and three-strain coculture with the linear division, respectively. This is the first report of efficient de novo production of kaempferide in a robust Escherichia coli coculture. The strategy of U-shaped pathway division in the coculture provides a promising way for improving the productivity of valuable and complex natural products.

Functional and structural insight into the flexibility of cytochrome P450 reductases from Sorghum bicolor and its implications for lignin composition

Plant NADPH-dependent cytochrome P450 reductase (CPR) is a multidomain enzyme that donates electrons for hydroxylation reactions catalyzed by class II cytochrome P450 monooxygenases involved in the synthesis of many primary and secondary metabolites. These P450 enzymes include trans-cinnamate-4-hydroxylase, p-coumarate-3′-hydroxylase, and ferulate-5-hydroxylase involved in monolignol biosynthesis. Because of its role in monolignol biosynthesis, alterations in CPR activity could change the composition and overall output of lignin. Therefore, to understand the structure and function of three CPR subunits from sorghum, recombinant subunits SbCPR2a, SbCPR2b, and SbCPR2c were subjected to X-ray crystallography and kinetic assays. Steady-state kinetic analyses demonstrated that all three CPR subunits supported the oxidation reactions catalyzed by SbC4H1 (CYP73A33) and SbC3′H (CYP98A1). Furthermore, comparing the SbCPR2b structure with the well-investigated CPRs from mammals enabled us to identify critical residues of functional importance and suggested that the plant flavin mononucleotide–binding domain might be more flexible than mammalian homologs. In addition, the elucidated structure of SbCPR2b included the first observation of NADP⁺ in a native CPR. Overall, we conclude that the connecting domain of SbCPR2, especially its hinge region, could serve as a target to alter biomass composition in bioenergy and forage sorghums through protein engineering.

Revisiting the Molecular Interactions between the Tumor Protein TCTP and the Drugs Sertraline/Thioridazine

TCTP protein is a pharmacological target in cancer and TCTP inhibitors such as sertraline have been evaluated in clinical trials. The direct interaction of TCTP with the drugs sertraline and thioridazine has been reported in vitro by SPR experiments to be in the ∼30-50 μM K_d range (Amson et al. Nature Med 2012), supporting a TCTP-dependent mode of action of the drugs on tumor cells. However, the molecular details of the interaction remain elusive although they are crucial to improve the efforts of on-going medicinal chemistry. In addition, TCTP can be phosphorylated by the Plk-1 kinase, which is indicative of poor prognosis in several cancers. The impact of phosphorylation on TCTP structure/dynamics and binding with therapeutical ligands remains unexplored. Here, we combined NMR, TSA, SPR, BLI and ITC techniques to probe the molecular interactions between TCTP with the drugs sertraline and thioridazine. We reveal that drug binding is much weaker than reported with an apparent ∼mM K_d and leads to protein destabilization that obscured the analysis of the published SPR data. We further demonstrate by NMR and SAXS that TCTP S46 phosphorylation does not promote tighter interaction between TCTP and sertraline. Accordingly, we question the supported model in which sertraline and thioridazine directly interact with isolated TCTP in tumor cells and discuss alternative modes of action for the drugs in light of current literature.

Biased cytochrome P450-mediated metabolism via small-molecule ligands binding P450 oxidoreductase

Metabolic control is mediated by the dynamic assemblies and function of multiple redox enzymes. A key element in these assemblies, the P450 oxidoreductase (POR), donates electrons and selectively activates numerous (>50 in humans and >300 in plants) cytochromes P450 (CYPs) controlling metabolism of drugs, steroids and xenobiotics in humans and natural product biosynthesis in plants. The mechanisms underlying POR-mediated CYP metabolism remain poorly understood and to date no ligand binding has been described to regulate the specificity of POR. Here, using a combination of computational modeling and functional assays, we identify ligands that dock on POR and bias its specificity towards CYP redox partners, across mammal and plant kingdom. Single molecule FRET studies reveal ligand binding to alter POR conformational sampling, which results in biased activation of metabolic cascades in whole cell assays. We propose the model of biased metabolism, a mechanism akin to biased signaling of GPCRs, where ligand binding on POR stabilizes different conformational states that are linked to distinct metabolic outcomes. Biased metabolism may allow designing pathway-specific therapeutics or personalized food suppressing undesired, disease-related, metabolic pathways.

