Cytochrome b5, its functions, structure and membrane topology

Bielectrode Strategy for Determination of CYP2E1 Catalytic Activity: Electrodes with Bactosomes and Voltammetric Determination of 6-Hydroxychlorzoxazone

We describe a bielectrode system for evaluation of the electrocatalytic activity of cytochrome P450 2E1 (CYP2E1) towards chlorzoxazone. One electrode of the system was employed to immobilize Bactosomes with human CYP2E1, cytochrome P450 reductase (CPR), and cytochrome b5 (cyt b5). The second electrode was used to quantify CYP2E1-produced 6-hydroxychlorzoxazone by its direct electrochemical oxidation, registered using square-wave voltammetry. Using this system, we determined the steady-state kinetic parameters of chlorzoxazone hydroxylation by CYP2E1 of Bactosomes immobilized on the electrode: the maximal reaction rate (Vmax) was 1.64 ± 0.08 min-1, and the Michaelis constant (KM) was 78 ± 9 μM. We studied the electrochemical characteristics of immobilized Bactosomes and have revealed that electron transfer from the electrode occurs both to the flavin prosthetic groups of CPR and the heme iron ions of CYP2E1 and cyt b5. Additionally, it has been demonstrated that CPR has the capacity to activate CYP2E1 electrocatalytic activity towards chlorzoxazone, likely through intermolecular electron transfer from the electrochemically reduced form of CPR to the CYP2E1 heme iron ion.

Direct interaction between the transmembrane helices stabilize cytochrome P450 2B4 and cytochrome b5 redox complex

The catalytic activity of cytochrome P450 2B4 (CYP2B4) is moderated by its cognate redox partner cytochrome b5 (Cyt-b5). The endoplasmic reticulum (ER) membrane and intermolecular transmembrane (TM) interaction between CYP2B4 and Cyt-b5 regulate the substrate catalysis and the reaction rate. This emphasizes the significance of elucidating the molecular basis of CYP2B4 and Cyt-b5 complexation in a membrane environment to better understand the enzymatic activity of CYP2B4. Our previous solid-state NMR studies revealed the membrane topology of the transmembrane domains of these proteins in the free and complex forms. Here, we show the cross-angle complex formation by the single-pass TM domains of CYP2B4 and Cyt-b5, which is mainly driven by several salt-bridges (E2-R128, R21-D104 and K25-D104), using a multi-microsecond molecular dynamic simulation. Additionally, the leucine-zipper residues (L8, L12, L15, L18 and L19 from CYP2B4) and π-stacking between H23 and F20 residues of CYP2B4 and W110 of Cyt-b5 are identified to stabilize the TM-TM complex in the ER membrane. The simulated tilts of the helices in the free and in the complex are in excellent agreement with solid-state NMR results. The TM-TM packing influences a higher order structural stability when compared to the complex formed by the truncated soluble domains of these two proteins. MM/PBSA based binding free energy estimates nearly 100-fold higher binding affinity (ΔG = -2810.68 ± 696.44 kJ/mol) between the soluble domains of the full-length CYP2B4 and Cyt-b5 when embedded in lipid membrane as compared to the TM-domain-truncated soluble domains (ΔG = -27.406 ± 10.32 kJ/mol). The high-resolution full-length CYP2B4-Cyt-b5 complex structure and its dynamics in a native ER membrane environment reported here could aid in the development of approaches to effectively modulate the drug-metabolism activity of CYP2B4.

Free ferrous ions sustain activity of mammalian stearoyl-CoA desaturase-1

Mammalian stearoyl-CoA desaturase-1 (SCD1) introduces a double-bond to a saturated long-chain fatty acid in a reaction catalyzed by a diiron center. The diiron center is well-coordinated by conserved histidine residues and is thought to remain with the enzyme. However, we find here that SCD1 progressively loses its activity during catalysis and becomes fully inactive after nine turnovers. Further studies show that the inactivation of SCD1 is due to the loss of an iron (Fe) ion in the diiron center, and that the addition of free ferrous ions (Fe²⁺) sustains the enzymatic activity. Using SCD1 labeled with Fe isotope, we further show that free Fe²⁺ is incorporated into the diiron center only during catalysis. We also discover that the diiron center in SCD1 has prominent electron paramagnetic resonance signals in its diferric state, indicative of distinct coupling between the two ferric ions. These results reveal that the diiron center in SCD1 is structurally dynamic during catalysis and that labile Fe²⁺ in cells could regulate SCD1 activity, and hence lipid metabolism.

Free ferrous ions sustain activity of mammalian stearoyl-CoA desaturase-1

Mammalian stearoyl-CoA desaturase-1 (SCD1) introduces a double-bond to a saturated long-chain fatty acid and the reaction is catalyzed by a diiron center, which is well-coordinated by conserved histidine residues and is thought to remain with enzyme. However, we find that SCD1 progressively loses its activity during catalysis and becomes fully inactive after nine turnovers. Further studies show that the inactivation of SCD1 is due to the loss of an iron (Fe) ion in the diiron center, and that the addition of free ferrous ions (Fe ²⁺ ) sustains the enzymatic activity. Using SCD1 labeled with Fe isotope, we further show that free Fe ²⁺ is incorporated into the diiron center only during catalysis. We also discover that the diiron center in SCD1 has prominent electron paramagnetic resonance signals in its diferric state, indicative of distinct coupling between the two ferric ions. These results reveal that the diiron center in SCD1 is structurally dynamic during catalysis and that labile Fe ²⁺ in cells could regulate SCD1 activity, and hence lipid metabolism.

Proteomic characterization of spontaneously regrowing spinal cord following injury in the teleost fish Apteronotus leptorhynchus, a regeneration-competent vertebrate

In adult mammals, spontaneous repair after spinal cord injury (SCI) is severely limited. By contrast, teleost fish successfully regenerate injured axons and produce new neurons from adult neural stem cells after SCI. The molecular mechanisms underlying this high regenerative capacity are largely unknown. The present study addresses this gap by examining the temporal dynamics of proteome changes in response to SCI in the brown ghost knifefish (Apteronotus leptorhynchus). Two-dimensional difference gel electrophoresis (2D DIGE) was combined with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and tandem mass spectrometry (MS/MS) to collect data during early (1 day), mid (10 days), and late (30 days) phases of regeneration following caudal amputation SCI. Forty-two unique proteins with significant differences in abundance between injured and intact control samples were identified. Correlation analysis uncovered six clusters of spots with similar expression patterns over time and strong conditional dependences, typically within functional families or between isoforms. Significantly regulated proteins were associated with axon development and regeneration; proliferation and morphogenesis; neuronal differentiation and re-establishment of neural connections; promotion of neuroprotection, redox homeostasis, and membrane repair; and metabolism or energy supply. Notably, at all three time points examined, significant regulation of proteins involved in inflammatory responses was absent.

