ferrous-CO Forms Research Articles

Analysis of Heme Iron Coordination in DGCR8: The Heme-Binding Component of the Microprocessor Complex.

DGCR8 is the RNA-binding partner of the nuclease Drosha. Their complex (the "Microprocessor") is essential for processing of long, primary microRNAs (pri-miRNAs) in the nucleus. Binding of heme to DGCR8 is essential for pri-miRNA processing. On the basis of the split Soret ultraviolet-visible (UV-vis) spectrum of ferric DGCR8, bis-thiolate sulfur (cysteinate, Cys(-)) heme iron coordination of DGCR8 heme iron was proposed. We have characterized DGCR8 heme ligation using the Δ276 DGCR8 variant and combined electron paramagnetic resonance (EPR), magnetic circular dichroism (MCD), electron nuclear double resonance, resonance Raman, and electronic absorption spectroscopy. These studies indicate DGCR8 bis-Cys heme iron ligation, with conversion from bis-thiolate (Cys(-)/Cys(-)) axial coordination in ferric DGCR8 to bis-thiol (CysH/CysH) coordination in ferrous DGCR8. Pri-miRNA binding does not perturb ferric DGCR8's optical spectrum, consistent with the axial ligand environment being separated from the substrate-binding site. UV-vis absorption spectra of the Fe(II) and Fe(II)-CO forms indicate discrete species exhibiting peaks with absorption coefficients substantially larger than those for ferric DGCR8 and that previously reported for a ferrous form of DGCR8. Electron-nuclear double resonance spectroscopy data exclude histidine or water as axial ligands for ferric DGCR8 and favor bis-thiolate coordination in this form. UV-vis MCD and near-infrared MCD provide data consistent with this conclusion. UV-vis MCD data for ferrous DGCR8 reveal features consistent with bis-thiol heme iron coordination, and resonance Raman data for the ferrous-CO form are consistent with a thiol ligand trans to the CO. These studies support retention of DGCR8 cysteine coordination upon reduction, a conclusion distinct from those of previous studies of a different ferrous DGCR8 isoform.

Arg375 tunes tetrahydrobiopterin functions and modulates catalysis by inducible nitric oxide synthase

NO synthase enzymes (NOS) support unique single-electron transitions of a bound H4B cofactor during catalysis. Previous studies showed that both the pterin structure and surrounding protein residues impact H4B redox function during catalysis. A conserved Arg residue (Arg375 in iNOS) forms hydrogen bonds with the H4B ring. In order to understand the role of this residue in modulating the function of H4B and overall NO synthesis of the enzyme, we generated and characterized three mutants R375D, R375K and R375N of the oxygenase domain of inducible NOS (iNOSoxy). The mutations affected the dimer stability of iNOSoxy and its binding affinity toward substrates and H4B to varying degrees. Optical spectra of the ferric, ferrous, ferrous dioxy, ferrous-NO, ferric-NO, and ferrous-CO forms of each mutant were similar to the wild-type. However, mutants displayed somewhat lower heme midpoint potentials and faster ferrous heme-NO complex reactivity with O2. Unlike the wild-type protein, mutants could not oxidize NOHA to nitrite in a H2O2-driven reaction. Mutation could potentially change the ferrous dioxy decay rate, H4B radical formation rate, and the amount of the Arg hydroxylation during single turnover Arg hydroxylation reaction. All mutants were able to form heterodimers with the iNOS G450A full-length protein and displayed lower NO synthesis activities and uncoupled NADPH consumption. We conclude that the conserved residue Arg375 (1) regulates the tempo and extent of the electron transfer between H4B and ferrous dioxy species and (2) controls the reactivity of the heme-based oxidant formed after electron transfer from H4B during steady state NO synthesis and H2O2-driven NOHA oxidation. Thus, Arg375 modulates the redox function of H4B and is important in controlling the catalytic function of NOS enzymes.