P450 variations bifurcate the early terpene indole alkaloid pathway in Catharanthus roseus and Camptotheca acuminata

Divergent terpene indole alkaloid (TIA) pathways in Catharanthus roseus and Camptotheca acuminata generate vinblastine and vincristine, and camptothecin, respectively. In contrast to Catharanthus which feeds secologanin (from methylated loganin) into its species-specific late pathway, Camptotheca feeds secologanic acid (from unmethylated loganic acid) into its late pathway. Having identified putative Camptotheca secologanic acid synthases (SLASs) and cytochrome P450 reductases (CPRs) in transcriptome databases, we have demonstrated that two P450s, CYP72A564 and CYP72A565, are capable of utilizing both loganic acid and loganin to generate secologanic acid and secologanin. We have extended the previous report of these activities by CYP72A565 and CYP72A610 (Yang et al., 2019) by demonstrating that both Arabidopsis CPRs (ATR1, ATR2) couple with these CYP72A proteins in yeast microsomal assays and that purified Camptotheca CPR1 couples with them in in vitro reconstitution assays. Kinetic analyses of purified full-length Camptotheca SLASs have indicated that both process loganic acid with nearly identical catalytic rates and efficiencies as measured by their k_cat and k_cat/K_M. In contrast, CYP72A564 processes loganin with two-fold greater efficiency than CYP72A565 correlating with the former's 3-fold greater affinity for loganin. The closely-related CYP72A730 does not bind or process either compound. Molecular modeling of these three proteins and comparisons with Catharanthus secologanin synthase (SLS) have identified key differences that likely determine their SLAS versus SLS selectivities. Our ability to reconstitute these SLAS/SLS activities provides valuable tools for further examinations of the residues involved in substrate recognition and determinations of their unusual mechanism of C-C bond scission.

Interaction Modes of Microsomal Cytochrome P450s with Its Reductase and the Role of Substrate Binding

The activity of microsomal cytochromes P450 (CYP) is strictly dependent on the supply of electrons provided by NADPH cytochrome P450 oxidoreductase (CPR). The variant nature of the isoform-specific proximal interface of microsomal CYPs implies that the interacting interface between the two proteins is degenerated. Recently, we demonstrated that specific CPR mutations in the FMN-domain (FD) may induce a gain in activity for a specific CYP isoform. In the current report, we confirm the CYP isoform dependence of CPR's degenerated binding by demonstrating that the effect of four of the formerly studied FD mutants are indeed exclusive of a specific CYP isoform, as verified by cytochrome c inhibition studies. Moreover, the nature of CYP's substrate seems to have a modulating role in the CPR:CYP interaction. In silico molecular dynamics simulations of the FD evidence that mutations induces very subtle structural alterations, influencing the characteristics of residues formerly implicated in the CPR:CYP interaction or in positioning of the FMN moiety. CPR seems therefore to be able to form effective interaction complexes with its structural diverse partners via a combination of specific structural features of the FD, which are functional in a CYP isoform dependent manner, and dependent on the substrate bound.

Conformational Equilibrium of NADPH-Cytochrome P450 Oxidoreductase Is Essential for Heme Oxygenase Reaction

Heme oxygenase (HO) catalyzes heme degradation using electrons supplied by NADPH-cytochrome P450 oxidoreductase (CPR). Electrons from NADPH flow first to FAD, then to FMN, and finally to the heme in the redox partner. Previous biophysical analyses suggest the presence of a dynamic equilibrium between the open and the closed forms of CPR. We previously demonstrated that the open-form stabilized CPR (ΔTGEE) is tightly bound to heme-HO-1, whereas the reduction in heme-HO-1 coupled with ΔTGEE is considerably slow because the distance between FAD and FMN in ΔTGEE is inappropriate for electron transfer from FAD to FMN. Here, we characterized the enzymatic activity and the reduction kinetics of HO-1 using the closed-form stabilized CPR (147CC514). Additionally, we analyzed the interaction between 147CC514 and heme-HO-1 by analytical ultracentrifugation. The results indicate that the interaction between 147CC514 and heme-HO-1 is considerably weak, and the enzymatic activity of 147CC514 is markedly weaker than that of CPR. Further, using cryo-electron microscopy, we confirmed that the crystal structure of ΔTGEE in complex with heme-HO-1 is similar to the relatively low-resolution structure of CPR complexed with heme-HO-1 in solution. We conclude that the "open-close" transition of CPR is indispensable for electron transfer from CPR to heme-HO-1.