Cytochrome b 5: A versatile electron carrier and regulator for plant metabolism

Cytochrome b ₅ (CB5) is a small heme-binding protein, known as an electron donor delivering reducing power to the terminal enzymes involved in oxidative reactions. In plants, the CB5 protein family is substantially expanded both in its isoform numbers and cellular functions, compared to its yeast and mammalian counterparts. As an electron carrier, plant CB5 proteins function not only in fatty acid desaturation, hydroxylation and elongation, but also in the formation of specialized metabolites such as flavonoids, phenolic esters, and heteropolymer lignin. Furthermore, plant CB5s are found to interact with different non-catalytic proteins such as ethylene signaling regulator, cell death inhibitor, and sugar transporters, implicating their versatile regulatory roles in coordinating different metabolic and cellular processes, presumably in respect to the cellular redox status and/or carbon availability. Compared to the plentiful studies on biochemistry and cellular functions of mammalian CB5 proteins, the cellular and metabolic roles of plant CB5 proteins have received far less attention. This article summarizes the fragmentary information pertaining to the discovery of plant CB5 proteins, and discusses the conventional and peculiar functions that plant CB5s might play in different metabolic and cellular processes. Gaining comprehensive insight into the biological functions of CB5 proteins could offer effective biotechnological solutions to tailor plant chemodiversity and cellular responses to environment stimuli.

Transmembrane helices mediate the formation of a stable ternary complex of b5R, cyt b5, and SCD1

Mammalian cytochrome b₅ (cyt b₅) and cytochrome b₅ reductase (b₅R) are electron carrier proteins for membrane-embedded oxidoreductases. Both b₅R and cyt b₅ have a cytosolic domain and a single transmembrane (TM) helix. The cytosolic domains of b₅R and cyt b₅ contain cofactors required for electron transfer, but it is not clear if the TM helix has function beyond being an anchor to the membrane. Here we show that b₅R and cyt b₅ form a stable binary complex, and so do cyt b₅ and stearoyl-CoA desaturase-1 (SCD1). We also show that b₅R, cyt b₅ and SCD1 form a stable ternary complex. We demonstrate that the TM helices are required for the assembly of stable binary and ternary complexes where electron transfer rates are greatly enhanced. These results reveal a role of the TM helix in cyt b₅ and b₅R, and suggest that an electron transport chain composed of a stable ternary complex may be a general feature in membrane-embedded oxidoreductases that require cyt b₅ and b₅R.

The transmembrane domains of mammalian cytochrome b₅ (cyt b₅), cyt b₅ reductase (b₅R), and stearoyl-CoA desaturase-1 (SCD1) form stable binary complexes between cyt b₅/b₅R or cyt b₅/SCD1 and a ternary complex, which enhance electron transfer rates.

Biochemical Investigation of Membrane-Bound Cytochrome b5 and the Catalytic Domain of Cytochrome b5 Reductase from Arabidopsis thaliana

The endoplasmic reticulum (ER) membrane of plant cells contains several enzymes responsible for the biosynthesis of a diverse range of molecules essential for plant growth and holds potential for industrial applications. Many of these enzymes are dependent on electron transfer proteins to sustain their catalytic cycles. In plants, two crucial ER-bound electron transfer proteins are cytochrome b5 and cytochrome b5 reductase, which catalyze the stepwise transfer of electrons from NADH to redox enzymes such as fatty acid desaturases, cytochrome P450s, and plant aldehyde decarbonylase. Despite the high significance of plant cytochrome b5 and cytochrome b5 reductase, they have eluded detailed characterization to date. Here, we overexpressed the full-length membrane-bound cytochrome b5 isoform B from the model plant Arabidopsis thaliana in Escherichia coli, purified the protein employing detergents as well as styrene-maleic acid (SMA) copolymers, and biochemically characterized the protein. The SMA-encapsulated cytochrome b5 exhibits a discoidal shape and the characteristic features of the active heme-bound state. We also overexpressed and purified the soluble domain of cytochrome b5 reductase from A. thaliana, establishing its activity, stability, and kinetic parameters. Further, we demonstrated that the plant cytochrome b5, purified in detergents and styrene maleic acid lipid particles (SMALPs), readily accepts electrons from the cognate plant cytochrome b5 reductase and distant electron mediators such as plant NADPH-cytochrome P450 oxidoreductase and cyanobacterial NADPH-ferredoxin reductase. We also measured the kinetic parameters of cytochrome b5 reductase for cytochrome b5. Our studies are the first to report the purification and detailed biochemical characterization of the plant cytochrome b5 and cytochrome b5 reductase from the bacterial overexpression system.

Redox metabolism for improving whole-cell P450-catalysed terpenoid biosynthesis

The growing preference for producing cytochrome P450-mediated natural products in microbial systems stems from the challenging nature of the organic chemistry approaches. The P450 enzymes are redox-dependent proteins, through which they source electrons from reducing cofactors to drive their activities. Widely researched in biochemistry, most of the previous studies have extensively utilised expensive cell-free assays to reveal mechanistic insights into P450 functionalities in presence of commercial redox partners. However, in the context of microbial bioproduction, the synergic activity of P450- reductase proteins in microbial systems have not been largely investigated. This is mainly due to limited knowledge about their mutual interactions in the context of complex systems. Hence, manipulating the redox potential for natural product synthesis in microbial chassis has been limited. As the potential of redox state as crucial regulator of P450 biocatalysis has been greatly underestimated by the scientific community, in this review, we re-emphasize their pivotal role in modulating the in vivo P450 activity through affecting the product profile and yield. Particularly, we discuss the applications of widely used in vivo redox engineering methodologies for natural product synthesis to provide further suggestions for patterning on P450-based terpenoids production in microbial platforms.

Optimization of Multiparameters for Increased Yields of Cytochrome B5 in Bioreactors

The production of recombinant proteins is gaining increasing importance as the market requests high quality proteins for several applications. However, several process parameters affect both the growth of cells and product yields. This study uses high throughput systems and statistical methods to assess the influence of fermentation conditions in lab-scale bioreactors. Using this methodology, it was possible to find the best conditions to produce cytochrome b5 with recombinant cells of Escherichia coli. Using partial least squares, the height-to-diameter ratio of the bioreactor, aeration rate, and PID controller parameters were found to contribute significantly to the final biomass and cytochrome concentrations. Hence, we could use this information to fine-tune the process parameters, which increased cytochrome production and yield several-fold. Using aeration of 1 vvm, a bioreactor with a height-to-ratio of 2.4 and tuned PID parameters, a production of 72.72 mg/L of cytochrome b5 in the culture media, and a maximum of product to biomass yield of 24.97 mg/g could be achieved.

