Formation of nitric oxide synthase-iron(II) nitrosoalkane complexes: severe restriction of access to the iron(II) site in the presence of tetrahydrobiopterin

Insights into Nitrosoalkane Binding to Myoglobin Provided by Crystallography of Wild-Type and Distal Pocket Mutant Derivatives

Nitrosoalkanes (R-N═O; R = alkyl) are biological intermediates that form from the oxidative metabolism of various amine (RNH2) drugs or from the reduction of nitroorganics (RNO2). RNO compounds bind to and inhibit various heme proteins. However, structural information on the resulting Fe-RNO moieties remains limited. We report the preparation of ferrous wild-type and H64A sw MbII-RNO derivatives (λmax 424 nm; R = Me, Et, Pr, iPr) from the reactions of MbIII-H2O with dithionite and nitroalkanes. The apparent extent of formation of the wt Mb derivatives followed the order MeNO > EtNO > PrNO > iPrNO, whereas the order was the opposite for the H64A derivatives. Ferricyanide oxidation of the MbII-RNO derivatives resulted in the formation of the ferric MbIII-H2O precursors with loss of the RNO ligands. X-ray crystal structures of the wt MbII-RNO derivatives at 1.76-2.0 Å resoln. revealed N-binding of RNO to Fe and the presence of H-bonding interactions between the nitroso O-atoms and distal pocket His64. The nitroso O-atoms pointed in the general direction of the protein exterior, and the hydrophobic R groups pointed toward the protein interior. X-ray crystal structures for the H64A mutant derivatives were determined at 1.74-1.80 Å resoln. An analysis of the distal pocket amino acid surface landscape provided an explanation for the differences in ligand orientations adopted by the EtNO and PrNO ligands in their wt and H64A structures. Our results provide a good baseline for the structural analysis of RNO binding to heme proteins possessing small distal pockets.

Insight into the preferential N-binding versus O-binding of nitrosoarenes to ferrous and ferric heme centers

Nitrosoarenes (ArNOs) are toxic metabolic intermediates that bind to heme proteins to inhibit their functions. Although much of their biological functions involve coordination to the Fe centers of hemes, the factors that determine N-binding or O-binding of these ArNOs have not been determined. We utilize X-ray crystallography and density functional theory (DFT) analyses of new representative ferrous and ferric ArNO compounds to provide the first theoretical insight into preferential N-binding versus O-binding of ArNOs to hemes. Our X-ray structural results favored N-binding of ArNO to ferrous heme centers, and O-binding to ferric hemes. Results of the DFT calculations rationalize this preferential binding on the basis of the energies of associated spin-states, and reveal that the dominant stabilization forces in the observed ferrous N-coordination and ferric O-coordination are dπ-pπ* and dσ-pπ*, respectively. Our results provide, for the first time, an explanation why in situ oxidation of the ferrous-ArNO compound to its ferric state results in the observed subsequent dissociation of the ligand.

The nitrosoamphetamine metabolite is accommodated in the active site of human hemoglobin: Spectroscopy and crystal structure

Amphetamine-based (Amph) drugs are metabolized in humans to their hydroxylamine (AmphNHOH) and nitroso (AmphNO) derivatives. The latter metabolites are known to bind to the Fe centers of cytochrome P450 and other heme enzymes to inhibit their activities. Although these AmphNHOH/AmphNO metabolites are present in vivo, their interactions with the blood protein hemoglobin (Hb) and the muscle protein (Mb) have been largely discounted due to a perception that the relatively small heme active sites of Hb and Mb will not be able to accommodate the large AmphNO group. We report the 2.15 Å resolution X-ray crystal structure of the AmphNO adduct of adult human hemoglobin as the Hb [α-Fe^III(H₂O)][β-Fe^II(AmphNO)] derivative. We show that the binding of AmphNO to the β subunit is enabled by an E helix movement and stabilization of ligand binding by H-bonding with the distal His63 residue. We also observe an AmphNHOH group in the Xe2 pocket in close proximity to the α heme site in this derivative. Additionally, UV-vis spectroscopy was used to characterize this and related wt and mutant Mb adducts. Importantly, our X-ray crystal structure of this Hb-nitrosoamphetamine complex represents the first crystal structure of a wild-type heme protein adduct of any amphetamine metabolite. Our results provide a framework for further studies of AmphNHOH/AmphNO interactions with Hb and Mb as viable processes that potentially contribute to the overall biological inorganic chemistry of amphetamine drugs.