The Role of the FMN-Domain of Human Cytochrome P450 Oxidoreductase in Its Promiscuous Interactions With Structurally Diverse Redox Partners

NADPH cytochrome P450 oxidoreductase (CPR) is the obligatory electron supplier that sustains the activity of microsomal cytochrome P450 (CYP) enzymes. The variant nature of the isoform-specific proximal interface of microsomal CYPs indicates that CPR is capable of multiple degenerated interactions with CYPs for electron transfer, through different binding mechanisms, and which are still not well-understood. Recently, we showed that CPR dynamics allows formation of open conformations that can be sampled by its structurally diverse redox partners in a CYP-isoform dependent manner. To further investigate the role of the CPR FMN-domain in effective binding of CPR to its diverse acceptors and to clarify the underlying molecular mechanisms, five different CPR-FMN-domain random mutant libraries were created. These libraries were screened for mutants with increased activity when combined with specific CYP-isoforms. Seven CPR-FMN-domain mutants were identified, supporting a gain in activity for CYP1A2 (P117H, G144C, A229T), 2A6 (P117L/L125V, G175D, H183Y), or 3A4 (N151D). Effects were evaluated using extended enzyme kinetic analysis, cytochrome b 5 competition, ionic strength effect on CYP activity, and structural analysis. Mutated residues were located either in or adjacent to several acidic amino acid stretches - formerly indicated to be involved in CPR:CYP interactions - or close to two tyrosine residues suggested to be involved in FMN binding. Several of the identified positions co-localize with mutations found in naturally occurring CPR variants that were previously shown to cause CYP-isoform-dependent effects. The mutations do not seem to significantly alter the geometry of the FMN-domain but are likely to cause very subtle alterations leading to improved interaction with a specific CYP. Overall, these data suggest that CYPs interact with CPR using an isoform specific combination of several binding motifs of the FMN-domain.

Biochemical and structural insights into the cytochrome P450 reductase from Candida tropicalis

Cytochrome P450 reductases (CPRs) are diflavin oxidoreductases that supply electrons to type II cytochrome P450 monooxygenases (CYPs). In addition, it can also reduce other proteins and molecules, including cytochrome c, ferricyanide, and different drugs. Although various CPRs have been functionally and structurally characterized, the overall mechanism and its interaction with different redox acceptors remain elusive. One of the main problems regarding electron transfer between CPRs and CYPs is the so-called “uncoupling”, whereby NAD(P)H derived electrons are lost due to the reduced intermediates’ (FAD and FMN of CPR) interaction with molecular oxygen. Additionally, the decay of the iron-oxygen complex of the CYP can also contribute to loss of reducing equivalents during an unproductive reaction cycle. This phenomenon generates reactive oxygen species (ROS), leading to an inefficient reaction. Here, we present the study of the CPR from Candida tropicalis (CtCPR) lacking the hydrophobic N-terminal part (Δ2–22). The enzyme supports the reduction of cytochrome c and ferricyanide, with an estimated 30% uncoupling during the reactions with cytochrome c. The ROS produced was not influenced by different physicochemical conditions (ionic strength, pH, temperature). The X-ray structures of the enzyme were solved with and without its cofactor, NADPH. Both CtCPR structures exhibited the closed conformation. Comparison with the different solved structures revealed an intricate ionic network responsible for the regulation of the open/closed movement of CtCPR.

Biochemical and structural insights into the cytochrome P450 reductase from Candida tropicalis

Cytochrome P450 reductases (CPRs) are diflavin oxidoreductases that supply electrons to type II cytochrome P450 monooxygenases (CYPs). In addition, it can also reduce other proteins and molecules, including cytochrome c, ferricyanide, and different drugs. Although various CPRs have been functionally and structurally characterized, the overall mechanism and its interaction with different redox acceptors remain elusive. One of the main problems regarding electron transfer between CPRs and CYPs is the so-called "uncoupling", whereby NAD(P)H derived electrons are lost due to the reduced intermediates' (FAD and FMN of CPR) interaction with molecular oxygen. Additionally, the decay of the iron-oxygen complex of the CYP can also contribute to loss of reducing equivalents during an unproductive reaction cycle. This phenomenon generates reactive oxygen species (ROS), leading to an inefficient reaction. Here, we present the study of the CPR from Candida tropicalis (CtCPR) lacking the hydrophobic N-terminal part (Δ2-22). The enzyme supports the reduction of cytochrome c and ferricyanide, with an estimated 30% uncoupling during the reactions with cytochrome c. The ROS produced was not influenced by different physicochemical conditions (ionic strength, pH, temperature). The X-ray structures of the enzyme were solved with and without its cofactor, NADPH. Both CtCPR structures exhibited the closed conformation. Comparison with the different solved structures revealed an intricate ionic network responsible for the regulation of the open/closed movement of CtCPR.