Endocrine and molecular milieus of ovarian follicles are diversely affected by human chorionic gonadotropin and gonadotropin-releasing hormone in prepubertal and mature gilts

Different strategies are used to meet optimal reproductive performance or manage reproductive health. Although exogenous human chorionic gonadotropin (hCG) and gonadotropin-releasing hormone (GnRH) agonists (A) are commonly used to trigger ovulation in estrous cycle synchronization, little is known about their effect on the ovarian follicle. Here, we explored whether hCG- and GnRH-A-induced native luteinizing hormone (LH) can affect the endocrine and molecular milieus of ovarian preovulatory follicles in pigs at different stages of sexual development. We collected ovaries 30 h after hCG/GnRH-A administration from altrenogest and pregnant mare serum gonadotropin (eCG)-primed prepubertal and sexually mature gilts. Several endocrine and molecular alternations were indicated, including broad hormonal trigger-induced changes in follicular fluid steroid hormones and prostaglandin levels. However, sexual maturity affected only estradiol levels. Trigger- and/or maturity-dependent changes in the abundance of hormone receptors (FSHR and LHCGR) and proteins associated with lipid metabolism and steroidogenesis (e.g., STAR, HSD3B1, and CYP11A1), prostaglandin synthesis (PTGS2 and PTGFS), extracellular matrix remodeling (MMP1 and TIMP1), protein folding (HSPs), molecular transport (TF), and cell function and survival (e.g., VIM) were observed. These data revealed different endocrine properties of exogenous and endogenous gonadotropins, with a potent progestational/androgenic role of hCG and estrogenic/pro-developmental function of LH.

Mechanism of electron transfers mediated by cytochromes c and b5 in mitochondria and endoplasmic reticulum: classical and murburn perspectives

We explore the mechanism of electron transfers mediated by cytochrome c, a soluble protein involved in mitochondrial oxidative phosphorylation and cytochrome b₅, a microsomal membrane protein acting as a redox aide in xenobiotic metabolism. We found minimal conservation in the sequence and surface amino acid residues of cytochrome c/b₅ proteins among divergent species. Therefore, we question the evolutionary logic for electron transfer (ET) occurring through affinity binding via recognition of specific surface residues/topography. Also, analysis of putative protein-protein interactions in the crystal structures of these proteins and their redox partners did not point to any specific interaction logic. A comparison of the kinetic and thermodynamic constants of wildtype vs. mutants did not provide strong evidence to support the binding-based ET paradigm, but indicated support for diffusible reactive species (DRS)-mediated process. Topographically divergent cytochromes from one species have been substituted for reaction with proteins from other species, implying the involvement of non-specific interactions. We provide a viable alternative (murburn concept) to classical protein-protein binding-based long range ET mechanism. To account for the promiscuity of interactions and solvent-accessible hemes, we propose that the two proteins act as non- specific redox capacitors, mediating one-electron redox equilibriums involving DRS and unbound ions.Communicated by Ramaswamy H. Sarma.

Substrate-Specific Allosteric Effects on the Enhancement of CYP17A1 Lyase Efficiency by Cytochrome b5

CYP17A1 is an essential human steroidogenic enzyme, which catalyzes two sequential reactions leading to the formation of androstenedione from progesterone and dehydroepiandrosterone from pregnenolone. The second reaction is the C17-C20 bond scission, which is strongly dependent on the presence of cytochrome b₅ and displays a heretofore unexplained more pronounced acceleration when 17OH-progesteone (17OH-PROG) is a substrate. The origin of the stimulating effect of cytochrome b₅ on C-C bond scission catalyzed by CYP17A1 is still debated as mostly due to either the acceleration of the electron transfer to the P450 oxy complex or allosteric effects of cytochrome b₅ favoring active site conformations that promote lyase activity. Using resonance Raman spectroscopy, we compared the effect of Mn-substituted cytochrome b₅ (Mn-Cytb₅) on the oxy complex of CYP17A1 with both proteins co-incorporated in lipid nanodiscs. For CYP17A1 with 17OH-PROG, a characteristic shift of the Fe-O mode is observed in the presence of Mn-b₅, indicating reorientation of a hydrogen bond between the 17OH group of the substrate from the terminal to the proximal oxygen atom of the Fe-O-O moiety, a configuration favorable for the lyase catalysis. For 17OH-pregnenolone, no such shift is observed, the favorable H-bonding orientation being present even without Mn-Cytb₅. These new data provide a precise allosteric interpretation for the more pronounced acceleration seen for the 17OH-PROG substrate.

Structure and Mechanism of a Unique Diiron Center in Mammalian Stearoyl-CoA Desaturase

Stearoyl-CoA desaturase 1 (SCD1) is a membrane-embedded metalloenzyme that catalyzes the formation of a double bond on a saturated acyl-CoA. SCD1 has a diiron center and its proper function requires an electron transport chain composed of NADH (or NADPH), cytochrome b5 reductase (b5R), and cytochrome b5 (cyt b5). Since SCD1 is a key regulator in fat metabolism and is required for survival of cancer cells, there is intense interest in targeting SCD1 for various metabolic diseases and cancers. Crystal structures of human and mouse SCD1 were reported recently; however, both proteins have two zinc ions instead of two iron ions in the catalytic center, and as a result, the enzymes are inactive. Here we report a general approach for incorporating iron into heterologously expressed proteins in HEK293 cells. We produced mouse SCD1 that contains a diiron center and visualized its diiron center by solving its crystal structure to 3.5 Å. We assembled the entire electron transport chain using the purified soluble domains of cyt b5 and b5R, and the purified mouse SCD1, and we showed that three proteins coordinate to produce proper products. These results established an in vitro system that allows precise perturbations of the electron transport chain for the understanding of the catalytic mechanism in SCD1.

Genome-Wide Identification and Analysis of the Cytochrome B5 Protein Family in Chinese Cabbage (Brassica rapa L. ssp. Pekinensis)

Cytochrome B5 (CB5) family proteins play an important role in various oxidation/reduction reactions in cells as the electron donor and are involved in a variety of biotic and abiotic stress processes. However, the function of the CB5s in Brassica rapa is still unclear. In this study, we carried out genome-wide identification, characterization, and expression analysis of BrCB5s in different tissues under adversities and stresses. It was identified that fifteen BrCB5s were distributed on different chromosomes, which were classified into seven groups (A-G) according to its phylogenetic relationship. Phylogenetic analysis of the CB5 protein sequences from six species showed that the BrCB5s conduct a close evolutionary process with the CB5s of Arabidopsis thaliana and far from those of Oryza sativa. Protein interaction analysis showed that 40 interaction patterns were predicted including two Sucrose Transporter 4 subfamily proteins (SUT 4) and Fatty Acid Hydroxylase 2 protein (FAH 2) can interact with most members of BrCB5s. The expression profile analysis indicated that BrCB5s were differentially expressed in different tissues, and the transcript abundances were significantly different under various abiotic stresses and plant hormone treatments. Our study provides a basis for a better understanding of the characteristics and biological functions of the CB5 family genes in Chinese cabbage during plant development, especially in plant responses to multiple stresses.

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.

