Rapid calmodulin-dependent interdomain electron transfer in neuronal nitric-oxide synthase measured by pulse radiolysis

Autoregulation and Autoinhibition of the Main NO Synthase Isoforms (Brief Review)

Nitric oxide (II) (NO) is the most important mediator of a wide range of physiological and pathophysiological processes. It is synthesized by NO synthases (NOSs), which have three main isoforms differing from each other in terms of activation and inhibition features, levels of NO production, subcellular localization, etc. At the same time, all isoforms are structurally very similar, and these differences are determined by NOS autoregulatory elements. The article presents an analysis of the autoregulatory and autoinhibitory mechanisms of the NOS reductase domain that determine differences in the productivity of isoforms, as well as their dependence on the concentration of Ca2+ ions. The main regulatory elements in NOS that modulate the electron transfer from flavin to heme include calmodulin (CaM), an autoinhibitory insert (AI), and the C-terminal tail (C-tail). Hydrophobic interactions of CaM with the surface of the NOS oxidase domain are assumed to facilitate electron transfer from flavin mononucleotide (FMN). CaM binding causes a change in the inter-domain distances, a shift of AI and the C-tail, and, as a result, a decrease in their inhibitory effect. CaM also shifts the conformational equilibrium of the reductase domain towards more open conformations, reduces the lifetime of conformations, their stereometric distribution, and accelerates the flow of electrons through the reductase domain. The AI element, apparently, induces a conformational change that hinders electron transfer within the reductase domain, similar to the hinge domain in cytochrome P450. Together with CaM, the C-tail regulates the electron flow between flavins, the distance and relative orientation of isoalloxane rings, and also modulates the electron flow from FMN to the terminal acceptor. Together with the C-tail, AI also predetermines the dependence of neuronal and endothelial forms of NOS on the concentration of Ca2+ ions, and the C-tail length affects differences in the productivity of NO synthesis. The inhibitory effect of the C-tail is likely to be reduced by CaM binding due to the C-tail shift due to the electrostatic repulsive forces of the negatively charged phosphate and aspartate residues. The autoregulatory elements of NOS require further study, since the mechanisms of their interaction are complex and multidirectional, and hence provide a wide range of characteristics of the observed isoforms.

Fields, biochemistry fast autooxidation of a Bis-Histidyl-ligated globin from the anhydrobiotic tardigrade, ramazzottius varieornatus, by molecular oxygen

Tardigrades, a phylum of meiofaunal organisms, exhibit extraordinary tolerance to various environmental conditions, including extreme temperatures (-272 to 151 °C) and exposure to ionizing radiation. Proteins from anhydrobiotic tardigrades with homology to known proteins from other organisms are new potential targets for structural genomics. Recently, we reported spectroscopic and structural characterization of a hexacoordinated hemoglobin (Kumaglobin [Kgb]) found in an anhydrobiotic tardigrade. In the absence of its exogenous ligand, Kgb displays hexacoordination with distal and proximal histidines. In this work, we analyzed binding of the molecular oxygen ligand following reduction of heme in Kgb using a pulse radiolysis technique. Radiolytically generated hydrated electrons (eaq-) reduced the heme iron of Kgb within 20 µs. Subsequently, ferrous heme reacted with O2 to form a ferrous-dioxygen intermediate with a second-order rate constant of 3.0 × 106 M-1 s-1. The intermediate was rapidly (within 0.1 s) autooxidized to the ferric form. Redox potential measurements revealed an E'0 of -400 mV (vs. SHE) in the ferric/ferrous couple. Our results suggest that Kgb may serve as a physiological generator of O2·- via redox signaling and/or electron transfer.

