Characterization of lysyl residues of NADH-cytochrome b5 reductase implicated in charge-pairing with active-site carboxyl residues of cytochrome b5 by site-directed mutagenesis of an expression vector for the flavoprotein

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.

Peroxidase-like activity of cytochrome b5 is triggered upon hemichrome formation in alkaline pH

In alkaline media (pH12) a catalytic peroxidase activity of cytochrome b5 was found associated to a different conformational state. Upon incubation at this pH, cytochrome b5 electronic absorption spectrum was altered, with disappearance of characteristic bands of cytochrome b5 at pH7.0. The appearance of new electronic absorption bands and EPR measurements support the formation of a cytochrome b5 class B hemichrome with an acquired ability to bind polar ligands. This hemichrome is characterized by a negative formal redox potential and the same folding properties than cytochrome b5 at pH7. The acquired peroxidase-like activity of cytochrome b5 found at pH12, driven by a hemichrome formation, suggests a role of this protein in peroxidation products propagation.

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.

Phospholipid membranes affect tertiary structure of the soluble cytochrome b5 heme-binding domain

The influence of charged phospholipid membranes on the conformational state of the water-soluble fragment of cytochrome b5 has been investigated by a variety of techniques at neutral pH. The results of this work provide the first evidence that aqueous solutions with high phospholipid/protein molar ratios (pH 7.2) induce the cytochrome to undergo a structural transition from the native conformation to an intermediate state with molten-globule like properties that occur in the presence of an artificial membrane surface and that leads to binding of the protein to the membrane. At other phospholipid/protein ratios, equilibrium was observed between cytochrome free in solution and cytochrome bound to the surface of vesicles. Inhibition of protein binding to the vesicles with increasing ionic strength indicated for the most part an electrostatic contribution to the stability of cytochrome b5-vesicle interactions at pH 7.2. The possible physiological role of membrane-induced conformational change in the structure of cytochrome b5 upon the interaction with its redox partners is discussed.

Expression and characterization of a functional canine variant of cytochrome b5 reductase

Cytochrome b5 reductase (cb5r), a member of the flavoprotein transhydrogenase family of oxidoreductase enzymes, catalyzes the transfer of reducing equivalents from the physiological electron donor, NADH, to two molecules of cytochrome b5. We have determined the correct nucleotide sequence for the putative full-length, membrane-associated enzyme from Canis familiaris, and have generated a heterologous expression system for production of a histidine-tagged variant of the soluble, catalytic diaphorase domain, comprising residues I33 to F300. Using a simple two-step chromatographic procedure, the recombinant diaphorase domain has been purified to homogeneity and demonstrated to be a simple flavoprotein with a molecular mass of 31,364 (m/z) that retained both NADH:ferricyanide reductase and NADH:cytochrome b5 reductase activities. The recombinant protein contained a full complement of FAD and exhibited absorption and CD spectra comparable to those of a recombinant form of the rat cytochrome b5 reductase diaphorase domain generated using an identical expression system, suggesting similar protein folding. Oxidation-reduction potentiometric titrations yielded a standard midpoint potential (Eo') for the FAD/FADH2 couple of -273+/-5 mV which was identical to the value obtained for the corresponding rat domain. Thermal denaturation studies revealed that the canine domain exhibited stability comparable to that of the rat protein, confirming similar protein conformations. Initial-rate kinetic studies revealed the canine diaphorase domain retained a marked preference for NADH versus NADPH as reducing substrate and exhibited kcat's of 767 and 600 s(-1) for NADH:ferricyanide reductase and NADH:cytochrome b5 reductase activities, respectively, with Km's of 7, 8, and 12 microM for NADH, K3Fe(CN)6, and cytochrome b5, respectively. Spectral-binding constants (Ks) determined for a variety of NAD+ analogs indicated the highest and lowest affinities were observed for APAD+ (Ks=71 microM) and PCA+ (Ks=>31 mM), respectively, and indicated the binding contributions of the various portions of the pyridine nucleotide. These results provide the first correct sequence for the full-length, membrane-associated form of C. familiaris cb5r and provide a direct comparison of the enzymes from two phylogenetic sources using identical expression systems that indicate that both enzymes have comparable spectroscopic, kinetic, thermodynamic, and structural properties.

