Cytochrome b5 and NADH-cytochrome-b5 reductase from sipunculan erythrocytes; a methemerythrin reduction system from Phascolopsis gouldii

Protonation of the Hydroperoxo Intermediate of Cytochrome P450 2B4 Is Slower in the Presence of Cytochrome P450 Reductase Than in the Presence of Cytochrome b5

Microsomal cytochromes P450 (P450) require two electrons and two protons for the oxidation of substrates. Although the two electrons can be provided by cytochrome P450 reductase, the second electron can also be donated by cytochrome b5 (b5). The steady-state activity of P450 2B4 is increased up to 10-fold by b5. To improve our understanding of the molecular basis of the stimulatory effect of b5 and to test the hypothesis that b5 stimulates catalysis by more rapid protonation of the anionic ferric hydroperoxo heme intermediate of P450 (Fe³⁺OOH)^- and subsequent formation of the active oxidizing species (Fe⁺⁴═O POR^•+), we have freeze-quenched the reaction mixture during a single turnover following reduction of oxyferrous P450 2B4 by each of its redox partners, b5 and P450 reductase. The electron paramagnetic resonance spectra of the freeze-quenched reaction mixtures lacked evidence of a hydroperoxo intermediate when b5 was the reductant presumably because hydroperoxo protonation and catalysis occurred within the dead time of the instrument. However, when P450 reductase was the reductant, a hydroperoxo P450 intermediate was observed. The effect of b5 on the enzymatic efficiency in D₂O and the kinetic solvent isotope effect under steady-state conditions are both consistent with the ability of b5 to promote rapid protonation of the hydroperoxo species and more efficient catalysis. In summary, by binding to the proximal surface of P450, b5 stimulates the activity of P450 2B4 by enhancing the rate of protonation of the hydroperoxo intermediate and formation of Compound I, the active oxidizing species, which allows less time for side product formation.

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.

The nature of aqueous tunneling pathways between electron-transfer proteins

Structured water molecules near redox cofactors were found recently to accelerate electron-transfer (ET) kinetics in several systems. Theoretical study of interprotein electron transfer across an aqueous interface reveals three distinctive electronic coupling mechanisms that we describe here: (i) a protein-mediated regime when the two proteins are in van der Waals contact; (ii) a structured water-mediated regime featuring anomalously weak distance decay at relatively close protein-protein contact distances; and (iii) a bulk water-mediated regime at large distances. Our analysis explains a range of otherwise puzzling biological ET kinetic data and provides a framework for including explicit water-mediated tunneling effects on ET kinetics.

Heterologous expression of Ascaris suum cytochrome b5 precursor protein: a histidine-tagged full-length presequence is correctly processed to transport the mature protein to the periplasm of Escherichia coli

The cytochrome b(5) of the body wall of adult Ascaris suum, a porcine parasitic nematode, is a novel type of cytochrome b(5). It is a soluble protein that lacks the COOH-terminal membrane-anchoring domain found in erythrocyte cytochrome b(5), but possesses an NH(2)-terminal extension (presequence) of 30 amino acids that are missing from the 82-residue protein purified from the nematode tissues [Yu, Y., Yamasaki, H., Kita, K., and Takamiya, S., 1996, Arch. Biochem. Biophys. 328, 165-172]. The nematode cytochrome b(5) is, therefore, probably synthesized as a precursor protein whose presequence is cleaved to form a mature protein, but the localization of the mature protein is still unknown. To investigate the processing of the putative precursor protein, the wild-type precursor of nematode cytochrome b(5) with a complete presequence (b5wt) and its NH(2) terminus-truncated derivatives, b5Delta18 and b5Delta28, with 18 and 28 residues deleted, respectively, were expressed using pET-28a(+) vector in Escherichia coli. As expected, all transformants, tb5wt, tb5Delta18, and tb5Delta28, produced recombinant proteins with a histidine-tagged NH(2)-terminal extension. However, only the recombinant protein with the full-length presequence, produced in tb5wt, was correctly processed and transported to the periplasm, from which the majority of the induced product was purified as a mature protein chemically and functionally identical to the native protein purified from the nematode body wall. These results clearly show that the nematode histidine-tagged presequence functions as a signal peptide in E. coli.

