Purification and properties of a membrane-bound NADH-cytochrome-b5 reductase from erythrocytes of the sipunculid worm, Phascolopsis gouldii

Methemoglobin reduction by NADH-cytochrome b(5) reductase in Propsilocerus akamusi larvae

For oxygen respiration, a methemoglobin (metHb) reduction system is needed in the cell because metHb cannot bind oxygen. We examined the insect Propsilocerus akamusi larvae to elucidate the metHb reduction system in an organism that inhabits an oxygen-deficient environment. NADH-dependent reduction of metHb in coelomic fluid suggested the coexistence of cytochrome b5 reductase (b5R) and cytochrome b5 with hemoglobin in the fluid and that these proteins were involved in physiological metHb reduction in the larvae. The presence of b5R was revealed by purifying b5R to homogeneity from the midge larvae. Using purified components, we showed that larval metHb was reduced via the NADH-b5R (FAD)-cytochrome b5-metHb pathway, a finding consistent with that in aerobic vertebrates, specifically humans and rabbits, and b5R function between mammal and insect was conserved. b5R was identified as a monomeric FAD-containing enzyme; it had a molecular mass of 33.2 kDa in gel-filtration chromatography and approximately 37 kDa in SDS-PAGE analysis. The enzyme's optimal pH and temperature were 6.4 and 25 °C, respectively. The apparent Km and Vmax values were 345 μM and 160 μmol min(-1) mg(-1), respectively, for ferricyanide and 328 μM and 500 μmol min(-1) mg(-1), respectively, for 2,6-dichlorophenolindophenol. The enzyme reaction was inhibited by benzoate, p-hydroxymercuribenzoate, iodoacetamide, and iodoacetate, and was not inhibited by metal ions or EDTA.

Purified NADH-cytochrome b5 reductase is a novel superoxide anion source inhibited by apocynin: sensitivity to nitric oxide and peroxynitrite

Cytochrome b5 reductase (Cb5R) is a pleiotropic flavoprotein that catalyzes multiple one-electron reduction reactions with various redox partners in cells. In earlier work from our laboratory, we have shown its implication in the generation of reactive oxygen species (ROS), primarily a superoxide anion overshoot peak, which plays a major role as a triggering event for the acceleration of apoptosis in cerebellar granule neurons in culture. However, the results obtained in that work did not allow us to exclude the possibility that this superoxide anion production could be derived from Cb5R acting in concert with other cellular components. In this work, we have purified Cb5R from pig liver and we have experimentally shown that this enzyme catalyzed NADH-dependent production of superoxide anion, assayed with cytochrome c and nitroblue tetrazolium as detection reagents for this particular ROS. The basic kinetic parameters for this novel NADH-dependent activity of Cb5R at 37°C are Vmax = 3.0 ± 0.5 μmol/min/mg of purified Cb5R and KM(NADH) = 2.8 ± 0.3 μM NADH. In addition, we report that apocynin, a widely used inhibitor of nonmitochondrial ROS production in mammalian cell cultures and tissues, is a potent inhibitor of purified Cb5R activity at the concentrations used in the experiments done with cell cultures. In the presence of apocynin the KM(NADH) value of Cb5R increases, and docking simulations indicate that apocynin can bind to a site near to or partially overlapping the NADH binding site of Cb5R. Other ROS, such as nitric oxide and peroxynitrite, have inhibitory effects on purified Cb5R, providing the basis for a feedback cellular protection mechanism through modulation of excessive extramitochondrial superoxide anion production by Cb5R. Both kinetic assays and docking simulations suggest that nitric oxide-induced nitrosylation (including covalent adduction of nitroso functional groups) of Cb5R cysteines and peroxynitrite-induced tyrosine nitration and cysteine oxidation modified the conformation of the NADH binding domain leading to a decreased affinity of Cb5R for NADH.

