MAP kinases differentially bind and phosphorylate NOS3 via two unique NOS3 sites

Nitric oxide synthase 3 (NOS3) is a major vasoprotective enzyme that catalyzes the conversion of l-arginine to nitric oxide (NO) in response to a significant number of signaling pathways. Here, we provide evidence that NOS3 interactions with MAP kinases have physiological relevance. Binding interactions of NOS3 with c-Jun N-terminal kinase (JNK1α1 ), p38α, and ERK2 were characterized using optical biosensing with full-length NOS3 and NOS3 specific peptides and phosphopeptides. Like p38α and ERK2, JNK1α1 exhibited high-affinity binding to full-length NOS3 (KD 15 nm). Rate constants exhibited fast-on, slow-off binding (kon  = 4106 m-1 s-1 ; koff  = 6.2 × 10-5  s-1 ). Further analysis using synthetic NOS3 peptides revealed two MAP kinase binding sites unique to NOS3. p38α evinced similar affinity with both NOS3 binding sites. For ERK2 and JNK1α1, the affinity at the two sites differed. However, NOS3 peptides with a phosphate at either S114 or S633 did not meaningfully interact with the kinases. Immunoblotting revealed that each kinase phosphorylated NOS3 with a unique pattern. JNK1α1 predominantly phosphorylated NOS3 at S114, ERK2 at S600, and p38α phosphorylated both residues. In vitro production of NO was unchanged by phosphorylation at these sites. In human microvascular endothelial cells, endogenous interactions of all the MAP kinases with NOS3 were captured using proximity ligation assay in resting cells. Our results underscore the importance of MAP kinase interactions, identifying two unique NOS3 interaction sites with potential for modulation by MAP kinase phosphorylation (S114) and other signaling inputs, like protein kinase A (S633).

Five decades of research on mitochondrial NADH-quinone oxidoreductase (complex I)

NADH-quinone oxidoreductase (complex I) is the largest and most complicated enzyme complex of the mitochondrial respiratory chain. It is the entry site into the respiratory chain for most of the reducing equivalents generated during metabolism, coupling electron transfer from NADH to quinone to proton translocation, which in turn drives ATP synthesis. Dysfunction of complex I is associated with neurodegenerative diseases such as Parkinson’s and Alzheimer’s, and it is proposed to be involved in aging. Complex I has one non-covalently bound FMN, eight to 10 iron-sulfur clusters, and protein-associated quinone molecules as electron transport components. Electron paramagnetic resonance (EPR) has previously been the most informative technique, especially in membrane in situ analysis. The structure of complex 1 has now been resolved from a number of species, but the mechanisms by which electron transfer is coupled to transmembrane proton pumping remains unresolved. Ubiquinone-10, the terminal electron acceptor of complex I, is detectable by EPR in its one electron reduced, semiquinone (SQ) state. In the aerobic steady state of respiration the semi-ubiquinone anion has been observed and studied in detail. Two distinct protein-associated fast and slow relaxing, SQ signals have been resolved which were designated SQNf and SQNs. This review covers a five decade personal journey through the field leading to a focus on the unresolved questions of the role of the SQ radicals and their possible part in proton pumping.

A mathematical model of endothelial nitric oxide synthase activation with time delay exhibiting Hopf bifurcation and oscillations

Nitric oxide (NO) is a gaseous compound that serves as a signaling molecule in cellular interactions. In the vasculature, NO is synthesized from endogenous agents by endothelial nitric oxide synthase (eNOS) where it plays key roles in several functions related to homeostasis, adaptation, and development. Recent experimental studies have revealed cycles of increasing and decreasing NO production when eNOS is stimulated by factors such as glucose or insulin. We offer a mathematical model of a generic amino acid receptor site on eNOS wherein this species is subject to activation/deactivation by a pair of interactive kinase and phosphatase species. The enzyme kinetic model is presented as a system of ordinary differential equations including time delay to allow for various intermediate, unspecified complexes. We show that under conditions on the model parameters, varying the delay time may give rise to a Hopf bifurcation. Properties of the bifurcating solutions are explored via a center manifold reduction, and a numerical illustration is provided.