Identification of cytochrome b5 CYTB-5.1 and CYTB-5.2 in C. elegans; evidence for differential regulation of SCD

Unsaturated fatty acids (UFAs) play crucial roles in living organisms regarding development, energy metabolism, stress resistance, etc. The biosynthesis of UFAs starts from the introduction of the first double bond by stearoyl-CoA desaturase (SCD), converting saturated fatty acids (SFAs) to monounsaturated fatty acids (MUFAs). This desaturation is considered to be an aerobic process that requires cytochrome b5 reductase, cytochrome b5 and SCD. However, this enzyme system remains elusive in Caenorhabditis elegans. Here, we show that inactivation by RNAi knockdown or mutation (gk442189) of putative cytochrome b5 genes cytb-5.1 led to reduced conversion of C18:0 to C18:1(n-9) by SCD desaturases FAT-6/7 in C. elegans. On the contrary, cytb-5.2RNAi and cytb-5.2(gk113588) mutant worms showed decreased conversion of C16:0 to C16:1(n-7) by FAT-5 desaturase. Dietary supplementation with C18:1(n-9) and C18:2(n-6) also showed that CYTB-5.1 is likely required for the activity of FAT-6/7 desaturases, but not for FAT-1 to FAT-4 desaturases. Interestingly, co-immunoprecipitation (Co-IP) demonstrated that either FAT-7 or FAT-5 has ability to interact with both CYTB-5.1 and CYTB-5.2. Moreover, RNAi knockdown of cytb-5.1 upregulates the transcriptional and translational expression of fat-5 to fat-7, which may be due to the feedback induction by reduced C18:1(n-9) and downstream fatty acids. Furthermore, both CYTB-5.1 and CYTB-5.2 are involved in fat accumulation, fertility and lifespan in worms, which may be independent of changes in fatty acid compositions. Collectively, these findings for the first time reveal the differential regulation of various SCDs by distinct cytochrome b5 CYTB-5.1 and CYTB-5.2 in the biosynthesis of UFAs in C. elegans.

Kinetic and Structural Characterization of the Effects of Membrane on the Complex of Cytochrome b 5 and Cytochrome c

Cytochrome b₅ (cytb₅) is a membrane protein vital for the regulation of cytochrome P450 (cytP450) metabolism and is capable of electron transfer to many redox partners. Here, using cyt c as a surrogate for cytP450, we report the effect of membrane on the interaction between full-length cytb₅ and cyt c for the first time. As shown through stopped-flow kinetic experiments, electron transfer capable cytb₅ - cyt c complexes were formed in the presence of bicelles and nanodiscs. Experimentally measured NMR parameters were used to map the cytb₅-cyt c binding interface. Our experimental results identify differences in the binding epitope of cytb₅ in the presence and absence of membrane. Notably, in the presence of membrane, cytb₅ only engaged cyt c at its lower and upper clefts while the membrane-free cytb₅ also uses a distal region. Using restraints generated from both cytb₅ and cyt c, a complex structure was generated and a potential electron transfer pathway was identified. These results demonstrate the importance of studying protein-protein complex formation in membrane mimetic systems. Our results also demonstrate the successful preparation of novel peptide-based lipid nanodiscs, which are detergent-free and possesses size flexibility, and their use for NMR structural studies of membrane proteins.

PP2A Controls Genome Integrity by Integrating Nutrient-Sensing and Metabolic Pathways with the DNA Damage Response

Mec1^ATR mediates the DNA damage response (DDR), integrating chromosomal signals and mechanical stimuli. We show that the PP2A phosphatases, ceramide-activated enzymes, couple cell metabolism with the DDR. Using genomic screens, metabolic analysis, and genetic and pharmacological studies, we found that PP2A attenuates the DDR and that three metabolic circuits influence the DDR by modulating PP2A activity. Irc21, a putative cytochrome b5 reductase that promotes the condensation reaction generating dihydroceramides (DHCs), and Ppm1, a PP2A methyltransferase, counteract the DDR by activating PP2A; conversely, the nutrient-sensing TORC1-Tap42 axis sustains DDR activation by inhibiting PP2A. Loss-of-function mutations in IRC21, PPM1, and PP2A and hyperactive tap42 alleles rescue mec1 mutants. Ceramides synergize with rapamycin, a TORC1 inhibitor, in counteracting the DDR. Hence, PP2A integrates nutrient-sensing and metabolic pathways to attenuate the Mec1^ATR response. Our observations imply that metabolic changes affect genome integrity and may help with exploiting therapeutic options and repositioning known drugs.

Role of cytochrome b5 in the modulation of the enzymatic activities of cytochrome P450 17α-hydroxylase/17,20-lyase (P450 17A1)

Cytochrome b5 (cyt b5) is a small hemoprotein that plays a significant role in the modulation of activities of an important steroidogenic enzyme, cytochrome P450 17α-hydroxylase/17,20-lyase (P450 17A1, CYP17A1). Located in the zona fasciculata and zona reticularis of the adrenal cortex and in the gonads, P450 17A1 catalyzes two different reactions in the steroidogenic pathway; the 17α-hydroxylation and 17,20-lyase, in the endoplasmic reticulum of these respective tissues. The activities of P450 17A1 are regulated by cyt b5 that enhances the 17,20-lyase reaction by promoting the coupling of P450 17A1 and cytochrome P450 reductase (CPR), allosterically. Cyt b5 can also act as an electron donor to enhance the 16-ene-synthase activity of human P450 17A1. In this review, we discuss the many roles of cyt b5 and focus on the modulation of CYP17A1 activities by cyt b5 and the mechanisms involved.

Marine Prasinoviruses and Their Tiny Plankton Hosts: A Review

Viruses play a crucial role in the marine environment, promoting nutrient recycling and biogeochemical cycling and driving evolutionary processes. Tiny marine phytoplankton called prasinophytes are ubiquitous and significant contributors to global primary production and biomass. A number of viruses (known as prasinoviruses) that infect these important primary producers have been isolated and characterised over the past decade. Here we review the current body of knowledge about prasinoviruses and their interactions with their algal hosts. Several genes, including those encoding for glycosyltransferases, methyltransferases and amino acid synthesis enzymes, which have never been identified in viruses of eukaryotes previously, have been detected in prasinovirus genomes. The host organisms are also intriguing; most recently, an immunity chromosome used by a prasinophyte in response to viral infection was discovered. In light of such recent, novel discoveries, we discuss why the cellular simplicity of prasinophytes makes for appealing model host organism-virus systems to facilitate focused and detailed investigations into the dynamics of marine viruses and their intimate associations with host species. We encourage the adoption of the prasinophyte Ostreococcus and its associated viruses as a model host-virus system for examination of cellular and molecular processes in the marine environment.

The cytochromeb5CybE is regulated by iron availability and is crucial for azole resistance inA. fumigatus

Cytochrome P450 enzymes (P450) play essential roles in redox metabolism in all domains of life including detoxification reactions and sterol biosynthesis.

Induced expression of microsomal cytochrome b5 determined at mRNA and protein levels in rats exposed to ellipticine, benzo[a]pyrene, and 1-phenylazo-2-naphthol (Sudan I)

ABSTRACT

The microsomal protein cytochrome b5 , which is located in the membrane of the endoplasmic reticulum, has been shown to modulate many reactions catalyzed by cytochrome P450 (CYP) enzymes. We investigated the influence of exposure to the anticancer drug ellipticine and to two environmental carcinogens, benzo[a]pyrene (BaP) and 1-phenylazo-2-naphthol (Sudan I), on the expression of cytochrome b5 in livers of rats, both at the mRNA and protein levels. We also studied the effects of these compounds on their own metabolism and the formation of DNA adducts generated by their activation metabolite(s) in vitro. The relative amounts of cytochrome b5 mRNA, measured by real-time polymerase chain reaction analysis, were induced by the test compounds up to 11.7-fold in rat livers. Western blotting using antibodies raised against cytochrome b5 showed that protein expression was induced by up to sevenfold in livers of treated rats. Microsomes isolated from livers of exposed rats catalyzed the oxidation of ellipticine, BaP, and Sudan I and the formation of DNA adducts generated by their reactive metabolite(s) more effectively than hepatic microsomes isolated from control rats. All test compounds are known to induce CYP1A1. This induction is one of the reasons responsible for increased oxidation of these xenobiotics by microsomes. However, induction of cytochrome b5 can also contribute to their enhanced metabolism.