Cytochrome b5 reductase: the roles of the recessive congenital methemoglobinemia mutants P144L, L148P, and R159*

Recessive congenital methemoglobinemia (RCM, OMIM 250800) arises from defects in either the erythrocytic or microsomal forms of the flavoprotein, cytochrome b5 reductase (cb5r) and was the first disease to be directly associated with a specific enzyme deficiency. Of the 33 verified mutations in cb5r that give rise to either the type I (erythrocytic) or type II (generalized) forms of RCM, three of the mutations, corresponding to P144L, L148P, and R159*, are located in a segment of the primary sequence composed of residues G143 to V171 which serves as a "hinge" or "linker" region between the FAD- and NADH-binding lobes of the protein. With the exception of R159*, which produces a truncated non-functional cb5r resulting in type II RCM, the type I methemoglobinemias resulting from the P144L or L148P mutations have been proposed to be due to decreased enzyme stability. Utilizing a recombinant form of the rat cb5r enzyme, we have generated the P144L, L148P, and P144L/L148P mutants, purified the resulting proteins to homogeneity and characterized their spectroscopic, kinetic, and thermodynamic properties. The three mutant proteins retained full complements of FAD with the P144L and L148P variants being spectroscopically indistinguishable from wild-type cb5r. In contrast, kinetic analyses revealed that the P144L, L148P, and P144L/L148P variants retained only 28, 31, and 8% of wild-type NADH:cytochrome b5 reductase activity, respectively, together with significant alterations in affinity for both NADH and NAD+. In addition, FAD oxidation-reduction potentials were 32, 19, and 65 mV more positive for the mutants than the corresponding FAD/FADH2 couple in native cb5r (E0'=-272 mV). Thermal and proteolytic stability measurements indicated that all three mutants were less stable than the wild-type protein while differential spectroscopy indicated altered pyridine nucleotide binding in all three variants. These results demonstrate that the "hinge" region is important in maintaining the correct orientation of the flavin- and pyridine nucleotide-binding lobes within the protein for efficient electron transfer and that the P144L and L148P mutations disrupt the normal registration of the FAD- and NADH-binding lobes resulting in altered affinities for both the physiological reducing substrate, NADH and its product, NAD+.

Functional interaction of cytochrome P450 with its redox partners: a critical assessment and update of the topology of predicted contact regions

The problem of donor-acceptor recognition has been the most important and intriguing one in the area of P450 research. The present review outlines the topological background of electron-transfer complex formation, showing that the progress in collaborative investigations, combining physical techniques with chemical-modification and immunolocalization studies as well as site-directed mutagenesis experiments, has increasingly enabled the substantiation of hypothetical work resulting from homology modelling of P450s. Circumstantial analysis reveals the contact regions for redox proteins to cluster on the proximal face of P450s, constituting parts of the highly conserved, heme-binding core fold. However, more variable structural components located in the periphery of the hemoprotein molecules also participate in donor docking. The cross-reactivity of electron carriers, purified from pro- and eukaryotic sources, with a diversity of P450 species points at a possible evolutionary conservation of common anchoring domains. While electrostatic mechanisms appear to dominate orientation toward each other of the redox partners to generate pre-collisional encounter complexes, hydrophobic forces are likely to foster electron transfer events by through-bonding or pi-stacking interactions. Moreover, electron-tunneling pathways seem to be operative as well. The availability of new P450 crystal structures together with improved validation strategies will undoubtedly permit the production of increasingly satisfactory three-dimensional donor-acceptor models serving to better understand the molecular principles governing functional association of the redox proteins.