Functional role of leucine-103 in myohemerythrin

Hemerythrins (Hrs) and myohemerythrins (Mhrs) are nonheme iron proteins that function as O2 carriers in four marine invertebrate phyla. Available amino acid sequences and X-ray structures indicate that a conserved leucine, residue 103 in the Themiste zostericola Mhr sequence, occupies a site distal to the Fe-O-Fe center. The side-chain methyl groups of the analogous leucine in Themiste dyscrita oxyHr are in van der Waals contact with bound O2 in the X-ray crystal structure, and this residue may therefore play a role in stabilizing bound dioxygen with respect to autoxidation. In order to test this hypothesis, the gene for T. zostericola Mhr was synthesized and expressed in Escherichia coli. Two mutant Mhrs, L103V and L103N, were also prepared. Optical spectra and kinetics data for these three proteins are presented. Importantly, neither mutant forms a stable oxy adduct; instead, rapid autoxidation results in formation of the corresponding met forms. In addition, the L103N Mhr displays unusually rapid reduction kinetics, suggesting that the amide functionality of Asn-103 destabilizes most bound ligands and additionally promotes rapid semi-metR <==> semi-metO isomerization.

Purification and characterization of cytochrome b5 reductase from the house fly, Musca domestica

NADH-cytochrome b5 reductase (b5R) from the house fly was purified through solubilization of microsomes with Triton X-100 followed by DEAE, carboxylmethyl and 5'-ADP affinity column chromatography. Yields of 9% with a 320-fold increase in NADH-ferricyanide reductase specific activity and 2% with a 76-fold increase in NADH-cytochrome b5R specific activity were obtained. Two forms of b5R, a major form with the apparent molecular mass of 31 kDa and a minor form of 33 kDa, were obtained. Both forms of purified b5R could reduce cytochrome b5 and both could use NADH or NADPH as an electron donor, although NADH was more efficient. Kinetics of the b5R activities were also studied. The 31-kDa b5R consists of integral of 291 amino acids with the NH2-terminal sequence of Thr-Ala-Arg-Leu-Arg-Thr-Leu-Ile-Asp-Ala. An antiserum developed against the 31-kDa b5R recognized both forms of b5R. Using this polyclonal antiserum as a probe, immunologically reactive proteins were found in microsomes from five species of Diptera, mouse and rat liver but not in spider mites nor insects from other orders.

The cytochrome b5-fold: an adaptable module

The family of b5-like cytochromes encompasses, besides cytochrome b5 itself, hemoprotein domains covalently associated with other redox proteins, in flavocytochrome b2 (L-lactate dehydrogenase), sulfite oxidase and assimilatory nitrate reductase. A comparison of about 40 amino acid sequences deposited in data banks shows that eight residues are invariant and about 15 positions carry strongly conservative substitutions. Examination of the location of these invariant and conserved positions in the light of the three-dimensional structures of beef cytochrome b5 and S cerevisiae flavocytochrome b2 suggests a strongly conserved protein structure for the b5-like heme-binding domain throughout evolution. Numerous NMR studies have demonstrated the existence of a positional isomerism for the heme, which involves both a 180 degree-rotation around the heme alpha,gamma-meso carbon atoms and a rotation through an axis normal to the heme plane at the iron. NMR studies did not detect significant differences in protein structure between reduced and oxidized states, or between species. The role of a number of side chains was probed by site-directed mutagenesis. Studies of complex formation and of electron transfer rates between cytochrome b5 and redox partners have led to the idea that complexation is driven by electrostatic forces, that it is generally the exposed heme edge which makes contact with electron donors and acceptors, but that there are multiple overlapping sites within this general area. For the bi- and trifunctional members of the family, extrapolation of available data would suggest a mobile heme-binding domain within a complex structure. In these cases the existence of a single interaction area for both electron donor and acceptor, or of two different ones, remains open to discussion.