Novel cell-penetrating peptide-adaptors effect intracellular delivery and endosomal escape of protein cargos

The use of cell-penetrating peptides (CPPs) as biomolecular delivery vehicles holds great promise for therapeutic and other applications, but development has been stymied by poor delivery and lack of endosomal escape. We have developed a CPP-adaptor system capable of efficient intracellular delivery and endosomal escape of user-defined protein cargos. The cell-penetrating sequence of HIV transactivator of transcription was fused to calmodulin, which binds with subnanomolar affinity to proteins containing a calmodulin binding site. Our strategy has tremendous advantage over prior CPP technologies because it utilizes high-affinity non-covalent, but reversible coupling between CPP and cargo. Three different cargo proteins fused to a calmodulin binding sequence were delivered to the cytoplasm of eukaryotic cells and released, demonstrating the feasibility of numerous applications in living cells including alteration of signaling pathways and gene expression.

Kinetic and spectroscopic studies of bicupin oxalate oxidase and putative active site mutants

Ceriporiopsis subvermispora oxalate oxidase (CsOxOx) is the first bicupin enzyme identified that catalyzes manganese-dependent oxidation of oxalate. In previous work, we have shown that the dominant contribution to catalysis comes from the monoprotonated form of oxalate binding to a form of the enzyme in which an active site carboxylic acid residue must be unprotonated. CsOxOx shares greatest sequence homology with bicupin microbial oxalate decarboxylases (OxDC) and the 241-244DASN region of the N-terminal Mn binding domain of CsOxOx is analogous to the lid region of OxDC that has been shown to determine reaction specificity. We have prepared a series of CsOxOx mutants to probe this region and to identify the carboxylate residue implicated in catalysis. The pH profile of the D241A CsOxOx mutant suggests that the protonation state of aspartic acid 241 is mechanistically significant and that catalysis takes place at the N-terminal Mn binding site. The observation that the D241S CsOxOx mutation eliminates Mn binding to both the N- and C- terminal Mn binding sites suggests that both sites must be intact for Mn incorporation into either site. The introduction of a proton donor into the N-terminal Mn binding site (CsOxOx A242E mutant) does not affect reaction specificity. Mutation of conserved arginine residues further support that catalysis takes place at the N-terminal Mn binding site and that both sites must be intact for Mn incorporation into either site.

Calmodulin activates neuronal nitric oxide synthase by enabling transitions between conformational states

We recently showed that inducible nitric oxide synthase conformational intermediates can be resolved via FMN fluorescence lifetimes. Here we show that neuronal NOS activation by calmodulin removes constraints favoring a closed 'input state', increasing occupation of other states and facilitating conformational transitions. The 90 ps FMN input state lifetime distinguishes it from ∼4 ns 'open' states in which FMN does not interact strongly with other groups, or 0.9 ns output states in which FMN interacts with ferriheme. Enablement of the conformational cycle is an important paradigm for control in nNOS and related enzymes, and may extend to other control modalities.

FMN fluorescence in inducible NOS constructs reveals a series of conformational states involved in the reductase catalytic cycle

Nitric oxide synthases (NOSs) produce NO as a molecular signal in the nervous and cardiovascular systems and as a cytotoxin in the immune response. NO production in the constitutive isoforms is controlled by calmodulin regulation of electron transfer. In the tethered shuttle model for NOS reductase function, the FMN domain moves between NADPH dehydrogenase and oxygenase catalytic centers. Crystal structures of neuronal NOS reductase domain and homologs correspond to an 'input state', with FMN in close contact with FAD. We recently produced two domain 'output state' (oxyFMN) constructs showing calmodulin dependent FMN domain association with the oxygenase domain. FMN fluorescence is sensitive to enzyme conformation and calmodulin binding. The inducible NOS (iNOS) oxyFMN construct is more fluorescent than iNOS holoenzyme. The difference in steady state fluorescence is rationalized by the observation of a series of characteristic states in the two constructs, which we assign to FMN in different environments. OxyFMN and holoenzyme share open conformations with an average lifetime of ~4.3 ns. The majority state in holoenzyme has a short lifetime of ~90 ps, probably because of FAD-FMN interactions. In oxyFMN about 25-30% of the FMN is in a state with a lifetime of 0.9 ns, which we attribute to quenching by heme in the output state. Occupancy of the output state together with our previous kinetic results yields a heme edge to FMN distance estimate of 12-15 Å. These results indicate that FMN fluorescence is a valuable tool to study conformational states involved in the NOS reductase catalytic cycle.