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

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

Cytochrome b₅ reductase-cytochrome b₅ as an active P450 redox enzyme system in Phanerochaete chrysosporium: atypical properties and in vivo evidence of electron transfer capability to CYP63A2

Two central redox enzyme systems exist to reduce eukaryotic P450 enzymes, the P450 oxidoreductase (POR) and the cyt b₅ reductase-cyt b₅. In fungi, limited information is available for the cyt b(5) reductase-cyt b(5) system. Here we characterized the kinetic mechanism of (cyt b₅r)-cyt b₅ redox system from the model white-rot fungus Phanerochaete chrysosporium (Pc) and made a quantitative comparison to the POR system. We determined that Pc-cyt b₅r followed a "ping-pong" mechanism and could directly reduce cytochrome c. However, unlike other cyt b₅ reductases, Pc-cyt b₅r lacked the typical ferricyanide reduction activity, a standard for cyt b₅ reductases. Through co-expression in yeast, we demonstrated that the Pc-cyt b₅r-cyt b₅ complex is capable of transferring electrons to Pc-P450 CYP63A2 for its benzo(a)pyrene monooxygenation activity and that the efficiency was comparable to POR. In fact, both redox systems supported oxidation of an estimated one-third of the added benzo(a)pyrene amount. To our knowledge, this is the first report to indicate that the cyt b₅r-cyt b₅ complex of fungi is capable of transferring electrons to a P450 monooxygenase. Furthermore, this is the first eukaryotic quantitative comparison of the two P450 redox enzyme systems (POR and cyt b₅r-cyt b₅) in terms of supporting a P450 monooxygenase activity.

Oxymyohemerythrin: discriminating between O2 release and autoxidation

Myohemerythrin (Mhr) is a non-heme iron O2 carrier (with two irons in the active site) that is typically found in the retractor muscle of marine 'peanut' worms. OxyMhr may either release O2, or undergo an autoxidation reaction in which hydrogen peroxide is released and diferric metMhr is produced. The autoxidation reaction can also be promoted by the addition of certain anions to Mhr solutions. This work, using recombinant Themiste zostericola Mhrs, contrasts the results of environmental effects on these reactions. For the O2 release reaction, deltaVdouble dagger(21.5 degrees C) = +28+/-3 cm3 mol(-1), deltaHdouble dagger(1 atm) = +22+/-1 kcal mol(-1), and deltaSdouble dagger(1 atm) = +28+/-4 eu. The autoxidation reaction (pH 8.0, 21.5 degrees C, 1 atm) displays different kinetic parameters: deltaVdouble dagger = -8+/-2 cm3 mol(-1), deltaHdouble dagger = +24.1+/-0.7 kcal mol(-1), and deltaSdouble dagger = +1+/-1 eu. Autoxidation in the presence of sodium azide is orders of magnitude faster than solvolytic autoxidation. The deltaVdouble dagger parameters for azide anation and azide-assisted autoxidation reaction are +15+/-2 and +59+/-2 cm3 mol(-1), respectively, indicating that the rate-limiting steps for the Mhr autoxidation and anation reactions (including O2 uptake) are not associated with ligand binding to the Fe2 center. The L103V and L103N oxyMhr mutants autoxidize approximately 10(3)-10(5) times faster than the wild-type protein, emphasizing the importance of leucine-103, which may function as a protein 'gate' in stabilizing bound dioxygen.

Conversion of non-functional to functional iron following reconstitution of hemerythrin

A recent report from this laboratory (Zhang, J.-H., Kurtz, D.M., Jr., Xia, Y.-M. and Debrunner, P.G. (1991) Biochemistry 30, 583-589) described a procedure for reconstitution of a functional di-iron site in the octameric, non-heme iron O2-carrying protein, hemerythrin by addition of ferrous salts to apoprotein, followed by slow dilution of the denaturant. Although the resulting protein contained its full complement of iron, i.e., 2 Fe per subunit, about 30% of the iron was found to remain ferrous under ambient O2, i.e., this iron was incapable of forming an O2 adduct. In this report a method is described for obtaining essentially fully functional hemerythrin by passage of the freshly reconstituted protein through an [oxy/30% non-functional----met----deoxy----oxy redox cycle. UV/vis absorption and 57Fe Mössbauer spectroscopies show that little or no non-functional iron remains in the reconstituted oxyhemerythrin after the redox cycle. Quantitations of protein and diiron sites show that, during the first step of the redox cycle, the non-functional iron is converted to a form that is spectroscopically indistinguishable from that of native methemerythrin. Far-UV circular dichroism shows that the secondary structure of this reconstituted methemerythrin is essentially identical to that of native protein. Non-denaturing polyacrylamide gel electrophoresis shows that the size and charge of the native and reconstituted proteins before and after redox cycling are essentially identical. These results indicate that the non-functional iron is converted to a functional form by the redox cycling, and that the key step in this conversion is the [oxy/30% non-functional]----met transformation.