GRAPHICAL ABSTRACT

Bioinformatic identification of cytochrome b5 homologues from the parasitic nematode Ascaris suum and the free-living nematode Caenorhabditis elegans highlights the crucial role of A. suum adult-specific secretory cytochrome b₅ in parasitic adaptation

We previously reported that adult Ascaris suum possesses NADH-metmyoglobin and NADH-methaemoglobin reductase systems that are located in the cells of the body wall and in the extracellular perienteric fluid, respectively, which helps them adapt to environmental hypoxia by recovering the differential functions of myoglobin and haemoglobin. A. suum cytochrome b5, an adult-specific secretory protein and an essential component of the NADH-metmyo (haemo) globin reductase system, has been extensively studied, and its unique nature has been determined. However, the relationship between A. suum cytochrome b5 and the canonical cytochrome b5 proteins, from the free-living nematode Caenorhabditis elegans is unclear. Here, we have characterised four cytochrome b5-like proteins from C. elegans (accession numbers: CAB01732, CCD68984, CAJ58492, and CAA98498) and three from A. suum (accession numbers: ADY48796, ADY46277, and ADY48338) and compared them with A. suum cytochrome b5 in silico. Bioinformatic and molecular analyses showed that CAA98498 from C. elegans is equivalent of A. suum cytochrome b5, which was not expressed as a mature mRNA. Further, the CAA98498 possessed no secretory signal peptide, which occurs in A. suum cytochrome b5 precursor. These results suggest that this free-living nematode does not need a haemoprotein such as the A. suum cytochrome b5 and highlight the crucial function of this A. suum adult-specific secretory cytochrome b5 in parasitic adaptation.

Transcriptome Analysis Reveals Key Flavonoid 3'-Hydroxylase and Flavonoid 3',5'-Hydroxylase Genes in Affecting the Ratio of Dihydroxylated to Trihydroxylated Catechins in Camellia sinensis

The ratio of dihydroxylated to trihydroxylated catechins (RDTC) is an important indicator of tea quality and biochemical marker for the study of genetic diversity. It is reported to be under genetic control but the underlying mechanism is not well understood. Flavonoid 3′-hydroxylase (F3′H) and flavonoid 3′,5′-hydroxylase (F3′5′H) are key enzymes involved in the formation of dihydroxylated and trihydroxylated catechins. The transcriptome and HPLC analysis of tea samples from Longjing43 and Zhonghuang2 under control and shading treatment were performed to assess the F3′H and F3′5′H genes that might affect RDTC. A total of 74.7 million reads of mRNA seq (2×101bp) data were generated. After de novo assembly, 109,909 unigenes were obtained, and 39,982 of them were annotated using 7 public databases. Four key F3′H and F3′5′H genes (including CsF3′5′H1, CsF3′H1, CsF3′H2 and CsF3′H3) were identified to be closely correlated with RDTC. Shading treatment had little effect on RDTC, which was attributed to the stable expression of these key F3′H and F3′5′H genes. The correlation of the coexpression of four key genes and RDTC was further confirmed among 13 tea varieties by real time PCR and HPLC analysis. The coexpression of three F3′H genes and a F3′5′H gene may play a key role in affecting RDTC in Camellia sinensis. The current results may establish valuable foundation for further research about the mechanism controlling catechin composition in tea.

Exposure of Chlamys farreri to tetrabromobisphenol A: accumulation and multibiomarker responses

Tetrabromobisphenol A (TBBPA) is currently the most widely used brominated flame retardant (BFR). To date, the toxic effects of TBBPA remains poorly understood in aquatic organisms, especially in bivalves. The objective of this experiment was to examine bioaccumulation and multibiomarker responses in the scallop Chlamys farreri exposed to TBBPA under laboratory conditions. The results showed that TBBPA was rapidly accumulated in and then eliminated from the gill and digestive gland of the scallops. TBBPA exposure invoked alterations in the detoxification system and induced oxidant stress and biomacromolecule damages in the gill and digestive gland of C. farreri. Additionally, glutathione-S-transferase (GST) activity, lipid peroxidation (LPO) level, cytochrome b5 (Cyt b5) content, and DNA strand break had good correlations with TBBPA accumulation levels in the gill and digestive gland of C. farreri. Summarizing, these results enabled us to hypothesize several toxic mechanisms of TBBPA and select potential biomarkers for TBBPA pollution monitoring.

Effect of Cytochrome b5 Content on the Activity of Polymorphic CYP1A2, 2B6, and 2E1 in Human Liver Microsomes

Human cytochrome b5 (Cyt b5) plays important roles in cytochrome P450 (CYP)-mediated drug metabolism. However, the expression level of Cyt b5 in normal human liver remains largely unknown. The effect of Cyt b5 on overall CYP activity in human liver microsomes (HLM) has rarely been reported and the relationship between Cyt b5 and the activity of polymorphic CYP has not been systematically investigated. In this study, we found that the median value of Cyt b5 protein was 270.01 pmol/mg from 123 HLM samples, and 12- and 19-fold individual variation was observed in Cyt b5 mRNA and protein levels, respectively. Gender and smoking clearly influenced Cyt b5 content. In addition, we found that Cyt b5 protein levels significantly correlated with the overall activity of CYP1A2, 2B6, and 2E1 in HLM. However, when the CYP activities were sorted by single nucleotide polymorphisms (SNP), the effect of Cyt b5 protein on the kinetic parameters varied greatly. There were significant correlations between Cyt b5 content and V_max and CL_int of CYP1A2 wild-types (3860GG, 2159GG, and 5347CC) as well as homozygous mutants (163AA and 3113GG). In contrast to V_max and CL_int, the K_m of CYP2B6 516GG and 785AA genotypes was inversely associated with Cyt b5 content. Correlations between Cyt b5 content and V_max and CL_int of CYP2E1 -1293GG, -1293GC, 7632TT, 7632TA, -333TT, and -352AA genotypes were also observed. In conclusion, Cyt b5 expression levels varied considerably in the Chinese cohort from this study. Cyt b5 had significant impact on the overall activity of CYP1A2, 2B6, and 2E1 in HLM and the effects of Cyt b5 protein on polymorphic CYP1A2, 2B6, and 2E1 activity were SNP-dependent. These findings suggest that Cyt b5 plays an important role in CYP-mediated activities in HLM and may possibly be a contributing factor for the individual variation observed in CYP enzyme activities.