Nitrosoamphetamine binding to myoglobin and hemoglobin: Crystal structure of the H64A myoglobin-nitrosoamphetamine adduct

N-hydroxyamphetamine (AmphNHOH) is an oxidative metabolite of amphetamine and methamphetamine. It is known to form inhibitory complexes upon binding to heme proteins. However, its interactions with myoglobin (Mb) and hemoglobin (Hb) have not been reported. We demonstrate that the reactions of AmphNHOH with ferric Mb and Hb generate the respective heme-nitrosoamphetamine derivatives characterized by UV-vis spectroscopy. We have determined the X-ray crystal structure of the H64A Mb-nitrosoamphetamine complex to 1.73 Å resolution. The structure reveals the N-binding of the nitroso-d-amphetamine isomer, with no significant H-bonding interactions between the ligand and the distal pocket amino acid residues.

Spectroscopic, catalytic and binding properties of Bacillus subtilis NO synthase-like protein: comparison with other bacterial and mammalian NO synthases

Genome sequencing has shown the presence of genes coding for NO-synthase (NOS)-like proteins in bacteria. The roles and properties of these proteins remain unclear. UV-visible spectroscopy was used to characterize the recombinant NOS-like protein from Bacillus subtilis (bsNOS) in its ferric and ferrous states in the presence of various Fe(III)- and Fe(II)-heme-ligands and of a series of L-arginine (L-arg) analogs. BsNOS exhibited several spectroscopic and binding properties in common with Bacillus anthracis NOS (baNOS) that were clearly different from those of tetrahydrobiopterin (H4B)-free mammalian NOS oxygenase domains (mNOS(oxys)) and of Staphylococcus aureus NOS (saNOS). Interestingly, bsNOS and baNOS that do not contain H4B exhibited properties much closer to those of H4B-containing mNOS(oxys). Moreover, bsNOS was found to efficiently catalyze the oxidation of L-arginine into L-citrulline by H(2)O(2), whereas H4B-free mNOS(oxys) exhibited low activities for this reaction.

EPR and ENDOR characterization of the reactive intermediates in the generation of NO by cryoreduced oxy-nitric oxide synthase from Geobacillus stearothermophilus

Cryoreduction EPR/ENDOR/step-annealing measurements with substrate complexes of oxy-gsNOS (3; gsNOS is nitric oxide synthase from Geobacillus stearothermophilus) confirm that Compound I (6) is the reactive heme species that carries out the gsNOS-catalyzed (Stage I) oxidation of L-arginine to N-hydroxy-L-arginine (NOHA), whereas the active species in the (Stage II) oxidation of NOHA to citrulline and HNO/NO(-) is the hydroperoxy-ferric form (5). When 3 is reduced by tetrahydrobiopterin (BH4), instead of an externally supplied electron, the resulting BH4(+) radical oxidizes HNO/NO(-) to NO. In this report, radiolytic one-electron reduction of 3 and its complexes with Arg, Me-Arg, and NO(2)Arg was shown by EPR and (1)H and (14,15)N ENDOR spectroscopies to generate 5; in contrast, during cryoreduction of 3/NOHA, the peroxo-ferric-gsNOS intermediate (4/NOHA) was trapped. During annealing at 145 K, ENDOR shows that 5/Arg and 5/Me-Arg (but not 5/NO(2)Arg) generate a Stage I primary product species in which the OH group of the hydroxylated substrate is coordinated to Fe(III), characteristic of 6 as the active heme center. Analysis shows that hydroxylation of Arg and Me-Arg is quantitative. Annealing of 4/NOHA at 160 K converts it first to 5/NOHA and then to the Stage II primary enzymatic product. The latter contains Fe(III) coordinated by water, characteristic of 5 as the active heme center. It further contains quantitative amounts of citrulline and HNO/NO(-); the latter reacts with the ferriheme to form the NO-ferroheme upon further annealing. Stage I delivery of the first proton of catalysis to the (unobserved) 4 formed by cryoreduction of 3 involves a bound water that may convey a proton from L-Arg, while the second proton likely derives from the carboxyl side chain of Glu 248 or the heme carboxylates; the process also involves proton delivery by water(s). In the Stage II oxidation of NOHA, the proton that converts 4/NOHA to 5/NOHA likely is derived from NOHA itself, a conclusion supported by the pH invariance of the process. The present results illustrate how the substrate itself modulates the nature and reactivity of intermediates along the monooxygenase reaction pathway.