Modulation of Redox Potential and Alteration in Reactivity via the Peroxide Shunt Pathway by Mutation of Cytochrome P450 around the Proximal Heme Ligand

In the thermophilic cytochrome P450 from the thermoacidophilic crenarchaeon Sulfolobus tokodaii strain 7 (P450st), a phenylalanine residue at position 310 and an alanine residue at position 320 are located close to the heme thiolate ligand, Cys317. Single site-directed mutants F310A and A320Q and double mutant F310A/A320Q have been constructed. All mutant enzymes as well as wild-type (WT) P450st were expressed at high levels. The substitution of F310 with Ala and of A320 with Gln induced shifts in redox potential and blue shifts in Soret absorption of ferrous-CO forms, while spectral characterization showed that in the resting state, the mutants almost retained the structural integrity of the active site. The redox potential of the heme varied as follows: -481 mV (WT), -477 mV (A320Q), -453 mV (F310A), and -450 mV (F310A/A320Q). The trend in the Soret band of the ferrous-CO form was as follows: 450 nm (WT) < 449 nm (A320Q) < 446 nm (F310A) < 444 nm (F310A/A320Q). These results established that the reduction potential and electron density on the heme iron are modulated by the Phe310 and Ala320 residues in P450st. The electron density on the heme decreases in the following order: WT > A320Q > F310A > F310A/A320Q. The electron density on the heme iron infers an essential role in P450 activity. The decrease in electron density interferes with the formation of a high-valent oxo-ferryl species called Compound I. However, steady-state turnover rates of styrene epoxidation with H2O2 show the following trend: WT approximately equal to A320Q < F310A approximately equal to F310A/A320Q. The shunt pathway which can provide the two electrons and oxygen required for a P450 reaction instead of NAD(P)H and dioxygen can rule out the first and second heme reduction in the catalytic process. Because the electron density on the heme iron might be deeply involved in the k cat values in this system, the intermediate Compound 0 which is the precursor species of Compound I mainly appears to participate dominantly in epoxidation with H2O2.

A Tryptophan that Modulates Tetrahydrobiopterin-Dependent Electron Transfer in Nitric Oxide Synthase Regulates Enzyme Catalysis by Additional Mechanisms

Nitric oxide synthases (NOSs) are flavo-heme enzymes that require (6R)-tetrahydrobiopterin (H(4)B) for activity. Our single-catalytic turnover study with the inducible NOS oxygenase domain showed that a conserved Trp that interacts with H(4)B (Trp457 in mouse inducible NOS) regulates the kinetics of electron transfer between H(4)B and an enzyme heme-dioxy intermediate, and this in turn alters the kinetics and extent of Arg hydroxylation [Wang, Z.-Q., et al. (2001) Biochemistry 40, 12819-12825]. To investigate the impact of these effects on NADPH-driven NO synthesis by NOS, we generated and characterized the W457A mutant of inducible NOS and the corresponding W678A and W678F mutants of neuronal NOS. Mutant defects in protein solubility and dimerization were overcome by purifying them in the presence of sufficient Arg and H(4)B, enabling us to study their physical and catalytic profiles. Optical spectra of the ferric, ferrous, heme-dioxy, ferrous-NO, ferric-NO, and ferrous-CO forms of each mutant were similar to that of the wild type. However, the mutants had higher apparent K(m) values for H(4)B and in one mutant for Arg (W457A). They all had lower NO synthesis activities, uncoupled NADPH consumption, and a slower and less prominent buildup of enzyme heme-NO complex during steady-state catalysis. Further analyses showed the mutants had normal or near-normal heme midpoint potential and heme-NO complex reactivity with O(2), but had somewhat slower ferric heme reduction rates and markedly slower reactivities of their heme-dioxy intermediate. We conclude that the conserved Trp (1) has similar roles in two different NOS isozymes and (2) regulates delivery of both electrons required for O(2) activation (i.e., kinetics of ferric heme reduction by the NOS flavoprotein domain and reduction of the heme-dioxy intermediate by H(4)B). However, its regulation of H(4)B electron transfer is most important because this ensures efficient coupling of NADPH oxidation and NO synthesis by NOS.

Roles of the proximal hydrogen bonding network in cytochrome P450cam-catalyzed oxygenation.