Tonoplast subcellular localization of maize cytochrome b5 reductases

Plant cytochrome b5 reductases (b5R) are assumed to be part of an ER-associated redox chain that oxidizes NADH to provide electrons via cytochrome b5 (cyt b5) to ER-associated fatty acyl desaturase and related hydroxylases, as in mammalian cells. Here we report on cDNA cloning of a novel maize b5R, NFR II, strongly related to a previously cloned cDNA, NFR I (Bagnaresi et al., 1999, Biochem. J. 338, 499-505). Maize b5R isoforms are produced by a small multi-gene family. The NFR cDNAs were shown to encode active b5Rs by heterologous expression in yeast. Both reductases, in addition to Fe3+-chelates, efficiently reduced Cu2+-chelates. Using a polyclonal antibody able to recognize both NFR I and NFR II isoforms, no ER or mitochondrial localization could be detected in maize roots. Unexpectedly, maize b5Rs were found to be targeted to the tonoplast. Using the most specific assay to measure NFR activity, we confirmed that the highest NFR specific activity is associated with tonoplast-enriched maize root fractions. Tonoplast targeting is not consistent with a role in desaturase reactions or with the other functions ascribed to date to plant b5R. This indicates that alternative ER-associated electron donors for desaturases need to be sought, and that plant b5Rs may have previously unexpected functions.

Targeting proteins to the lumen of endoplasmic reticulum using N-terminal domains of 11beta-hydroxysteroid dehydrogenase and the 50-kDa esterase

Previous studies identified two intrinsic endoplasmic reticulum (ER) proteins, 11beta-hydroxysteroid dehydrogenase, isozyme 1 (11beta-HSD) and the 50-kDa esterase (E3), sharing some amino acid sequence motifs in their N-terminal transmembrane (TM) domains. Both are type II membrane proteins with the C terminus projecting into the lumen of the ER. This finding implied that the N-terminal TM domains of 11beta-HSD and E3 may constitute a lumenal targeting signal (LTS). To investigate this hypothesis we created chimeric fusions using the putative targeting sequences and the reporter gene, Aequorea victoria green fluorescent protein. Transfected COS cells expressing LTS-green fluorescent protein chimeras were examined by fluorescent microscopy and electron microscopic immunogold labeling. The orientation of expressed chimeras was established by immunocytofluorescent staining of selectively permeabilized COS cells. In addition, protease protection assays of membranes in the presence and absence of detergents was used to confirm lumenal or the cytosolic orientation of the constructed chimeras. To investigate the general applicability of the proposed LTS, we fused the N terminus of E3 to the N terminus of the NADH-cytochrome b5 reductase lacking the myristoyl group and N-terminal 30-residue membrane anchor. The orientation of the cytochrome b5 reductase was reversed, from cytosolic to lumenal projection of the active domain. These observations establish that an amino acid sequence consisting of short basic or neutral residues at the N terminus, followed by a specific array of hydrophobic residues terminating with acidic residues, is sufficient for lumenal targeting of single-pass proteins that are structurally and functionally unrelated.

Systematic mutations of highly conserved His49 and carboxyl-terminal of recombinant porcine liver NADH-cytochrome b5 reductase solubilized domain

The cDNA encoding solubilized porcine liver NADH-cytochrome b5 reductase catalytic domain (Pb5R) was cloned and overexpressed in Escherichia coli. A highly conserved His49 and a C-terminal Phe272 of Pb5R, which are located near the isoalloxazine moiety of the FAD, were systematically modulated by site-directed mutagenesis. Large structural change was not detected on the absorption and circular dichroism spectra of mutant proteins. Drastic changes in enzymatic properties were not observed, but the apparent Km value for soluble form of porcine liver cytochrome b5 (Pb5) was affected by the substitutions of His49 with glutamic acid and with lysine, deletion of C-terminal Phe272, and addition of Gly273. The values of the catalytic constant (kcat) were obviously decreased by the substitution of His49 with glutamic acid or the addition of Gly273. In these two mutants, the rate for reduction of FAD was decreased, and the rate for autoxidation of reduced FAD was increased. These results showed that His49 and C-terminal carboxyl group in Pb5R are not critical for the electron transfer to Pb5, but the electrostatic environmental changes at these positions could affect the recognition of Pb5 and modulate the catalytic function of the enzyme by changing the stability of reduced FAD.