Myoglobin as an Efficient Electrocatalyst for Nitromethane Reduction

Xenobiotic metabolizing heme enzymes are thought to take a crucial part in the activation of a variety of carcinogens, including nitro compounds, through catalytic electron-transfer reactions, especially under anaerobic conditions. Myoglobin (Mb), as a model heme enzyme, is found to act as an efficient electrocatalyst for the reduction of nitromethane in thin surfactant films on pyrolytic graphite electrodes. The electrocatalytic process is characterized by cyclic voltammetry. The Mb-Fe(II)-nitrosomethane complex, a possible intermediate in the catalysis, is characterized spectroscopically in the surfactant film on indium tin oxide electrodes. Bulk electrolysis indicates the formation of mainly methylhydroxylamine as an end aqueous product. A rationale for the catalysis invokes the highly reduced Fe(I) state of myoglobin in surfactant film; the latter engages in efficient inner-sphere electron transfers to the nitro compound coupled to proton transfers.

Crystal structures of ferrous horse heart myoglobin complexed with nitric oxide and nitrosoethane

The interactions of nitric oxide (NO) and organic nitroso compounds with heme proteins are biologically important, and adduct formation between NO-containing compounds and myoglobin (Mb) have served as prototypical systems for studies of these interactions. We have prepared crystals of horse heart (hh) MbNO from nitrosylation of aqua-metMb crystals, and we have determined the crystal structure of hh MbNO at a resolution of 1.9 A. The Fe-N-O angle of 147 degrees in hh MbNO is larger than the corresponding 112 degrees angle previously determined from the crystal structure of sperm whale MbNO (Brucker et al., Proteins 1998;30:352-356) but is similar to the 150 degrees angle determined from a MS XAFS study of a frozen solution of hh MbNO (Rich et al., J Am Chem Soc 1998;120:10827-10836). The Fe-N(O) bond length of 2.0 A (this work) is longer than the 1.75 A distance determined from the XAFS study and suggests distal pocket influences on FeNO geometry. The nitrosyl N atom is located 3.0 A from the imidazole N(epsilon) atom of the distal His64 residue, suggesting electrostatic stabilization of the FeNO moiety by His64. The crystal structure of the nitrosoethane adduct of ferrous hh Mb was determined at a resolution of 1.7 A. The nitroso O atom of the EtNO ligand is located 2.7 A from the imidazole N(epsilon) atom of His64, suggesting a hydrogen bond interaction between these groups. To the best of our knowledge, the crystal structure of hh Mb(EtNO) is the first such determination of a nitrosoalkane adduct of a heme protein.

Kinetics of CO and NO ligation with the Cys(331)-->Ala mutant of neuronal nitric-oxide synthase

Nitric-oxide synthases (NOS) catalyze the conversion of l-arginine to NO, which then stimulates many physiological processes. In the active form, each NOS is a dimer; each strand has both a heme-binding oxygenase domain and a reductase domain. In neuronal NOS (nNOS), there is a conserved cysteine motif (CX(4)C) that participates in a ZnS(4) center, which stabilizes the dimer interface and/or the flavoprotein-heme domain interface. Previously, the Cys(331) --> Ala mutant was produced, and it proved to be inactive in catalysis and to have structural defects that disrupt the binding of l-Arg and tetrahydrobiopterin (BH(4)). Because binding l-Arg and BH(4) to wild type nNOS profoundly affects CO binding with little effect on NO binding, ligand binding to the mutant was characterized as follows. 1) The mutant initially has behavior different from native protein but reminiscent of isolated heme domain subchains. 2) Adding l-Arg and BH(4) has little effect immediately but substantial effect after extended incubation. 3) Incubation for 12 h restores behavior similar but not quite identical to that of wild type nNOS. Such incubation was shown previously to restore most but not all catalytic activity. These kinetic studies substantiate the hypothesis that zinc content is related to a structural rather than a catalytic role in maintaining active nNOS.