Structural and functional roles of the hydrogen bonding network that surrounds the heme-thiolate coordination of P450(cam) from Pseudomonas putida were investigated. A hydrogen bond between the side chain amide of Gln360 and the carbonyl oxygen of the axial Cys357 was removed in Q360L. The side chain hydrogen bond and the electrostatic interaction between the polypeptide amide proton of Gln360 and the sulfur atom of Cys357 were simultaneously removed in Q360P. The increased electron donation of the axial thiolate in Q360L and Q360P was evidenced by negative shifts of their reduction potentials by 45 and 70 mV, respectively. Together with the results on L358P in which the amide proton at position 358 was removed (Yoshioka, S., Takahashi, S., Ishimori, K., Morishima, I. J. Inorg. Biochem. 2000, 81, 141-151), we propose that the side chain hydrogen bond and the electrostatic interaction of the amide proton with the thiolate ligand cause approximately 45 and approximately 35 mV of positive shifts, respectively, of the redox potential of the heme in P450(cam). The resonance Raman spectra of the ferrous-CO form of the Q360 mutants showed a downshifted Fe-CO stretching mode at 482 approximately 483 cm(-)(1) compared with that of wild-type P450(cam) at 484 cm(-)(1). The Q360 mutants also showed the upshift by 4 approximately 5 cm(-)(1) of the Fe-NO stretching mode in the ferrous-NO form. These Raman results indicate the increase in the sigma-electron donation of the thiolate ligand in the reduced state of the Q360 mutants and were in contrast to the increased pi-back-donation of the thiolate in L358P having an upshifted Fe-CO stretching mode at 489 cm(-)(1). The catalytic activities of the Q360 mutants for the unnatural substrates were similar to those of the wild-type enzyme, indicating that the increased sigma-electron donation does not promote the O-O bond heterolysis in the Q360 mutants, although the increased pi-electron donation in L358P promoted the heterolysis of the O-O bond. We conclude that the functions of the proximal hydrogen bonding network in P450(cam) are to stabilize the heme-thiolate coordination, and to regulate the redox potential of the heme iron. Furthermore, we propose that the pi-electron donation, not the sigma-electron donation, of the thiolate ligand promotes the heterolysis of the O-O bond of dioxygen.

Sol-gel encapsulated horseradish peroxidase: a catalytic material for peroxidation.

This study addresses the viability of sol-gel encapsulated HRP (HRP:sol-gel) as a recyclable solid-state catalytic material. Ferric, ferric-CN, ferrous, and ferrous-CO forms of HRP:sol-gel were investigated by resonance Raman and UV-visible methods. Electronic and vibrational spectroscopic changes associated with changes in spin state, oxidation state, and ligation of the heme in HRP:sol-gel were shown to correlate with those of HRP in solution, showing that the heme remains a viable ligand-binding complex. Furthermore, the high-valent HRP:sol-gel intermediates, compound I and compound II, were generated and identified by time-resolved UV-visible spectroscopy. Catalytic activity of the HRP:sol-gel material was demonstrated by enzymatic assays by using I(-), guaiacol, and ABTS as substrates. Encapsulated HRP was shown to be homogeneously distributed throughout the sol-gel host. Differences in turnover rates between guaiacol and I(-) implicate mass transport of substrate through the silicate matrix as a defining parameter in the peroxidase activity of HRP:sol-gel. HRP:sol-gel was reused as a peroxidation catalyst for multiple reaction cycles without loss of activity, indicating that such materials show promise as reusable catalytic materials.

Enzymatic reaction of hydrogen peroxide-dependent peroxygenase cytochrome P450s: kinetic deuterium isotope effects and analyses by resonance Raman spectroscopy.

Cytochromes P450SP(alpha) (CYP152B1) and P450BS(beta) (CYP152A1), which are isolated from Sphingomonas paucimobilis and Bacillus subtilis, respectively, belong to the P450 superfamily, but catalyze hydroxylation reactions, in which an oxygen atom from H2O2 is efficiently introduced into fatty acids (e.g., myristic acid). P450SP(alpha) produces the alpha-hydroxylated (alpha-OH) products at 100%, while P450BS(beta) produces alpha- and beta-hydroxylated (beta-OH) products at 33 and 67%, respectively. Using deuterium-substituted fatty acids ([2,2-d2]-myristic acid and d27-myristic acid) as a substrate, the peroxygenase reactions of the two bacterial P450s were investigated. In the P450SP(alpha) reaction, we observed an intermolecular noncompetitive kinetic isotope effect on Vmax (DV = 4.1) when [2,2-d2]-myristic acid was used, suggesting that an isotopically sensitive step involving the alpha-hydrogen of the fatty acid is present in the catalytic cycle. On the other hand, D(V/K) was masked, in sharp contrast to the features of usual monooxygenases P450. The characteristic kinetic features can be interpreted in terms of the faster product formation than the substrate dissociation. A similar kinetic isotope effect was observed [DV = 4.9, D(V/K) approximately 1] for the P450BS(beta) reaction, when d27-myristic acid was used as a substrate, indicating that the reaction mechanism is the same for both peroxygenases. The resonance Raman spectral data of P450BS(beta) in the ferric and ferrous-CO forms in the presence and absence of myristic acid demonstrated that the catalytic pocket of the enzyme is polar, so that the location of the carboxylate of the substrate close to the sixth ligand of the heme could be allowed. On the basis of these results on the kinetic isotope effects and spectroscopy, we discuss the possible mechanisms of the alpha- and beta-hydroxylation of fatty acids catalyzed by peroxygenases P450SP(alpha) and P450BS(beta).