Conformational Motion of Ferredoxin Enables Efficient Electron Transfer to Heme in the Full-Length P450TT

Cytochrome P450 monooxygenases (P450s) are versatile biocatalysts used in natural products biosynthesis, xenobiotic metabolisms, and biotechnologies. In P450s, the electrons required for O₂ activation are supplied by NAD(P)H through stepwise electron transfers (ETs) mediated by redox partners. While much is known about the machinery of the catalytic cycle of P450s, the mechanisms of long-range ET are largely unknown. Very recently, the first crystal structure of full-length P450_TT was solved. This enables us to decipher the interdomain ET mechanism between the [2Fe-2S]-containing ferredoxin and the heme, by use of molecular dynamics simulations. In contrast to the "distal" conformation characterized in the crystal structure where the [2Fe-2S] cluster is ∼28 Å away from heme-Fe, our simulations demonstrated a "proximal" conformation of [2Fe-2S] that is ∼17 Å [and 13.7 Å edge-to-edge] away from heme-Fe, which may enable the interdomain ET. Key residues involved in ET pathways and interdomain complexation were identified, some of which have already been verified by recent mutation studies. The conformational transit of ferredoxin between "distal" and "proximal" was found to be controlled mostly by the long-range electrostatic interactions between the ferredoxin domain and the other two domains. Furthermore, our simulations show that the full-length P450_TT utilizes a flexible ET pathway that resembles either P450_Scc or P450_cam. Thus, this study provides a uniform picture of the ET process between reductase domains and heme domain.

Reaction Intermediates of Nitric Oxide Synthase from Deinococcus radiodurans as Revealed by Pulse Radiolysis: Evidence for Intramolecular Electron Transfer from Biopterin to FeII-O2 Complex

Nitric oxide synthase (NOS) is a cytochrome P450-type mono-oxygenase that catalyzes the oxidation of l-arginine (Arg) to nitric oxide (NO) through a reaction intermediate N-hydroxy-l-arginine (NHA). The mechanism underlying the reaction catalyzed by NOS from Deinococcus radiodurans was investigated using pulse radiolysis. Radiolytically generated hydrated electrons reduced the heme iron of NOS within 2 μs. Subsequently, ferrous heme reacted with O2 to form a ferrous-dioxygen intermediate with a second-order rate constant of 2.8 × 108 M-1 s-1. In the tetrahydrofolate (H4F)-bound enzyme, the ferrous-dioxygen intermediate was found to decay an another intermediate with a first-order rate constant of 2.2 × 103 s-1. The spectrum of the intermediate featured an absorption maximum at 440 nm and an absorption minimum at 390 nm. In the absence of H4F, this step did not proceed, suggesting that H4F was reduced with the ferrous-dioxygen intermediate to form a second intermediate. The intermediate further converted to the original ferric form with a first-order rate constant of 4 s-1. A similar intermediate could be detected after pulse radiolysis in the presence of NHA, although the intermediate decayed more slowly (0.5 s-1). These data suggested that a common catalytically active intermediate involved in the substrate oxidation of both Arg and NHA may be formed during catalysis. In addition, we investigated the solvent isotope effects on the kinetics of the intermediate after pulse radiolysis. Our experiments revealed dramatic kinetic solvent isotope effects on the conversion of the intermediate to the ferric form, of 10.5 and 2.5 for Arg and NHA, respectively, whereas the faster phases were not affected. These data suggest that the proton transfer in DrNOS is the rate-limiting reaction of the intermediate with the substrates.

A cross-domain charge interaction governs the activity of NO synthase

NO synthase (NOS) enzymes perform interdomain electron transfer reactions during catalysis that may rely on complementary charge interactions at domain-domain interfaces. Guided by our previous results and a computer-generated domain-docking model, we assessed the importance of cross-domain charge interactions in the FMN-to-heme electron transfer in neuronal NOS (nNOS). We reversed the charge of three residues (Glu-762, Glu-816, and Glu-819) that form an electronegative triad on the FMN domain and then individually reversed the charges of three electropositive residues (Lys-423, Lys-620, and Lys-660) on the oxygenase domain (NOSoxy), to potentially restore a cross-domain charge interaction with the triad, but in reversed polarity. Charge reversal of the triad completely eliminated heme reduction and NO synthesis in nNOS. These functions were partly restored by the charge reversal at oxygenase residue Lys-423, but not at Lys-620 or Lys-660. Full recovery of heme reduction was probably muted by an accompanying change in FMN midpoint potential that made electron transfer to the heme thermodynamically unfavorable. Our results provide direct evidence that cross-domain charge pairing is required for the FMN-to-heme electron transfer in nNOS. The unique ability of charge reversal at position 423 to rescue function indicates that it participates in an essential cross-domain charge interaction with the FMN domain triad. This supports our domain-docking model and suggests that it may depict a productive electron transfer complex formed during nNOS catalysis.