Metal substitutions at the diiron sites of hemerythrin and myohemerythrin: contributions of divalent metals to stability of a four-helix bundle protein

A general method is described for substitution of Mn(II) and Co(II) into the diiron sites of hemerythrin and myohemerythrin. Characterizations of these metal-substituted proteins show that their structures closely resemble those of the native proteins. In particular, the four-helix bundle structure appears to be maintained. The apomyohemerythrin retains most of the native helix content but is considerably less stable to denaturation than are the metal-containing proteins. The relative affinities of M(II) for apohemerythrin--namely, Co greater than Fe greater than Mn--parallel the stabilities of the M2myohemerythrins to denaturation by guanidinium chloride. These results indicate that for myohemerythrin (i) the majority of the helical structure found in the native protein does not require incorporation of M(II) and (ii) stabilization of the native structure relative to the fully unfolded structure appears to be due predominantly to M(II)-protein interactions, at least for M = Fe and Co. Incorporation of M(II) also generates unfolding cooperativity in myohemerythrin. This cooperativity can be attributed to interhelical interactions, which are prevented in the apoprotein by solvation of the seven metal ligand residues. The results are consistent with a minimal model for folding/unfolding of myohemerythrin and hemerythrin subunits consisting of the sequential equilibria, N in equilibrium with I in equilibrium with D, between native, intermediate, and fully unfolded states, respectively. The properties of apomyohemerythrin make it a candidate for the intermediate state, I.

Myohemerythrin from the sipunculid, Phascolopsis gouldii: purification, properties and amino acid sequence

Two previously unknown isoforms, labelled iso I and iso II, of the oxygen-carrying protein, myohemerythrin, have been isolated from carcasses of the sipunculid worm, Phascolopsis gouldii. The two isoforms have non-identical N-terminal amino acid sequences and slightly different absorption spectra in the met form. Far-ultraviolet circular dichroism shows that iso I contains approximately 69% alpha-helix. The complete amino acid sequence for iso I was obtained. The molecular weight calculated from this amino acid sequence and including the active site Fe-O-Fe unit, is 13,829. All of the physical and chemical properties of iso I noted above, including the amino acid sequence, are very similar to those of T. zostericola myohemerythrin. Except for the amino acid sequence, these properties are also very similar to that of a subunit in hemerythrin, the octameric analog found in hemerythrocytes. Only 58 of the 113 residues in P. gouldii hemerythrin are conserved in iso I. Sequence comparisons were used to help identify residues responsible for maintaining the common tertiary and diiron site structures in hemerythrin and myohemerythrin. The seven iron ligand residues previously identified in crystal structures of hemerythrin and myohemerythrin are conserved in iso I. However, none of the ten residue pairs previously identified as engaging in direct salt-bridge or hydrogen bond interactions between subunits in the hemerythrin octamer are conserved in iso I.

Purification and characterization of a soybean flour-inducible ferredoxin reductase of Streptomyces griseus

We have purified an NADH-dependent ferredoxin reductase from crude extracts of Streptomyces griseus cells grown in soybean flour-enriched medium. The purified protein has a molecular weight of 60,000 as determined by sodium dodecyl sulfate gel electrophoresis. The enzyme requires Mg2+ ion for catalytic activity in reconstituted assays, and its spectral properties resemble those of many other flavin adenine dinucleotide-containing flavoproteins. A relatively large number of hydrophobic amino acid residues are found by amino acid analysis, and beginning with residue 7, a consensus flavin adenine dinucleotide binding sequence, GXGXXGXXXA, is revealed in this protein. In the presence of NADH, the ferredoxin reductase reduces various electron acceptors such as cytochrome c, potassium ferricyanide, dichlorophenolindophenol, and nitroblue tetrazolium. However, only cytochrome c reduction by the ferredoxin reductase is enhanced by the addition of ferredoxin. In the presence of NADH, S. griseus ferredoxin and cytochrome P-450soy, the ferredoxin reductase mediates O dealkylation of 7-ethoxycoumarin.