Proximal cysteine residue is essential for the enzymatic activities of cytochrome P450cam.

To investigate the functional and structural roles of the proximal thiolate ligand in cytochrome P450cam, we prepared the C357H mutant of the enzyme in which the axial cysteine residue (Cys357) was replaced with a histidine residue. We obtained the unstable C357H mutant by developing a new preparation procedure involving in vitro folding of P450cam from the inclusion bodies. The C357H mutant in the ferrous-CO form exhibited the Soret peak at 420 nm and the Fe-CO stretching line at 498 cm-1, indicating a neutral histidine residue as the axial ligand. However, another internal ligand is coordinated to the heme iron as the sixth ligand in the ferric and ferrous forms of the C357H mutant, suggesting the collapse of the substrate-binding site. The C357H mutant showed no catalytic activity for camphor hydroxylation and the reduced heterolytic/homolytic ratio of the O-O bond scission in the reaction with cumene hydroperoxide. The present observations indicate that the thiolate coordination in P450cam is important for the construction of the heme pocket and the heterolysis of the O-O bond.

Roles of the axial push effect in cytochrome P450cam studied with the site-directed mutagenesis at the heme proximal site

To examine the roles of the axial thiolate in cytochrome P450-catalyzed reactions, a mutant of cytochrome P450cam, L358P, was prepared to remove one of the conserved amide protons that are proposed to neutralize the negative charge of the thiolate sulfur. The increased push effect of the thiolate in L358P was evidenced by the reduced reduction potential of the heme. The 15N-NMR and resonance Raman spectra of the mutant in the ferric–CN – and in the ferrous–CO forms, respectively, also supported the increased push effect. The maintenance of stereo- and regioselectivities for d-camphor hydroxylation by the mutant suggests the minimum structural change at the distal site. The heterolysis/homolysis ratios of cumene hydroperoxide were the same for wild-type and L358P. However, we observed the enhanced monooxygenations of the unnatural substrates using dioxygen and electrons supplied from the reconstituted system, which indicate the significant role of the push effect in dioxygen activation. We interpret that the enhanced push effect inhibits the protonation of the inner oxygen atom and/or promotes the protonation of the outer oxygen atom in the putative iron-hydroperoxo intermediate (Fe 3+–O–OH) of P450cam. This work is the first experimental indication of the significance of the axial cysteine for the P450 reactivity.

Dynamic light-scattering and preliminary crystallographic studies of the sensor domain of the haem-based oxygen sensor FixL from Rhizobium meliloti

FixL is a transmitter protein in a two-component system which acts as an oxygen sensor when the symbiotic Rhizobia resides in root nodules of host plants. The oxygen-sensor domain of Rhizobium meliloti FixL (RmFixLH) was purified by His-tag affinity and isoelectronic focusing chromatographies, without the use of gel-filtration chromatography. Dynamic light-scattering measurements of RmFixLH thus obtained revealed it to be monodispersive and present as a homodimer in solution. A single crystal of RmFixLH in the met (Fe3+) form was grown in 100 mM acetic acid/NaOH buffer at pH 4.6 in the presence of 200 mM ammonium acetate, using 40%(w/v) PEG 4000 as a precipitant. A crystal of the ferrous CO form of RmFixLH was also prepared by reduction of the met crystal with Na2S2O4 in an atmosphere of CO. The crystals (0.2 x 0.05 x 0.01 mm) belong to the monoclinic system (C2) with unit-cell parameters a = 60.94, b = 37.44, c = 54.14 A, beta = 115.29 degrees and diffract X-rays to 1.7 A resolution at station BL44B2 of SPring-8, Japan. Bijvoet difference Patterson maps show a clear peak corresponding to the haem iron in RmFixLH.