Thermodynamic characterization of five key kinetic parameters that define neuronal nitric oxide synthase catalysis

NO synthase (NOS) enzymes convert L-arginine to NO in two sequential reactions whose rates (k(cat1) and k(cat2)) are both limited by the rate of ferric heme reduction (k(r)). An enzyme ferric heme-NO complex forms as an immediate product complex and then undergoes either dissociation (at a rate that we denote as k(d)) to release NO in a productive manner, or reduction (k(r)) to form a ferrous heme-NO complex that must react with O2 (at a rate that we denote as k(ox)) in a NO dioxygenase reaction that regenerates the ferric enzyme. The interplay of these five kinetic parameters (k(cat1), k(cat2), k(r), k(d) and k(ox)) determines NOS specific activity, O2 concentration response, and pulsatile versus steady-state NO generation. In the present study, we utilized stopped-flow spectroscopy and single catalytic turnover methods to characterize the individual temperature dependencies of the five kinetic parameters of rat neuronal NOS. We then incorporated the measured kinetic values into computer simulations of the neuronal NOS reaction using a global kinetic model to comprehensively model its temperature-dependent catalytic behaviours. The results obtained provide new mechanistic insights and also reveal that the different temperature dependencies of the five kinetic parameters significantly alter neuronal NOS catalytic behaviours and NO release efficiency as a function of temperature.

The Heme-Based Oxygen-Sensor Phosphodiesterase Ec DOS (DosP): Structure-Function Relationships

Escherichia coli Direct Oxygen Sensor (Ec DOS, also known as Ec DosP) is a heme-based O2-sensing phosphodiesterase from Escherichia coli that catalyzes the conversion of cyclic-di-GMP to linear di-GMP. Cyclic-di-GMP is an important second messenger in bacteria, highlighting the importance of understanding structure-function relationships of Ec DOS. Ec DOS is composed of an N-terminal heme-bound O2-sensing PAS domain and a C-terminal phosphodiesterase catalytic domain. Notably, its activity is markedly enhanced by O2 binding to the heme Fe(II) complex in the PAS sensor domain. X-ray crystal structures and spectroscopic and catalytic characterization of the wild-type and mutant proteins have provided important structural and functional clues to understanding the molecular mechanism of intramolecular catalytic regulation by O2 binding. This review summarizes the intriguing findings that have obtained for Ec DOS.

Control of electron transfer and catalysis in neuronal nitric-oxide synthase (nNOS) by a hinge connecting its FMN and FAD-NADPH domains

In nitric-oxide synthases (NOSs), two flexible hinges connect the FMN domain to the rest of the enzyme and may guide its interactions with partner domains for electron transfer and catalysis. We investigated the role of the FMN-FAD/NADPH hinge in rat neuronal NOS (nNOS) by constructing mutants that either shortened or lengthened this hinge by 2, 4, and 6 residues. Shortening the hinge progressively inhibited electron flux through the calmodulin (CaM)-free and CaM-bound nNOS to cytochrome c, whereas hinge lengthening relieved repression of electron flux in CaM-free nNOS and had no impact or slowed electron flux through CaM-bound nNOS to cytochrome c. How hinge length influenced heme reduction depended on whether enzyme flavins were pre-reduced with NADPH prior to triggering heme reduction. Without pre-reduction, changing the hinge length was deleterious; with pre-reduction, the hinge shortening was deleterious, and hinge lengthening increased heme reduction rates beyond wild type. Flavin fluorescence and stopped-flow kinetic studies on CaM-bound enzymes suggested hinge lengthening slowed the domain-domain interaction needed for FMN reduction. All hinge length changes lowered NO synthesis activity and increased uncoupled NADPH consumption. We conclude that several aspects of catalysis are sensitive to FMN-FAD/NADPH hinge length and that the native hinge allows a best compromise among the FMN domain interactions and associated electron transfer events to maximize NO synthesis and minimize uncoupled NADPH consumption.