Simultaneous purification and characterization of cytochrome b5 reductase and cytochrome b5 from sheep liver

Cytochrome b5 was purified from detergent solubilized sheep liver microsomes by using three successive DEAE-cellulose, and Sephadex G-100 column chromatographies. It was purified 54-fold and the yield was 23.5% with respect to microsomes. The apparent Mr of cytochrome b5 was estimated to be 16,200 +/- 500 by SDS-PAGE. Absolute absorption spectrum of the purified cytochrome b5 showed maximal absorption at 412 nm and dithionite-reduced cytochrome b5 gave peaks at 557, 526.5 and 423 nm. The ability of the purified sheep liver cytochrome b5 to transfer electrons from NADH-cytochrome b5 reductase to cytochrome c was investigated. The K(m) and Vmax values were calculated to be 0.088 microM cytochrome b5 and 315.8 microM cytochrome c reduced/min/mg enzyme, respectively. Also the reduction of cytochrome b5 by reductase was studied and K(m) and Vmax values were determined to be 5 microM cytochrome b5 and 5200 nmol cytochrome b5 reduced/min/mg enzyme, respectively. The K(m) and Vmax values for the cofactor NADH in the presence of saturating concentration of cytochrome b5 were found to be 0.0017 mM NADH and 6944 nmol cytochrome b5 reduced/min/mg enzyme, respectively. NADH-cytochrome b5 reductase was also partially purified from the same source, detergent solubilized sheep liver microsomes, by using two successive DEAE-cellulose, and 5'-ADP-agarose affinity column chromatographies. It was purified 144-fold and the yield was 7% with respect to microsomes. The apparent monomer Mr of reductase was estimated to be 34,000 by SDS-PAGE. When ferricyanide was used as an electron acceptor, reductase showed maximum activity between 6.8 and 7.5. The K(m) and Vmax values of the enzyme for ferricyanide were calculated as 0.024 mM ferricyanide and 673 mumol ferricyanide reduced/min/mg enzyme, respectively. The K(m) and Vmax values for the cofactor NADH in the presence of saturating amounts of ferricyanide were found to be 0.020 mM NADH and 699 mumol ferricyanide reduced/min/mg enzyme, respectively.

Role of carboxyl residues surrounding heme of human cytochrome b5 in the electrostatic interaction with NADH-cytochrome b5 reductase

To identify the cytochrome b5 residues responsible for the electrostatic interaction with NADH-cytochrome b5 reductase (b5R), we prepared and characterized the cytochrome b5 mutants in which Glu41, Glu42, Glu63, Asp70, and Glu73 were replaced by Ala, utilizing site-directed mutagenesis and the expression system for cytochrome b5 in Escherichia coli. Apparent Km values of the wild type b5R for Glu42Ala cytochrome b5 and Asp70Ala cytochrome b5 were approximately three-fold and six-fold higher than that for the wild type cytochrome b5, respectively, while the kcat values for those mutants were not remarkably affected. In contrast, Glu41Ala, Glu63Ala, and Glu73Ala cytochrome b5 showed almost the same kinetic properties as the wild type cytochrome b5. Furthermore, kinetic studies on combinations of the cytochrome b5 and b5R mutants suggested the interaction between Glu42 and Asp70 of cytochrome b5 and Lys125 and Lys41 of b5R, respectively, in the reaction.

Electrostatic interaction between NADH-cytochrome b5 reductase and cytochrome b5 studied by site-directed mutagenesis

Electrostatic interaction between NADH-cytochrome b5 reductase and cytochrome b5 was studied by site-directed mutagenesis. The target residues for mutagenesis were selected on the basis of the previously reported chemical cross-linking study of these two proteins, which implicated possible charge-pair interactions between Lys-41, Lys-125, Lys-162, and Lys-163 of the enzyme, and Glu-47, Glu-48, Glu-52, Glu-60, Asp-64 (group A), and heme propionate of cytochrome b5. Mutant reductases that lost one of the above-listed Lys residues showed higher K(m) values for cytochrome b5 and lower kcat values than those of the wild type, suggesting that all of the examined Lys residues participate in binding with cytochrome b5 as reported previously. In contrast, a removal of one of (or even all of) the group A residues from cytochrome b5 by mutagenesis caused no significant effect on the catalytic properties of cytochrome b5. Additional elimination of another set of negative residues (Glu-41, Glu-42, Asp-57, and Glu-63 (Group B)), which are also located close to heme, elevated the K(m) value by more than five folds. These results suggest that there should be other acidic residue(s) than group A in cytochrome b5 which participate in binding with NADH-cytochrome b5 reductase.