Regulation of ozone-induced lung inflammation and injury by the β-galactoside-binding lectin galectin-3

Macrophages play a dual role in ozone toxicity, contributing to both pro- and anti-inflammatory processes. Galectin-3 (Gal-3) is a lectin known to regulate macrophage activity. Herein, we analyzed the role of Gal-3 in the response of lung macrophages to ozone. Bronchoalveolar lavage (BAL) and lung tissue were collected 24-72h after exposure (3h) of WT and Gal-3(-/-) mice to air or 0.8ppm ozone. In WT mice, ozone inhalation resulted in increased numbers of proinflammatory (Gal-3(+), iNOS(+)) and anti-inflammatory (MR-1(+)) macrophages in the lungs. While accumulation of iNOS(+) macrophages was attenuated in Gal-3(-/-) mice, increased numbers of enlarged MR-1(+) macrophages were noted. This correlated with increased numbers of macrophages in BAL. Flow cytometric analysis showed that these cells were CD11b(+) and consisted mainly (>97%) of mature (F4/80(+)CD11c(+)) proinflammatory (Ly6GLy6C(hi)) and anti-inflammatory (Ly6GLy6C(lo)) macrophages. Increases in both macrophage subpopulations were observed following ozone inhalation. Loss of Gal-3 resulted in a decrease in Ly6C(hi) macrophages, with no effect on Ly6C(lo) macrophages. CD11b(+)Ly6G(+)Ly6C(+) granulocytic (G) and monocytic (M) myeloid derived suppressor cells (MDSC) were also identified in the lung after ozone. In Gal-3(-/-) mice, the response of G-MDSC to ozone was attenuated, while the response of M-MDSC was heightened. Changes in inflammatory cell populations in the lung of ozone treated Gal-3(-/-) mice were correlated with reduced tissue injury as measured by cytochrome b5 expression. These data demonstrate that Gal-3 plays a role in promoting proinflammatory macrophage accumulation and toxicity in the lung following ozone exposure.

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.

Transcriptome Analysis Reveals Key Flavonoid 3'-Hydroxylase and Flavonoid 3',5'-Hydroxylase Genes in Affecting the Ratio of Dihydroxylated to Trihydroxylated Catechins in Camellia sinensis

The ratio of dihydroxylated to trihydroxylated catechins (RDTC) is an important indicator of tea quality and biochemical marker for the study of genetic diversity. It is reported to be under genetic control but the underlying mechanism is not well understood. Flavonoid 3'-hydroxylase (F3'H) and flavonoid 3',5'-hydroxylase (F3'5'H) are key enzymes involved in the formation of dihydroxylated and trihydroxylated catechins. The transcriptome and HPLC analysis of tea samples from Longjing43 and Zhonghuang2 under control and shading treatment were performed to assess the F3'H and F3'5'H genes that might affect RDTC. A total of 74.7 million reads of mRNA seq (2×101bp) data were generated. After de novo assembly, 109,909 unigenes were obtained, and 39,982 of them were annotated using 7 public databases. Four key F3'H and F3'5'H genes (including CsF3'5'H1, CsF3'H1, CsF3'H2 and CsF3'H3) were identified to be closely correlated with RDTC. Shading treatment had little effect on RDTC, which was attributed to the stable expression of these key F3'H and F3'5'H genes. The correlation of the coexpression of four key genes and RDTC was further confirmed among 13 tea varieties by real time PCR and HPLC analysis. The coexpression of three F3'H genes and a F3'5'H gene may play a key role in affecting RDTC in Camellia sinensis. The current results may establish valuable foundation for further research about the mechanism controlling catechin composition in tea.

Dietary accumulation of tetrabromobisphenol A and its effects on the scallop Chlamys farreri

Tetrabromobisphenol A (TBBPA) is currently the most widely-used brominated flame retardant (BFR) and has been proven to have a very high toxicity to aquatic organisms including bivalves. In this study, a laboratory experiment was conducted to obtain a better understanding of the role of algae food on the bioaccumulation of TBBPA and its effects on the suspension-feeding bivalve, the scallop, Chlamys farreri. Scallops were exposed to TBBPA via algae food alone or food+water for 10 days. Results showed that TBBPA was accumulated rapidly by scallops, reaching an approximate steady state in soft tissues within 3 days. The primary route of TBBPA accumulation was via the water, while dietary uptake was relatively minor. TBBPA exposure led to significant inhibition on microsomal cytochrome P450 and cytochrome b5 levels in gills and digestive gland, whereas GST activity and GSH level increased significantly, which indicated that TBBPA could be a suitable substrate to directly participate in phase II metabolism. TBBPA clearly induced the activity of SOD, suggesting the oxidant stress induced by TBBPA. This study suggests that dietary uptake was not the predominant uptake route for TBBPA in bivalves and also provides a preliminary basis for studying the detoxification and antioxidant responses of marine bivalves upon exposure to TBBPA.

¹H, ¹³C and ¹⁵N resonance assignments for the full-length mammalian cytochrome b₅ in a membrane environment

Microsomal cytochrome b5 plays a key role in the oxidation of a variety of exogenous and endogenous compounds, including drugs, fatty acids, cholesterol and steroid hormones. To better understand its functional properties in a membrane mimic environment, we carried out high-resolution solution NMR studies. Here we report resonance assignments for full-length rabbit cytochrome b5 embedded in dodecylphosphocholine micelles.

Role of protein-protein interactions in cytochrome P450-mediated drug metabolism and toxicity

Through their unique oxidative chemistry, cytochrome P450 monooxygenases (CYPs) catalyze the elimination of most drugs and toxins from the human body. Protein–protein interactions play a critical role in this process. Historically, the study of CYP–protein interactions has focused on their electron transfer partners and allosteric mediators, cytochrome P450 reductase and cytochrome b5. However, CYPs can bind other proteins that also affect CYP function. Some examples include the progesterone receptor membrane component 1, damage resistance protein 1, human and bovine serum albumin, and intestinal fatty acid binding protein, in addition to other CYP isoforms. Furthermore, disruption of these interactions can lead to altered paths of metabolism and the production of toxic metabolites. In this review, we summarize the available evidence for CYP protein–protein interactions from the literature and offer a discussion of the potential impact of future studies aimed at characterizing noncanonical protein–protein interactions with CYP enzymes.

Protein interactomics based on direct molecular fishing on paramagnetic particles: practical realization and further SPR validation

There is increasing evidence that proteins function in the cell as integrated stable or temporally formed protein complexes, interactomes. Previously, using model systems we demonstrated applicability of direct molecular fishing on paramagnetic particles for protein interactomics (Ershov et al. Proteomics, 2012, 12, 3295). In the present study, we have used a combination of affinity-based molecular fishing and subsequent MS for investigation of human liver proteins involved in interactions with immobilized microsomal cytochrome b5 (CYB5A), and also transthyretin and BSA as alternative affinity ligands (baits). The LC-MS/MS identification of prey proteins fished on these baits revealed three sets of proteins: 98, 120, and 220, respectively. Comparison analysis of these sets revealed only three proteins common for all the baits. In the case of paired analysis, the number of common proteins varied from 2 to 9. The binding capacity of some identified proteins has been validated by a SPR-based biosensor. All the investigated proteins effectively interacted with the immobilized CYB5A (Kd values ranged from 0.07 to 1.1 μM). Results of this study suggest that direct molecular fishing is applicable for analysis of protein-protein interactions (PPI) under normal and pathological conditions, in which altered PPIs are especially important.