Identification and functional and spectral characterization of a globin-coupled histidine kinase from Anaeromyxobacter sp. Fw109-5

Two-component signal transduction systems regulate numerous important physiological functions in bacteria. In this study we have identified, cloned, overexpressed, and characterized a dimeric full-length heme-bound (heme:protein, 1:1 stoichiometry) globin-coupled histidine kinase (AfGcHK) from Anaeromyxobacter sp. strain Fw109-5 for the first time. The Fe(III), Fe(II)-O(2), and Fe(II)-CO complexes of the protein displayed autophosphorylation activity, whereas the Fe(II) complex had no significant activity. A H99A mutant lost heme binding ability, suggesting that this residue is the heme proximal ligand. Moreover, His-183 was proposed as the autophosphorylation site based on the finding that the H183A mutant protein was not phosphorylated. The phosphate group of autophosphorylated AfGcHK was transferred to Asp-52 and Asp-169 of a response regulator, as confirmed from site-directed mutagenesis experiments. Based on the amino acid sequences and crystal structures of other globin-coupled oxygen sensor enzymes, Tyr-45 was assumed to be the O(2) binding site at the heme distal side. The O(2) dissociation rate constant, 0.10 s(-1), was substantially increased up to 8.0 s(-1) upon Y45L mutation. The resonance Raman frequencies representing ν(Fe-O2) (559 cm(-1)) and ν(O-O) (1149 cm(-1)) of the Fe(II)-O(2) complex of Y45F mutant AfGcHK were distinct from those of the wild-type protein (ν(Fe-O2), 557 cm(-1); ν(O-O), 1141 cm(-1)), supporting the proposal that Tyr-45 is located at the distal side and forms hydrogen bonds with the oxygen molecule bound to the Fe(II) complex. Thus, we have successfully identified and characterized a novel heme-based globin-coupled oxygen sensor histidine kinase, AfGcHK, in this study.

Structural and mechanistic aspects of flavoproteins: electron transfer through the nitric oxide synthase flavoprotein domain

Nitric oxide synthases belong to a family of dual-flavin enzymes that transfer electrons from NAD(P)H to a variety of heme protein acceptors. During catalysis, their FMN subdomain plays a central role by acting as both an electron acceptor (receiving electrons from FAD) and an electron donor, and is thought to undergo large conformational movements and engage in two distinct protein-protein interactions in the process. This minireview summarizes what we know about the many factors regulating nitric oxide synthase flavoprotein domain function, primarily from the viewpoint of how they impact electron input/output and conformational behaviors of the FMN subdomain.

Regulation of FMN subdomain interactions and function in neuronal nitric oxide synthase

Nitric oxide synthases (NOS) are modular, calmodulin- (CaM-) dependent, flavoheme enzymes that catalyze oxidation of l-arginine to generate nitric oxide (NO) and citrulline. During catalysis, the FMN subdomain cycles between interaction with an NADPH-FAD subdomain to receive electrons and interaction with an oxygenase domain to deliver electrons to the NOS heme. This process can be described by a three-state, two-equilibrium model for the conformation of the FMN subdomain, in which it exists in two distinct bound states (FMN-shielded) and one common unbound state (FMN-deshielded). We studied how each partner subdomain, the FMN redox state, and CaM binding may regulate the conformational equilibria of the FMN module in rat neuronal NOS (nNOS). We utilized four nNOS protein constructs of different subdomain composition, including the isolated FMN subdomain, and determined changes in the conformational state by measuring the degree of FMN shielding by fluorescence, electron paramagnetic resonance, or stopped-flow spectroscopic techniques. Our results suggest the following: (i) The NADPH-FAD subdomain has a far greater capacity to interact with the FMN subdomain than does the oxygenase domain. (ii) CaM binding has no direct effects on the FMN subdomain. (iii) CaM destabilizes interaction of the FMN subdomain with the NADPH-FAD subdomain but does not measurably increase its interaction with the oxygenase domain. Our results imply that a different set point and CaM regulation exists for either conformational equilibrium of the FMN subdomain. This helps to explain the unique electron transfer and catalytic behaviors of nNOS, relative to other dual-flavin enzymes.