Coupling of Redox and Structural States in Cytochrome P450 Reductase Studied by Molecular Dynamics Simulation

Cytochrome P450 reductase (CPR) is the key protein that regulates the electron transfer from NADPH to various heme-containing monooxygenases. CPR has two flavin-containing domains: one with flavin adenine dinucleotide (FAD), called FAD domain, and the other with flavin mononucleotide (FMN), called FMN domain. It is considered that the electron transfer occurs via FAD and FMN (NADPH → FAD → FMN → monooxygenase) and is regulated by an interdomain open-close motion. It is generally thought that the structural state is coupled with the redox state, which, however, has not yet been firmly established. In this report, we studied the coupling of the redox and the structural states by full-scale molecular dynamics (MD) simulation of CPR (total 86.4 μs). Our MD result showed that while CPR predominantly adopts the closed state both in the oxidized and reduced states, it exhibits a tendency to open in the reduced state. We also found a correlation between the FAD-FMN distance and the predicted FMN-monooxygenase distance, which is embedded in the equilibrium thermal fluctuation of CPR. Based on these results, a physical mechanism for the electron transfer by CPR is discussed.

Enzyme-fusion strategies for redirecting and improving carotenoid synthesis in S. cerevisiae

Spatial clustering of enzymes has proven an elegant approach to optimize metabolite transfer between enzymes in synthetic metabolic pathways. Among the multiple methods used to promote colocalisation, enzyme fusion is probably the simplest. Inspired by natural systems, we have explored the metabolic consequences of spatial reorganizations of the catalytic domains of Xanthophyllomyces dendrorhous carotenoid enzymes produced in Saccharomyces cerevisiae. Synthetic genes encoding bidomain enzymes composed of CrtI and CrtB domains from the natural CrtYB fusion were connected in the two possible orientations, using natural and synthetic linkers. A tridomain enzyme (CrtB, CrtI, CrtY) harboring the full β-carotene producing pathway was also constructed. Our results demonstrate that domain order and linker properties considerably impact both the expression and/or stability of the constructed proteins and the functionality of the catalytic domains, all concurring to either diminish or boost specific enzymatic steps of the metabolic pathway. Remarkably, the yield of β-carotene production doubled with the tridomain fusion while precursor accumulation decreased, leading to an improvement of the pathway efficiency, when compared to the natural system. Our data strengthen the idea that fusion of enzymatic domains is an appropriate technique not only to achieve spatial confinement and enhance the metabolic flux but also to produce molecules not easily attainable with natural enzymatic configurations, even with membrane bound enzymes.

Probing the Role of the Hinge Segment of Cytochrome P450 Oxidoreductase in the Interaction with Cytochrome P450

NADPH-cytochrome P450 reductase (CPR) is the unique redox partner of microsomal cytochrome P450s (CYPs). CPR exists in a conformational equilibrium between open and closed conformations throughout its electron transfer (ET) function. Previously, we have shown that electrostatic and flexibility properties of the hinge segment of CPR are critical for ET. Three mutants of human CPR were studied (S243P, I245P and R246A) and combined with representative human drug-metabolizing CYPs (isoforms 1A2, 2A6 and 3A4). To probe the effect of these hinge mutations different experimental approaches were employed: CYP bioactivation capacity of pre-carcinogens, enzyme kinetic analysis, and effect of the ionic strength and cytochrome b₅ (CYB5) on CYP activity. The hinge mutations influenced the bioactivation of pre-carcinogens, which seemed CYP isoform and substrate dependent. The deviations of Michaelis-Menten kinetic parameters uncovered tend to confirm this discrepancy, which was confirmed by CYP and hinge mutant specific salt/activity profiles. CPR/CYB5 competition experiments indicated a less important role of affinity in CPR/CYP interaction. Overall, our data suggest that the highly flexible hinge of CPR is responsible for the existence of a conformational aggregate of different open CPR conformers enabling ET-interaction with structural varied redox partners.