Structural and Functional Characterization of Nitric Oxide Reductase

The iron coordination structure of nitric oxide reductase (NOR) was investigated by iron K-edge EXAFS. The EXAFS of the ferrous form of NOR is similar to that of cytochrome P450cam (P450cam), but the EXAFS of its ferrous-CO forms exhibited a subtle but significant difference from those of P450cam. The structural basis of unique function of the NOR is discussed.

Spectroscopic evidence for a conformational transition in horseradish peroxidase at very low pH.

Resonance Raman (RR), electronic absorption, and circular dichroism (CD) spectroscopies of the ferric, ferrous, and ferrous-CO forms of horseradish peroxidase (HRP-C) at pH 3.1 are reported. The CD spectra in the UV region show only a small decrease in the alpha-helical content upon pH lowering, whereas dramatic changes are observed in the Soret region. The final form of ferric HRP-C is 5-coordinate high-spin heme whose histidine ligand is replaced by a water ligand with a polar character. The electronic and CD spectra show the presence of an intermediate form with a 6-coordinate heme. Therefore, the cleavage of the proximal Fe-imidazole bond is preceded by the binding of a distal water molecule. For the ferrous form of HRP-C, the pH-dependence of the absorption spectra revealed only the native form in the range pH 5-7 and an unfolded form with a Soret maximum at 383 nm at pH 3.1. An intermediate state, characterized by a Soret maximum at 424 nm, was observed only in a transient way, within a few milliseconds. A metastable and a final species are observed also for the ferrous-CO complex at pH 3.1, as proved by isosbestic points in the electronic absorption spectra. The two forms show different RR nu(Fe-C) and IR nu(CO) modes. The metastable form corresponds to a heme where histidine is replaced by water. The final form is due to the displacement of the water ligand by the proximal histidine. We propose a kinetic model to account for our results at pH 3.1 for the ferric, ferrous, and ferrous-CO forms.

Resonance Raman spectroscopy reveals novel ligation properties of the porcine myoglobin double mutant H64V/V68H

A resonance Raman spectroscopic study of the porcine myoglobin double mutant H64V/V68H has confirmed that the ferric form is bis-histidine ligated, has revealed that the bis-histidine ligation is retained on reduction to the ferrous form, and has demonstrated that CO can displace the ligated distal histidine to produce a ferrous CO form which has a low steady-state photolability, indicating that the replacement histidine blocks the CO escape route from the binding site.

Evidence that a formyl-substituted iron porphyrin is the prosthetic group of myeloperoxidase: magnetic circular dichroism similarity of the peroxidase to Spirographis heme-reconstituted myoglobin.

To probe the identity of the active site heme-type prosthetic group of myeloperoxidase, whose structure has not been established unambiguously [proposed structures are (i) a chlorin (dihydroporphyrin) or (ii) a formyl-substituted porphyrin such as present in heme a], Spirographis heme (2-formyl-4-vinyldeuteroheme IX) has been incorporated into apo-myoglobin as a possible iron porphyrin model. Comparison of parallel derivatives of these two green proteins with magnetic circular dichroism spectroscopy reveals considerable similarities between several derivatives of these proteins, including the pyridine hemochromogen, the native ferric, ferrous-oxy, and ferrous-CO forms. In contrast, the magnetic circular dichroism spectra of available iron chlorin (octaethylchlorin) model complexes in analogous ligation and oxidation states do not show any significant spectral similarities to myeloperoxidase. This finding provides important evidence in favor of a formyl-substituted porphyrin as the structure of the prosthetic group macrocycle of myeloperoxidase.

Effects of cyanogen bromide modification of the distal histidine on the spectroscopic and ligand binding properties of myoglobin: magnetic circular dichroism spectroscopy as a probe of distal water ligation in ferric high-spin histidine-bound heme proteins