Neutralizing a surface charge on the FMN subdomain increases the activity of neuronal nitric-oxide synthase by enhancing the oxygen reactivity of the enzyme heme-nitric oxide complex

Nitric-oxide synthases (NOSs) are calmodulin-dependent flavoheme enzymes that oxidize l-Arg to nitric oxide (NO) and l-citrulline. Their catalytic behaviors are complex and are determined by their rates of heme reduction (k(r)), ferric heme-NO dissociation (k(d)), and ferrous heme-NO oxidation (k(ox)). We found that point mutation (E762N) of a conserved residue on the enzyme's FMN subdomain caused the NO synthesis activity to double compared with wild type nNOS. However, in the absence of l-Arg, NADPH oxidation rates suggested that electron flux through the heme was slower in E762N nNOS, and this correlated with the mutant having a 60% slower k(r). During NO synthesis, little heme-NO complex accumulated in the mutant, compared with approximately 50-70% of the wild-type nNOS accumulating as this complex. This suggested that the E762N nNOS is hyperactive because it minimizes buildup of an inactive ferrous heme-NO complex during NO synthesis. Indeed, we found that k(ox) was 2 times faster in the E762N mutant than in wild-type nNOS. The mutational effect on k(ox) was independent of calmodulin. Computer simulation and experimental measures both indicated that the slower k(r) and faster k(ox) of E762N nNOS combine to lower its apparent K(m,O(2)) for NO synthesis by at least 5-fold, which in turn increases its V/K(m) value and enables it to be hyperactive in steady-state NO synthesis. Our work underscores how sensitive nNOS activity is to changes in the k(ox) and reveals a novel means for the FMN module or protein-protein interactions to alter nNOS activity.

Intramolecular electron transfer processes in Cu(B)-deficient cytochrome bo studied by pulse radiolysis

The Escherichia coli cytochrome bo is a heme-copper terminal ubiquinol oxidase, and functions as a redox-driven proton pump. We applied pulse radiolysis technique for studying the one-electron reduction processes in the Cu(B)-deficient mutant, His333Ala. We found that the Cu(B) deficiency suppressed the heme b-to-heme o electron transfer two order of the magnitude (4.0 x 10(2) s(-1)), as found for the wild-type enzyme in the presence of 1 mM KCN (3.0 x 10(2) s(-1)). Potentiometric analysis of the His333Ala mutant revealed the 40 mV decrease in the E(m) value for low-spin heme b and the 160 mV increase in the E(m) value of high-spin heme o. Our results indicate that Cu(B) not only serves as one-electron donor to the bound dioxygen upon the O-O bond cleavage, but also facilitates dioxygen reduction at the heme-copper binuclear centre by modulating the E(m) value of heme o through magnetic interactions. And the absence of a putative OH(-) bound to Cu(B) seems not to affect the uptake of the first chemical proton via K-channel in the His333Ala mutant.

Formation of spectral intermediate G-C and A-T anion complex in duplex DNA studied by pulse radiolysis

The dynamics of electron adducts of 2'-deoxynucleotides and oligonucelotides (ODNs) were measured spectroscopically by nanosecond pulse radiolysis. The radical anions of the nucleotides were produced within 10 ns by the reaction of hydrated electrons (e(aq)(-)) and were protonated to form the corresponding neutral radicals. At pH 7.0, the radical anion of deoxythymidine (dT(*-)) was protonated to form the neutral radical dT(H)(*) in the time range of microseconds. The rate constant for the protonation was determined as 1.8 x 10(10) M(-1) s(-1). In contrast, the neutral radical of dC(H)(*) was formed immediately after the pulse, suggesting that the protonation occurs within 10 ns. The transient spectra of excess electrons of the double-stranded ODNs 5'-TAATTTAATAT-3' (AT) and 5'-CGGCCCGGCGC-3' (GC) differed from those of pyrimidine radicals (C and T) and their composite. In contrast, the spectra of the electron adducts of the single-stranded ODNs GC and AT exhibited characteristics of C and T, respectively. These results suggest that, in duplex ODNs, the spectral intermediates of G-C and A-T anions complex were formed. On the microsecond time scale, the subsequent changes in absorbance of the ODN AT had a first-order rate constant of 4 x 10(4) s(-1), reflecting the protonation of T.