A novel cytochrome b5-like domain is linked to the carboxyl terminus of the Saccharomyces cerevisiae delta-9 fatty acid desaturase

Cytochrome b5 is an amphipathic mobile membrane protein that is predominantly located at the endoplasmic reticulum surface. It is an essential component of a number of membrane-bound redox systems. In animal and fungal cells cytochrome b5 is thought to be an electron donor for sterol modifying enzymes and fatty acid desaturases. Disruption of the Saccharomyces cytochrome b5 gene, however, yielded cells that had no nutritional requirement for either sterols or unsaturated fatty acids. Expression of sterol and fatty acid-modifying genes was increased in the cytochrome b5-disrupted cells, however, suggesting that cytochrome b5 may play some nonessential role in these functions. Unsaturated fatty acids in yeast are formed by Ole1p, an oxygen-dependent delta-9 fatty acid desaturase that is an intrinsic endoplasmic reticulum membrane protein. Although the yeast delta-9 fatty acid desaturase does not appear to require cytochrome b5, introduction of the rat liver stearoyl-CoA desaturase gene into an ole1-disrupted, cytochrome b5-disrupted yeast strain revealed that this enzyme specifically requires cytochrome b5 to function. Comparison of the coding sequences of the yeast and rat desaturase genes showed that the yeast protein contains a 113-amino acid carboxyl-terminal extension not found in the rat enzyme. That extension has regions of strong homology to cytochrome b5, particularly in the heme binding and electron transfer motifs. Truncation or disruption of the desaturase cytochrome b5-like domain in cells that contain the wild type diffusible b5 produced unsaturated fatty acid auxotrophy, suggesting that the cytochrome b5-like domain of Ole1p plays an essential role in the desaturase reaction.

Cytochrome b5, its functions, structure and membrane topology

The first part of the present communication reviews recent advances in our understanding of the known physiological functions of cytochrome b5. In addition, one section is devoted to a description of a recently discovered function of cytochrome b5, namely its involvement in the synthesis of the oncofetal antigen N-glycolylneuraminic acid. The second part of the article summarizes site-directed mutagenesis studies, primarily conducted in the author's laboratory, in both the catalytic heme-binding and membrane-binding domain of cytochrome b5. These studies have shown that: 1) the membrane binding domain of cytochrome b5 spans the bilayer; 2) cytochrome b5 lacking 19 COOH-terminal amino acids does not bind to membrane bilayers; and 3) specific amino acids in the membrane binding domain have been mutated and shown not to be essential for the function of cytochrome b5 with its redox partners.

Shedding and isolation of the delta 6-desaturase system from rat liver microsomes by application of high hydrostatic pressure

A procedure using high hydrostatic pressure without detergent has been applied in this study to subfractionate the components of the delta 6-desaturase system from the rat liver endoplasmic reticulum. Microsomes were suspended in a buffer containing liposomes made of phosphatidylcholine. The mixture was placed in a sealed pressure bomb and subjected to hydrostatic pressure of up to 1500 bars. Under these conditions the total desaturase activity was found in the liposomal fraction thus indicating that the three components of the desaturase system, namely NADH-cytochrome b5 reductase, cytochrome b5 and the delta 6-desaturase co-extracted. Size chromatography and FPLC of the released proteins followed by SDS-PAGE confirmed the independent release of the three components corresponding to the delta 6-desaturase, system. delta 6-Desaturase activity could be fully regenerated by mixing the aqueous dispersions of the three components without further purification. Our results indicate that these components are physically facing a similar lipid environment in the microsomal membrane.