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.

Mouse liver cytidine-5'-monophosphate-N-acetylneuraminic acid hydroxylase. Catalytic function and regulation

In this paper, we present the results of an investigation into the catalytic properties of CMP-Neu5Ac hydroxylase (Neu5Ac: N-acetylneuraminic acid) in high-speed supernatants of mouse liver. The enzyme was most active in Hepes/NaOH pH 7.4 and was markedly inhibited by relatively small increases in ionic strength, though the inhibition was abolished by desalting procedures. Several nonionic detergents could activate the hydroxylase to various degrees in a concentration-dependent manner. Ionic detergents and a number of phospholipids were, however, generally inert or inhibitory. The lack of inhibitory influence of a wide range of nucleotides revealed that CMP-Neu5Ac hydroxylase binds its sugar-nucleotide substrate with a high degree of specificity. Thus, even millimolar concentrations of several cytidine nucleotides elicited virtually negligible inhibition, though the reaction product, CMP-Neu5Gc (Neu5Gc: N-glycoloylneuraminic acid), was a weak inhibitor. The results also indicate that the enzyme is not regulated by any nucleotides or sugar-nucleotides. Dilution of high-speed supernatants with buffer gave rise to a decrease in the specific activity of the hydroxylase, implicating the involvement of more than one component in catalysis. Activity could be restored by the addition of a heat extract of the supernatant. The active principle in this extract was found to be a heat-stable protein with a molecular mass of about 17 kDa. Immunochemical studies allowed this protein to be identified as cytochrome b5 and it was shown that this electron carrier is essential for the activity of CMP-Neu5Ac hydroxylase. Inhibition studies using iron ligands and activation by exogenous iron salts suggest the involvement of a non-haem iron cofactor in the catalytic cycle of this hydroxylase. Cytochrome b5 may thus serve as an electron donor for this postulated cofactor.

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

The purification to homogeneity of the membrane-bound NADH-cytochrome-b5 reductase from erythrocytes of the sipunculid, Phascolopsis gouldii is reported. This highly purified reductase has allowed more detailed characterizations of its molecular and kinetic properties than was possible in a previous study (Utecht, R.E. and Kurtz, D.M., Jr. (1988) Biochim. Biophys. Acta 953, 164-178). The reductase has a molecular weight of 34,000 and contains FAD as the prosthetic group. In aqueous solution containing 0.5 vol% Triton X-100, the reductase forms an aggregate of Mr approximately 220,000. A higher purity preparation of P. gouldii erythrocyte b5 was also obtained. The combination of purified, solubilized reductase and cytochrome b5 was shown to catalyze the quantitative two-electron reduction of [Fe(III),Fe(III)]methemerythrin to [Fe(II),Fe(II)]deoxyhemerythrin by NADH. The P. gouldii NADH-cytochrome b5 reductase is the first from hemerythrin-containing erythrocytes to be purified and characterized. This methemerythrin reduction system appears to be analogous to methemoglobin reductases from vertebrate erythrocytes.

Investigation of the solution structures and mobility of oxidised and reduced cytochrome b5 by 2D NMR spectroscopy

Two-dimensional 1H NMR spectroscopy is used to examine the structure and mobility of cytochrome b5 in solution. The assignment of many residues and the interpretation of nuclear Overhauser effects (NOEs) in both redox states allow definition of secondary structural elements. Comparison with X-ray diffraction data shows that differences between crystal and solution structures are small. The dynamics of the protein are examined and the protein is shown to be more mobile than cytochrome c. The relationship of the structure and dynamics to the electron transfer function of cytochrome b5 is discussed.