Menaquinone as well as ubiquinone as a bound quinone crucial for catalytic activity and intramolecular electron transfer in Escherichia coli membrane-bound glucose dehydrogenase

Escherichia coli membrane-bound glucose dehydrogenase (mGDH), which is one of quinoproteins containing pyrroloquinoline quinone (PQQ) as a coenzyme, is a good model for elucidating the function of bound quinone inside primary dehydrogenases in respiratory chains. Enzymatic analysis of purified mGDH from cells defective in synthesis of ubiquinone (UQ) and/or menaquinone (MQ) revealed that Q-free mGDH has very low levels of activity of glucose dehydrogenase and UQ2 reductase compared with those of UQ-bearing mGDH, and both activities were significantly increased by reconstitution with UQ1. On the other hand, MQ-bearing mGDH retains both catalytic abilities at the same levels as those of UQ-bearing mGDH. A radiolytically generated hydrated electron reacted with the bound MQ to form a semiquinone anion radical with an absorption maximum at 400 nm. Subsequently, decay of the absorbance at 400 nm was accompanied by an increase in the absorbance at 380 nm with a first order rate constant of 5.7 x 10(3) s(-1). This indicated that an intramolecular electron transfer from the bound MQ to the PQQ occurred. EPR analysis revealed that characteristics of the semiquinone radical of bound MQ are similar to those of the semiquinone radical of bound UQ and indicated an electron flow from PQQ to MQ as in the case of UQ. Taken together, the results suggest that MQ is incorporated into the same pocket as that for UQ to perform a function almost equivalent to that of UQ and that bound quinone is involved at least partially in the catalytic reaction and primarily in the intramolecular electron transfer of mGDH.

Effect of base sequence and deprotonation of Guanine cation radical in DNA

The deprotonation of guanine cation radical (G+*) in oligonucleotides (ODNs) was measured spectroscopically by nanosecond pulse radiolysis. The G+* in ODN, produced by oxidation with SO4-*, deprotonates to form the neutral G radical (G(-H)*). In experiments using 5-substituted cytosine-modified ODN, substitution of the cytosine C5 hydrogen by a methyl group increased the rate constant of deprotonation, whereas replacement by bromine decreased the rate constant. Kinetic solvent isotope effects on the kinetics of deoxyguanosine (dG) and ODN duplexes were examined in H2O and D2O. The rate constant of formation of G(-H)* in dG was 1.7-fold larger in H2O than D2O, whereas the rate constant in the ODN duplex was 3.8-fold larger in H2O than D2O. These results suggest that the formation of G(-H)* from G+* in the ODN corresponds to the deprotonation of the oxidized hydrogen-bridged (G+*-C) base pair by a water molecule. The characteristic absorption maxima of G+* around 400 nm were shifted to a longer wavelength in the order of G<GG<GGG-containing ODNs. In contrast, the spectra of G(-H)* were not affected by the sequence and were essentially similar to that of free dG. These results suggest that the positive charge in G+* in ODN is delocalized over the extended pi orbitals of DNA base. The rate constant of the deprotonation was altered by the sequence of ODNs, where bases adjacent to guanine are important factors for deprotonation.

Structural Basis for Isozyme-specific Regulation of Electron Transfer in Nitric-oxide Synthase

Three nitric-oxide synthase (NOS) isozymes play crucial, but distinct, roles in neurotransmission, vascular homeostasis, and host defense, by catalyzing Ca(2+)/calmodulin-triggered NO synthesis. Here, we address current questions regarding NOS activity and regulation by combining mutagenesis and biochemistry with crystal structure determination of a fully assembled, electron-supplying, neuronal NOS reductase dimer. By integrating these results, we structurally elucidate the unique mechanisms for isozyme-specific regulation of electron transfer in NOS. Our discovery of the autoinhibitory helix, its placement between domains, and striking similarities with canonical calmodulin-binding motifs, support new mechanisms for NOS inhibition. NADPH, isozyme-specific residue Arg(1400), and the C-terminal tail synergistically repress NOS activity by locking the FMN binding domain in an electron-accepting position. Our analyses suggest that calmodulin binding or C-terminal tail phosphorylation frees a large scale swinging motion of the entire FMN domain to deliver electrons to the catalytic module in the holoenzyme.