Essential functions of iron-requiring proteins in DNA replication, repair and cell cycle control

Eukaryotic cells contain numerous iron-requiring proteins such as iron-sulfur (Fe-S) cluster proteins, hemoproteins and ribonucleotide reductases (RNRs). These proteins utilize iron as a cofactor and perform key roles in DNA replication, DNA repair, metabolic catalysis, iron regulation and cell cycle progression. Disruption of iron homeostasis always impairs the functions of these iron-requiring proteins and is genetically associated with diseases characterized by DNA repair defects in mammals. Organisms have evolved multi-layered mechanisms to regulate iron balance to ensure genome stability and cell development. This review briefly provides current perspectives on iron homeostasis in yeast and mammals, and mainly summarizes the most recent understandings on iron-requiring protein functions involved in DNA stability maintenance and cell cycle control.

Dynamic interaction between membrane-bound full-length cytochrome P450 and cytochrome b5 observed by solid-state NMR spectroscopy

Microsomal monoxygenase enzymes of the cytochrome-P450 family are found in all biological kingdoms, and play a central role in the breakdown of metabolic as well as xenobiotic, toxic and 70% of the drugs in clinical use. Full-length cytochrome-b5 has been shown to be important for the catalytic activity of cytochrome-P450. Despite the significance in understanding the interactions between these two membrane-associated proteins, only limited high-resolution structural information on the full-length cytochrome-P450 and the cytochromes-b5-P450 complex is available. Here, we report a structural study on a functional ~72-kDa cytochromes-b5-P450 complex embedded in magnetically-aligned bicelles without having to freeze the sample. Functional and solid-state NMR (Nuclear Magnetic Resonance) data reveal interactions between the proteins in fluid lamellar phase bilayers. In addition, our data infer that the backbone structure and geometry of the transmembrane domain of cytochrome-b5 is not significantly altered due to its interaction with cytochrome-P450, whereas the mobility of cytochrome-b5 is considerably reduced.

Association of cytochrome b5 with ETR1 ethylene receptor signaling through RTE1 in Arabidopsis

Ethylene plays important roles in plant growth, development and stress responses, and is perceived by a family of receptors that repress ethylene responses when ethylene is absent. Repression by the ethylene receptor ETR1 depends on an integral membrane protein, REVERSION TO ETHYLENE SENSITIVITY1 (RTE1), which acts upstream of ETR1 in the endoplasmic reticulum (ER) membrane and Golgi apparatus. To investigate RTE1 function, we screened for RTE1-interacting proteins using the yeast split-ubiquitin assay, which yielded the ER-localized cytochrome b(5) (Cb5) isoform D. Cb5s are small hemoproteins that perform electron transfer reactions in all eukaryotes, but their roles in plants are relatively uncharacterized. Using bimolecular fluorescence complementation (BiFC), we found that all four ER-localized Arabidopsis Cb5 isoforms (AtCb5–B, -C, -D and -E) interact with RTE1 in plant cells. In support of this interaction, atcb5 mutants exhibited phenotypic parallels with rte1 mutants in Arabidopsis. Phenotypes included partial suppression of etr1–2 ethylene insensitivity, and no suppression of RTE1-independent ethylene receptor isoforms. The single loss-of-function mutants atcb5–b, -c and -d appeared similar to the wild-type, but double mutant combinations displayed slight ethylene hypersensitivity. Over-expression of AtCb5–D conferred reduced ethylene sensitivity similar to that conferred by RTE1 over-expression, and genetic analyses suggested that AtCb5–D acts upstream of RTE1 in the ethylene response. These findings suggest an unexpected role for Cb5, in which Cb5 and RTE1 are functional partners in promoting ETR1-mediated repression of ethylene signaling.

Giardia intestinalis incorporates heme into cytosolic cytochrome b₅

The anaerobic intestinal pathogen Giardia intestinalis does not possess enzymes for heme synthesis, and it also lacks the typical set of hemoproteins that are involved in mitochondrial respiration and cellular oxygen stress management. Nevertheless, G. intestinalis may require heme for the function of particular hemoproteins, such as cytochrome b5 (cytb5). We have analyzed the sequences of eukaryotic cytb5 proteins and identified three distinct cytb5 groups: group I, which consists of C-tail membrane-anchored cytb5 proteins; group II, which includes soluble cytb5 proteins; and group III, which comprises the fungal cytb5 proteins. The majority of eukaryotes possess both group I and II cytb5 proteins, whereas three Giardia paralogs belong to group II. We have identified a fourth Giardia cytb5 paralog (gCYTb5-IV) that is rather divergent and possesses an unusual 134-residue N-terminal extension. Recombinant Giardia cytb5 proteins, including gCYTb5-IV, were expressed in Escherichia coli and exhibited characteristic UV-visible spectra that corresponded to heme-loaded cytb5 proteins. The expression of the recombinant gCYTb5-IV in G. intestinalis resulted in the increased import of extracellular heme and its incorporation into the protein, whereas this effect was not observed when gCYTb5-IV containing a mutated heme-binding site was expressed. The electrons for Giardia cytb5 proteins may be provided by the NADPH-dependent Tah18-like oxidoreductase GiOR-1. Therefore, GiOR-1 and cytb5 may constitute a novel redox system in G. intestinalis. To our knowledge, G. intestinalis is the first anaerobic eukaryote in which the presence of heme has been directly demonstrated.

Functional and structural characterisation of a viral cytochrome b5

Cytochrome b5 is a ubiquitous electron transport protein. The sequenced viral OtV-2 genome, which infects Ostreococcus tauri, was predicted to encode a putative cytochrome b5 enzyme. Using purified OtV-2 cytochrome b5 we confirm this protein has identical spectral properties to purified human cytochrome b5 and additionally that the viral enzyme can substitute for yeast cytochrome b5 in yeast cytochrome P450 51 mediated sterol 14α-demethylation. The crystal structure of the OtV-2 cytochrome b5 enzyme reveals a single domain, comprising four β sheets, four α helices and a haem moiety, which is similar to that found in larger eukaryotic cytochrome proteins. As a product of a horizontal gene transfer event involving a subdomain of the host fumarate reductase-like protein, OtV-2 cytochrome b5 appears to have diverged in function and is likely to have evolved an entirely new role for the virus during infection. Indeed, lacking a hydrophobic C-terminal anchor, OtV-2 encodes the first cytosolic cytochrome b5 characterised. The lack of requirement for membrane attachment (in contrast to all other microsomal cytochrome b5s) may be a reflection of the small size of the host cell, further emphasizes the unique nature of this virus gene product and draws attention to the potential importance of cytochrome b5 metabolic activity at the extremes of cellular scale.