Formation of iron(II)-nitrosoalkane complexes: a new activity of microperoxidase 8

Microperoxidase 8 (MP8) is a heme octapeptide, obtained by enzymatic hydrolysis of heart cytochrome c, in which a histidine is axially coordinated to the heme iron, and acts as its fifth ligand. It exhibits two kinds of activities: a peroxidase-like activity and a cytochrome P450-like activity. We here show that MP8 is not only able to oxidize various aliphatic and aromatic hydroxylamines with the formation of MP8-Fe(II)-nitrosoalkane or -arene complexes absorbing around 414 nm, but also that these complexes can be obtained by reduction of nitroalkanes. This is the first example of fully characterized iron(II)-metabolite complexes of MP8. Such complexes constitute good models for those obtained upon oxidation of amphetamine or macrolids by cytochromes P450. In addition, this is a new catalytic activity of MP8, which validates the use of this mini-enzyme as a convenient model for hemoproteins of interest in toxicology and pharmacology such as cytochromes P450 and peroxidases.

Azo reduction of methyl red by neuronal nitric oxide synthase: the important role of FMN in catalysis

Nitric oxide synthase (NOS) is composed of an oxygenase domain and a reductase domain. The reductase domain has NADPH, FAD, and FMN binding sites. Wild-type nNOS reduced the azo bond of methyl red with a turnover number of approximately 130 min(-1) in the presence of Ca(2+)/calmodulin (CaM) and NADPH under anaerobic conditions. Diphenyleneiodonium chloride (DPI), a flavin/NADPH binding inhibitor, completely inhibited azo reduction. The omission of Ca(2+)/CaM from the reaction system decreased the activity to 5%. The rate of the azo reduction with an FMN-deficient mutant was also 5% that of the wild type. NADPH oxidation rates for the wild-type and mutant enzymes were well coupled with azo reduction. Thus, we suggest that electrons delivered from the FMN of the nNOS enzyme reduce the azo bond of methyl red and that this reductase activity is controlled by Ca(2+)/CaM.

Mutational analysis of the tetrahydrobiopterin-binding site in inducible nitric-oxide synthase

Inducible nitric-oxide synthase (iNOS) is a hemeprotein that requires tetrahydrobiopterin (H4B) for activity. The influence of H4B on iNOS structure-function is complex, and its exact role in nitric oxide (NO) synthesis is unknown. Crystal structures of the mouse iNOS oxygenase domain (iNOSox) revealed a unique H4B-binding site with a high degree of aromatic character located in the dimer interface and near the heme. Four conserved residues (Arg-375, Trp-455, Trp-457, and Phe-470) engage in hydrogen bonding or aromatic stacking interactions with the H4B ring. We utilized point mutagenesis to investigate how each residue modulates H4B function. All mutants contained heme ligated to Cys-194 indicating no deleterious effect on general protein structure. Ala mutants were monomers except for W457A and did not form a homodimer with excess H4B and Arg. However, they did form heterodimers when paired with a full-length iNOS subunit, and these were either fully or partially active regarding NO synthesis, indicating that preserving residue identities or aromatic character is not essential for H4B binding or activity. Aromatic substitution at Trp-455 or Trp-457 generated monomers that could dimerize with H4B and Arg. These mutants bound Arg and H4B with near normal affinity, but Arg could not displace heme-bound imidazole, and they had NO synthesis activities lower than wild-type in both homodimeric and heterodimeric settings. Aromatic substitution at Phe-470 had no significant effects. Together, our work shows how hydrogen bonding and aromatic stacking interactions of Arg-375, Trp-457, Trp-455, and Phe-470 influence iNOSox dimeric structure, heme environment, and NO synthesis and thus help modulate the multiple effects of H4B.