Direct observation of guanine radical cation deprotonation in duplex DNA using pulse radiolysis

The dynamics of one-electron oxidation of guanine (G) base mononucleotide and that in DNA have been investigated by pulse radiolysis. The radical cation (G+*) of deoxyguanosine (dG), produced by oxidation with SO(4)-*, rapidly deprotonates to form the neutral G radical (G(-H)*) with a rate constant of 1.8 x 10(7) s(-1) at pH 7.0, as judged from transient spectroscopy. With experiments using different double-stranded oligonucleotides containing G, GG, and GGG sequences, the absorbance increases at 625 nm, characteristic of formation of the G(-H)*, were found to consist of two phases. The rate constants of the faster ( approximately 1.3 x 10(7) s(-1)) and slower phases ( approximately 3.0 x 10(6) s(-1)) were similar for the different oligonucleotides. On the other hand, in the oligonucleotide containing G located at the 5'- and 3'-terminal positions, only the faster phase was seen. These results suggest that the lifetime of the radical cation of the G:C base pair (GC+*), depending on its location in the DNA chain, is longer than that of free dG. In addition, the absorption spectral intermediates showed that hole transport to a specific G site within a 12-13mer double-stranded oligonucleotide is complete within 50 ns; that is, the rate of hole transport over 20 A is >10(7) s(-1).

Mechanistic studies on the intramolecular one-electron transfer between the two flavins in the human neuronal nitric-oxide synthase and inducible nitric-oxide synthase flavin domains

Neuronal nitric-oxide synthase (nNOS) differs from inducible NOS (iNOS) in both its dependence on the intracellular Ca2+ concentration and the production rate of NO. To investigate what difference(s) exist between the two NOS flavin domains at the electron transfer level, we isolated the recombinant human NOS flavin domains, which were co-expressed with human calmodulin (CaM). The flavin semiquinones, FADH* and FMNH*, in both NOSs participate in the regulation of one-electron transfer within the flavin domain. Each semiquinone can be identified by a characteristic absorption peak at 520 nm (Guan, Z.-W., and Iyanagi, T. (2003) Arch. Biochem. Biophys. 412, 65-76). NADPH reduction of the FAD and FMN redox centers by the CaM-bound flavin domains was studied by stopped-flow and rapid scan spectrometry. Reduction of the air-stable semiquinone (FAD-FMNH*) of both domains with NADPH showed that the extent of conversion of FADH2/FMNH* to FADH*/FMNH2 in the iNOS flavin domain was greater than that of the nNOS flavin domain. The reduction of both oxidized domains (FAD-FMN) with NADPH resulted in the initial formation of a small amount of disemiquinone, which then decayed. The rate of intramolecular electron transfer between the two flavins in the iNOS flavin domain was faster than that of the nNOS flavin domain. In addition, the formation of a mixture of the two- and four-electron-reduced states in the presence of excess NADPH was different for the two NOS flavin domains. The data indicate a more favorable formation of the active intermediate FMNH2 in the iNOS flavin domain.

Characterization of Drosophila nitric oxide synthase: a biochemical study

The heme and flavin-binding domains of Drosophila nitric oxide synthase (DNOS) were expressed in Escherichia coli using the expression vector pCW. The denatured molecular mass of the expressed protein was 152kDa along with a proteolytically cleaved product of 121kDa. The DNOS heme protein exhibited very low Ca(2+)/calmodulin-dependent NO synthase activity. The trypsin digestion patterns were different from nNOS. The full-length DNOS protein had high degree of stability against trypsin. The activity assay of trypsin-digested protein confirmed the same result. Urea dissociation profile of DNOS full-length protein showed that the reductase domain activity was much more susceptible towards urea than the oxygenase domain activity. Urea gradient gel of DNOS full-length protein established distinct transition of dissociation and unfolding in the range 3-4M urea. Reductase domain activity of full-length DNOS protein against external electron acceptors like cytochrome c indicated slow electron transfer from FMN. The bacterial expression of DNOS full-length protein represents an important development in structure-function studies of this enzyme and comparison with other mammalian NOS enzymes which is evolutionary significant.