Magnetic circular dichroism (MCD) spectroscopy has been utilized to characterize the change in coordination structure in native ferric sperm whale myoglobin upon cyanogen bromide-modification. Comparison of the MCD properties of the ferric high-spin state of cyanogen bromide-modified myoglobin (BrCN-Mb) with those of native ferric horseradish peroxidase and Aplysia myoglobin suggests that ferric BrCN-Mb is a potential MCD model for the pentacoordinate state of ferric high-spin histidine-ligated heme proteins. These five-coordinate heme proteins afford a relatively weak and unsymmetric signal in the Soret region of the MCD spectrum. In contrast, native ferric myoglobin and the benzohydroxamic acid adduct of ferric horseradish peroxidase show a strong and symmetric derivative-shaped Soret MCD signal which is indicative of hexacoordination with water and histidine axial ligands. Therefore it seems that MCD spectroscopy could be used to probe the presence of water ligated to the distal side of ferric high-spin heme proteins. The MCD spectra of the ferric-azide, ferrous-deoxy and ferrous-CO forms of BrCN-Mb have also been measured and compared to those of analogous native myoglobin complexes. The present MCD study has been extended to include new ligands, NO, thiocyanate and cyanate, which bind to ferric BrCN-Mb. With exogenous ligands such as CO, NO and thiocyanate, the coordination structures of the BrCN-Mb complexes are similar to those of the respective native myoglobin adducts. In the case of ferrous-deoxy and ferric-cyanate BrCN-Mb, however, the altered MCD spectra (and EPR for the latter) reveal changes in electronic structure which likely correlate with alterations of the coordination environment of these BrCN-Mb derivatives. Data are also presented which support the proposed tetrazole-bound structure for azide-treated BrCN-Mb (Hori, H., Fujii, M., Shiro, Y., Iizuka, T., Adachi, S. and Morishima, I. (1989) J. Biol. Chem. 264, 5715-5719) and the inability of the distal histidine of BrCN-Mb to stabilize the ferric ligand-bound state.

Effects of cholesterol analogues and inhibitors on the heme moiety of cytochrome P-450scc: a resonance Raman study.

Interactions of cholesterol analogues and inhibitors with the heme moiety of cytochrome P-450scc were examined by resonance Raman spectroscopy. The Raman spectra of ferric cytochrome P-450scc complexed with inhibitors such as cyanide, phenyl isocyanide, aminoglutethimide, and metyrapone were characteristic of low-spin state and were very similar. However, the effect of exchange of the sixth ligand from the oxygen atom (ferric low-spin state) to the nitrogen atom upon aminoglutethimide and metyrapone binding was seen as down-frequency shifts of the v3 band from 1503 to 1501 and 1502 cm-1, respectively, while cyanide and phenyl isocyanide binding caused an up-frequency shift of the v3 band to 1505 cm-1. The effects of cholesterol analogues [22(R)-hydroxycholesterol, 22(S)-hydroxycholesterol, 22-ketocholesterol, 20(S)-hydroxycholesterol, and 25-hydroxycholesterol] on a Fe2+-CO stretching frequency of cytochrome P-450scc in ferrous CO form were examined. The 22(R)-hydroxycholesterol complex could not give a clear Fe2+-CO stretching Raman band due to a strong photodissociability. 22(S)-Hydroxycholesterol and 25-hydroxycholesterol complexes gave the Raman bands at 487 and 483 cm-1, respectively, whereas 20(S)-hydroxycholesterol and 22-ketocholesterol complexes gave Fe2+-CO stretching frequencies (478 cm-1) almost identical with that without substrate (477 cm-1). These findings suggest the existence of the following physiologically important natures of the cytochrome P-450scc active site: (1) there is a strong steric interaction between heme-bound carbon monoxide and the 22(R)-hydroxyl group or the 22(R)-hydrogen of the steroid side chain and (2) the hydroxylation at the 20S position may cause a conformational change of the side-chain group relative to the heme.

An oxygen-binding flavohemoprotein from Alcaligenes eutrophus

A procedure is described for the purification of a soluble flavohemoprotein from the hydrogen bacterium Alcaligenes eutrophus. The isolated protein exists as a monomer with a molecular weight of approx. 43 000. The molecule contains two prosthetic groups, 1 mol each of noncovalently bound FAD and protoheme per monomer. The absorption spectra of the protein in its ferric, ferrous-deoxy and ferrous-carboxy forms are similar to those of hemoglobins, with the exception of the flavin contribution (absorption maxima — ferric form: 395, 456, 483, 645 nm; ferrous-deoxy form: 436, 560 nm; ferrous-CO form: 423, 539, 569 nm). The flavohemoprotein when reduced by NADH in aerobic solution is capable of binding oxygen reversibly. The stable oxygenated complex exhibits absorption maxima at 414, 541, and 576 nm. The protein catalyzes the reduction of various dyes and cytochrome c by NADH.