Preparation and characterization of Aloe vera acetate and electrospinning fibers as promising antibacterial properties materials

In this work, the anti-bacterial effect of Aloe vera derivate fibers produced by the electrospinning method was reported. Aloe vera Polyvinylpyrrolidone (Av/PVP) and Aloe vera acetate-Polyvinylpyrrolidone (AvAc/PVP) electrospun fibers were prepared with different concentrations and their microstructure and mechanical properties were studied. Various methods such as differential scanning calorimetry (DSC), thermogravimetry analysis (TGA), water contact angle (CA) tests, Fourier-Transform Nuclear Magnetic Resonance (FT-NMR), scanning electron microscope (SEM), X-ray diffraction (XRD), CHNSO and Fourier-Transform Infrared Spectroscopy (FT-IR) were used to characterize prepared samples. (Av/PVP) electrospun fibers were prepared with different concentrations (6-10 wt%) of PVP and 0.2 wt% Av blended and tested in medicinal herb for wound healing, antibacterial and anti-inflammatory properties. For further study, the effect of AvAc film on the properties of composite film was studied. AvAc increased the thermal stability and crystallite size percentage of samples. Antibacterial and antiviral test studies on the scaffold displayed no bacterial and viral growth. These results suggest that AvAc/PVP scaffolds could be promising candidates for wound healing applications.

Direct observation of multiple conformational states in Cytochrome P450 oxidoreductase and their modulation by membrane environment and ionic strength

Cytochrome P450 oxidoreductase (POR) is the primary electron donor in eukaryotic cytochrome P450 (CYP) containing systems. A wealth of ensemble biophysical studies of Cytochrome P450 oxidoreductase (POR) has reported a binary model of the conformational equilibrium directing its catalytic efficiency and biomolecular recognition. In this study, full length POR from the crop plant Sorghum bicolor was site-specifically labeled with Cy3 (donor) and Cy5 (acceptor) fluorophores and reconstituted in nanodiscs. Our single molecule fluorescence resonance energy transfer (smFRET) burst analyses of POR allowed the direct observation and quantification of at least three dominant conformational sub-populations, their distribution and occupancies. Moreover, the state occupancies were remodeled significantly by ionic strength and the nature of reconstitution environment, i.e. phospholipid bilayers (nanodiscs) composed of different lipid head group charges vs. detergent micelles. The existence of conformational heterogeneity in POR may mediate selective activation of multiple downstream electron acceptors and association in complexes in the ER membrane.

The Hinge Segment of Human NADPH-Cytochrome P450 Reductase in Conformational Switching: The Critical Role of Ionic Strength

NADPH-cytochrome P450 reductase (CPR) is a redox partner of microsomal cytochromes P450 and is a prototype of the diflavin reductase family. CPR contains 3 distinct functional domains: a FMN-binding domain (acceptor reduction), a linker (hinge), and a connecting/FAD domain (NADPH oxidation). It has been demonstrated that the mechanism of CPR exhibits an important step in which it switches from a compact, closed conformation (locked state) to an ensemble of open conformations (unlocked state), the latter enabling electron transfer to redox partners. The conformational equilibrium between the locked and unlocked states has been shown to be highly dependent on ionic strength, reinforcing the hypothesis of the presence of critical salt interactions at the interface between the FMN and connecting FAD domains. Here we show that specific residues of the hinge segment are important in the control of the conformational equilibrium of CPR. We constructed six single mutants and two double mutants of the human CPR, targeting residues G240, S243, I245 and R246 of the hinge segment, with the aim of modifying the flexibility or the potential ionic interactions of the hinge segment. We measured the reduction of cytochrome c at various salt concentrations of these 8 mutants, either in the soluble or membrane-bound form of human CPR. All mutants were found capable of reducing cytochrome c yet with different efficiency and their maximal rates of cytochrome c reduction were shifted to lower salt concentration. In particular, residue R246 seems to play a key role in a salt bridge network present at the interface of the hinge and the connecting domain. Interestingly, the effects of mutations, although similar, demonstrated specific differences when present in the soluble or membrane-bound context. Our results demonstrate that the electrostatic and flexibility properties of the hinge segment are critical for electron transfer from CPR to its redox partners.