CO binding to the isolated oxygenase domain of neuronal nitric oxide synthase: effects of inhibitors and mutations at the substrate-binding site

In order to understand the heme distal structure of neuronal nitric oxide synthase (nNOS), we studied the binding affinity of CO for the ferrous wild type enzyme and the Glu592Ala and Tyr588His substrate binding-site mutants (generated in the oxygenase domain, nNOSox) in the presence of substrates and inhibitors. The CO binding affinities (K(d) = 10-21 mM) of nNOSox in the presence of the substrates, L-Arg and NHA, or inhibitors such as NAME and agmatine were more than two-fold lower than in their absence (K(d) = 5 mM). The presence of NIL strongly inhibited CO binding and gave a K(d) of more than 100 mM. These effects were not observed for the Glu592Ala mutant. The trend in CO binding affinities observed for the Tyr588His mutant was similar to that of the wild type enzyme. The presence of the isolated reductase domain did not affect CO binding. We discuss the role of substrate and inhibitor binding in CO complexation as well as in catalysis.

Calmodulin activates electron transfer through neuronal nitric-oxide synthase reductase domain by releasing an NADPH-dependent conformational lock

Neuronal nitric-oxide synthase (nNOS) is activated by the Ca(2+)-dependent binding of calmodulin (CaM) to a characteristic polypeptide linker connecting the oxygenase and reductase domains. Calmodulin binding also activates the reductase domain of the enzyme, increasing the rate of reduction of external electron acceptors such as cytochrome c. Several unusual structural features appear to control this activation mechanism, including an autoinhibitory loop, a C-terminal extension, and kinase-dependent phosphorylation sites. Pre-steady state reduction and oxidation time courses for the nNOS reductase domain indicate that CaM binding triggers NADP(+) release, which may exert control over steady-state turnover. In addition, the second order rate constant for cytochrome c reduction in the absence of CaM was found to be highly dependent on the presence of NADPH. It appears that NADPH induces a conformational change in the nNOS reductase domain, restricting access to the FMN by external electron acceptors. CaM binding reverses this effect, causing a 30-fold increase in the second order rate constant. The results show a startling interplay between the two ligands, which both exert control over the conformation of the domain to influence its electron transfer properties. In the full-length enzyme, NADPH binding will probably close the conformational lock in vivo, preventing electron transfer to the oxygenase domain and the resultant stimulation of nitric oxide synthesis.

Electron supply and catalytic oxidation of nitrogen by cytochrome P450 and nitric oxide synthase

Cytochrome P450 and nitric oxide synthase (NOS) oxidize nitrogen atoms, although the substrates and transformations are highly restricted for NOS. The first reaction catalyzed by NOS is mediated by a P450-like ferryl species, although it is generated by a distinct process in which a tetrahydrobiopterin molecule in NOS serves as a transient electron donor. The second NOS reaction appears to be mediated by an iron dioxygen precursor of the ferryl species. The transient tetrahydrobiopterin radical formed in these reactions is quenched by electron transfer from the NOS flavin domain. Electron transfer from the flavins is controlled by the binding of calmodulin, the presence of peptide inserts in the flavin domain, the substrate structure, and phosphorylation of the enzyme.

The activation of neuronal nitric-oxide synthase by various divalent cations

Nitric-oxide synthase (NOS; EC 1.14.13.39) catalyzes the oxidation of L-arginine to nitric oxide (NO(.)) and L-citrulline via the intermediate N(omega)-hydroxy-L-arginine. Of the three distinct isoforms of NOS that have been characterized, the constitutive neuronal NOS (NOS I) generates NO(.) associated with long-term potentiation (LTP) and early brain development. All of the NOS isoforms contain an N-terminal oxidase and a C-terminal reductase domain connected by a Ca(2+)/calmodulin binding region. To activate NOS I, Ca(2+) has to bind to calmodulin, allowing electron transport through both domains. Calcium ions are tightly regulated in cells. However, a number of other metal ions that bind and activate calmodulin may also activate NOS I. One such metal ion may be Pb(2+), which is associated with neurobehavioral and psychological alterations, including the inhibition of LTP. The effect of various divalent cations on NOS I activity was tested, and the results presented herein demonstrate that Pb(2+) and Sr(2+) can activate NOS I to a level similar to that found for Ca(2+). Finally, there is a synergy between Pb(2+) and Ca(2+) resulting in maximal activation of NOS I using minimal concentrations of both metal ions.