Functional characterization of eight human CYP1A2 variants: the role of cytochrome b5

BACKGROUND

Interindividual variability in cytochrome P450 (CYP)-mediated xenobiotic metabolism is extensive. CYP metabolism requires two electrons, which can be donated by NADPH cytochrome P450 oxidoreductase (CYPOR) and/or cytochrome b5 (b5). Although substantial number of studies have reported on the function and effect of b5 in CYP-mediated catalysis, its mode of action is still not fully understood.

OBJECTIVE

The aim of this work was to examine the effect of b5 on the activities of eight natural-occurring variants of human CYP1A2, namely, T83M, S212C, S298R, G299S, I314V, I386F, C406Y, and R456H.

MATERIALS AND METHODS

An approach, as used in our former study was applied, coexpressing these polymorphic CYP1A2 variants separately with CYPOR and b5 in the bacterial cell model BTC-CYP. For each variant, 16 different activity parameters were measured, using eight different substrates. This heterogeneous data set was merged with the one of our former study (i.e. without b5) and a multivariate analysis was carried out.

RESULTS

This analysis indicated that b5 seems to have the ability to affect CYP1A2 variants to behave more like the wild-type variant. This was especially the case for variant I386F, for which the presence of b5 was crucial to show activity. Variants T83M and C406Y showed considerably different activity-profiles when in the presence of b5. Furthermore, our data seem to implicate CYP1A2 residue G299 in its interaction with CYPOR and/or b5.

CONCLUSION

Results indicate the ability of b5 to affect CYP1A2 variants to behave more like the wild-type variant, attenuating detrimental effects of structural mutations of these variants, seemingly through extensive allosteric effects.

A model of the membrane-bound cytochrome b5-cytochrome P450 complex from NMR and mutagenesis data

Microsomal cytochrome b5 (cytb5) is a membrane-bound protein that modulates the catalytic activity of its redox partner, cytochrome P4502B4 (cytP450). Here, we report the first structure of full-length rabbit ferric microsomal cytb5 (16 kDa), incorporated in two different membrane mimetics (detergent micelles and lipid bicelles). Differential line broadening of the cytb5 NMR resonances and site-directed mutagenesis data were used to characterize the cytb5 interaction epitope recognized by ferric microsomal cytP450 (56 kDa). Subsequently, a data-driven docking algorithm, HADDOCK (high ambiguity driven biomolecular docking), was used to generate the structure of the complex between cytP4502B4 and cytb5 using experimentally derived restraints from NMR, mutagenesis, and the double mutant cycle data obtained on the full-length proteins. Our docking and experimental results point to the formation of a dynamic electron transfer complex between the acidic convex surface of cytb5 and the concave basic proximal surface of cytP4502B4. The majority of the binding energy for the complex is provided by interactions between residues on the C-helix and β-bulge of cytP450 and residues at the end of helix α4 of cytb5. The structure of the complex allows us to propose an interprotein electron transfer pathway involving the highly conserved Arg-125 on cytP450 serving as a salt bridge between the heme propionates of cytP450 and cytb5. We have also shown that the addition of a substrate to cytP450 likely strengthens the cytb5-cytP450 interaction. This study paves the way to obtaining valuable structural, functional, and dynamic information on membrane-bound complexes.

Cytochrome b5: novel roles in steroidogenesis

Cytochrome b(5) (cyt-b(5)) is essential for the regulation of steroidogenesis and as such has been implicated in a number of clinical conditions. It is well documented that this small hemoprotein augments the 17,20-lyase activity of cytochrome P450 17α-hydroxylase/17,20-lyase (CYP17A1). Studies have revealed that this augmentation is accomplished by cyt-b(5) enhancing the interaction between cytochrome P450 reductase (POR) and CYP17A1. In this paper we present evidence that cyt-b(5) induces a conformational change in CYP17A1, in addition to facilitating the interaction between CYP17A1 and POR. We also review the recently published finding that cyt-b(5) allosterically augments the activity of 3β-hydroxysteroid dehydrogenase/Δ(5)-Δ(4) isomerase (3βHSD), a non cytochrome P450 enzyme, by increasing the enzymes affinity for its cofactor, NAD(+). The physiological importance of this finding, in terms of understanding adrenal androstenedione production, is examined. Finally, evidence that cyt-b(5) is able to form homomeric complexes in living cells is presented and discussed.

Cytochrome b5 and cytokeratin 17 are biomarkers in bronchoalveolar fluid signifying onset of acute lung injury

Acute lung injury (ALI) is characterized by pulmonary edema and acute inflammation leading to pulmonary dysfunction and potentially death. Early medical intervention may ameliorate the severity of ALI, but unfortunately, there are no reliable biomarkers for early diagnosis. We screened for biomarkers in a mouse model of ALI. In this model, inhalation of S. aureus enterotoxin A causes increased capillary permeability, cell damage, and increase protein and cytokine concentration in the lungs. We set out to find predictive biomarkers of ALI in bronchoalveolar lavage (BAL) fluid before the onset of clinical manifestations. A cutting edge proteomic approach was used to compare BAL fluid harvested 16 h post S. aureus enterotoxin A inhalation versus BAL fluid from vehicle alone treated mice. The proteomic PF 2D platform permitted comparative analysis of proteomic maps and mass spectrometry identified cytochrome b5 and cytokeratin 17 in BAL fluid of mice challenged with S. aureus enterotoxin A. Validation of cytochrome b5 showed tropic expression in epithelial cells of the bronchioles. Importantly, S. aureus enterotoxin A inhalation significantly decreased cytochrome b5 during the onset of lung injury. Validation of cytokeratin 17 showed ubiquitous expression in lung tissue and increased presence in BAL fluid after S. aureus enterotoxin A inhalation. Therefore, these new biomarkers may be predictive of ALI onset in patients and could provide insight regarding the basis of lung injury and inflammation.

OnpA, an unusual flavin-dependent monooxygenase containing a cytochrome b(5) domain

ortho-Nitrophenol 2-monooxygenase (EC 1.14.13.31) from Alcaligenes sp. strain NyZ215 catalyzes monooxygenation of ortho-nitrophenol to form catechol via ortho-benzoquinone. Sequence analysis of this onpA-encoded enzyme revealed that it contained a flavin-binding monooxygenase domain and a heme-binding cytochrome b(5) domain. OnpA was purified to homogeneity as a His-tagged protein and was considered a monomer, as determined by gel filtration. FAD and heme were identified by high-performance liquid chromatography (HPLC) and HPLC-mass spectrometry (HPLC-MS) as cofactors in this enzyme, and quantitative analysis indicated that 1 mol of the purified recombinant OnpA contained 0.66 mol of FAD and 0.20 mol of heme. However, the enzyme activity of OnpA was increased by 60% and 450% after addition of FAD and hemin, respectively, suggesting that the optimal stoichiometry was 1:1:1. In addition, site-directed mutagenesis experiments confirmed that two highly conserved histidines located in the cytochrome b(5) domain were associated with binding of the heme, and the cytochrome b(5) domain was involved in the OnpA activity. These results indicate that OnpA is an unusual FAD-dependent monooxygenase containing a fused cytochrome b(5) domain that is essential for its activity. Therefore, we here demonstrate a link between cytochrome b(5) and flavin-dependent monooxygenases.