Orchestrated Domain Movement in Catalysis by Cytochrome P450 Reductase

NADPH-cytochrome P450 reductase is a multi-domain redox enzyme which is a key component of the P450 mono-oxygenase drug-metabolizing system. We report studies of the conformational equilibrium of this enzyme using small-angle neutron scattering, under conditions where we are able to control the redox state of the enzyme precisely. Different redox states have a profound effect on domain orientation in the enzyme and we analyse the data in terms of a two-state equilibrium between compact and extended conformations. The effects of ionic strength show that the presence of a greater proportion of the extended form leads to an enhanced ability to transfer electrons to cytochrome c. Domain motion is intrinsically linked to the functionality of the enzyme, and we can define the position of the conformational equilibrium for individual steps in the catalytic cycle.

Restricting the conformational freedom of the neuronal nitric-oxide synthase flavoprotein domain reveals impact on electron transfer and catalysis

The signaling molecule nitric oxide (NO) is synthesized in animals by structurally related NO synthases (NOSs), which contain NADPH/FAD- and FMN-binding domains. During catalysis, NADPH-derived electrons transfer into FAD and then distribute into the FMN domain for further transfer to internal or external heme groups. Conformational freedom of the FMN domain is thought to be essential for the electron transfer (ET) reactions in NOSs. To directly examine this concept, we utilized a "Cys-lite" neuronal NOS flavoprotein domain and substituted Cys for two residues (Glu-816 and Arg-1229) forming a salt bridge between the NADPH/FAD and FMN domains in the conformationally closed structure to allow cross-domain disulfide bond formation or cross-linking by bismaleimides of various lengths. The disulfide bond cross-link caused a ≥95% loss of cytochrome c reductase activity that was reversible with DTT treatment, whereas graded cross-link lengthening gradually increased activity, thus defining the conformational constraints in the catalytic process. We used spectroscopic and stopped-flow techniques to further investigate how the changes in FMN domain conformational freedom impact the following: (i) the NADPH interaction; (ii) kinetics of electron loading (flavin reduction); (iii) stabilization of open versus closed conformational forms in two different flavin redox states; (iv) reactivity of the reduced FMN domain toward cytochrome c; (v) response to calmodulin binding; and (vi) the rates of interflavin ET and the FMN domain conformational dynamics. Together, our findings help explain how the spatial and temporal behaviors of the FMN domain impact catalysis by the NOS flavoprotein domain and how these behaviors are governed to enable electron flow through the enzyme.

Conformational States of Cytochrome P450 Oxidoreductase Evaluated by Förster Resonance Energy Transfer Using Ultrafast Transient Absorption Spectroscopy

NADPH-cytochrome P450 oxidoreductase (CYPOR) was shown to undergo large conformational rearrangements in its functional cycle. Using a new Förster resonance energy transfer (FRET) approach based on femtosecond transient absorption spectroscopy (TA), we determined the donor-acceptor distance distribution in the reduced and oxidized states of CYPOR. The unmatched time resolution of TA allowed the quantitative assessment of the donor-acceptor FRET, indicating that CYPOR assumes a closed conformation in both reduced and oxidized states in the absence of the redox partner. The described ultrafast TA measurements of FRET with readily available red-infrared fluorescent labels open new opportunities for structural studies in chromophore-rich proteins and their complexes.

Phosphorylation Controls Endothelial Nitric-oxide Synthase by Regulating Its Conformational Dynamics

The activity of endothelial NO synthase (eNOS) is triggered by calmodulin (CaM) binding and is often further regulated by phosphorylation at several positions in the enzyme. Phosphorylation at Ser¹¹⁷⁹ occurs in response to diverse physiologic stimuli and increases the NO synthesis and cytochrome c reductase activities of eNOS, thereby enhancing its participation in biological signal cascades. Despite its importance, the mechanism by which Ser¹¹⁷⁹ phosphorylation increases eNOS activity is not understood. To address this, we used stopped-flow spectroscopy and computer modeling approaches to determine how the phosphomimetic mutation (S1179D) may impact electron flux through eNOS and the conformational behaviors of its reductase domain, both in the absence and presence of bound CaM. We found that S1179D substitution in CaM-free eNOS had multiple effects; it increased the rate of flavin reduction, altered the conformational equilibrium of the reductase domain, and increased the rate of its conformational transitions. We found these changes were equivalent in degree to those caused by CaM binding to wild-type eNOS, and the S1179D substitution together with CaM binding caused even greater changes in these parameters. The modeling indicated that the changes caused by the S1179D substitution, despite being restricted to the reductase domain, are sufficient to explain the stimulation of both the cytochrome c reductase and NO synthase activities of eNOS. This helps clarify how Ser¹¹⁷⁹ phosphorylation regulates eNOS and provides a foundation to compare its regulation by other phosphorylation events.