MAP kinases bind endothelial nitric oxide synthase

Endothelial nitric oxide synthase (eNOS) contains a motif similar to recognition sequences in known MAPK binding partners. In optical biosensing experiments, eNOS bound p38 and ERK with ∼100 nM affinity and complex kinetics. Binding is diffusion-limited (k_on ∼ .15 × 10⁶ M⁻¹ s⁻¹). Neuronal NOS also bound p38 but exhibited much slower and weaker binding. p38-eNOS binding was inhibited by calmodulin. Evidence for a ternary complex was found when eNOS bound p38 was exposed to CaM, increasing the apparent dissociation rate. These observations strongly suggest a direct role for MAPK in regulation of NOS with implications for signaling pathways including angiogenesis and control of vascular tone.

Pacemaker-associated cyanosis in an adolescent: The answer hiding behind shadows

Lead thrombosis is a recognized complication of permanent transvenous pacemaker (PM) implantation. We present the interesting case of an adolescent with a dual-chamber PM presenting with fatigue and hypoxemia. Due to limitations of various imaging modalities, the diagnosis was difficult. She was eventually diagnosed with intracardiac PM lead thrombi obstructing tricuspid valve inflow. The pediatric literature on PM lead thrombosis is also briefly reviewed.

Rate, affinity and calcium dependence of nitric oxide synthase isoform binding to the primary physiological regulator calmodulin

Using interferometry-based biosensors the binding and release of endothelial and neuronal nitric oxide synthase (eNOS and nNOS) from calmodulin (CaM) was measured. In both isoforms, binding to CaM is diffusion limited and within approximately three orders of magnitude of the Smoluchowski limit imposed by orientation-independent collisions. This suggests that the orientation of CaM is facilitated by the charge arrays on the CaM-binding site and the complementary surface on CaM. Protein kinase C phosphorylation of eNOS T495, adjacent to the CaM-binding site, abolishes or greatly slows CaM binding. Kinases which increase the activity of eNOS did not stimulate the binding of CaM, which is already diffusion limited. The coupling of Ca(2+) binding and CaM/NOS binding equilibria links the affinity of CaM for NOS to the Ca(2+) dependence of CaM binding. Hence, changes in the Ca(2+) sensitivity of CaM binding always imply changes in the NOS-CaM affinity. It is possible, however, that in some regimes binding and activation are not synonymous, so that Ca(2+) sensitivity need not be tightly linked to CaM sensitivity of activation. This study is being extended using mutants to probe the roles of individual structural elements in binding and release.

Possible roles of two quinone molecules in direct and indirect proton pumps of bovine heart NADH-quinone oxidoreductase (complex I)

In many energy transducing systems which couple electron and proton transport, for example, bacterial photosynthetic reaction center, cytochrome bc(1)-complex (complex III) and E. coli quinol oxidase (cytochrome bo(3) complex), two protein-associated quinone molecules are known to work together. T. Ohnishi and her collaborators reported that two distinct semiquinone species also play important roles in NADH-ubiquinone oxidoreductase (complex I). They were called SQ(Nf) (fast relaxing semiquinone) and SQ(Ns) (slow relaxing semiquinone). It was proposed that Q(Nf) serves as a "direct" proton carrier in the semiquinone-gated proton pump (Ohnishi and Salerno, FEBS Letters 579 (2005) 4555), while Q(Ns) works as a converter between one-electron and two-electron transport processes. This communication presents a revised hypothesis in which Q(Nf) plays a role in a "direct" redox-driven proton pump, while Q(Ns) triggers an "indirect" conformation-driven proton pump. Q(Nf) and Q(Ns) together serve as (1e(-)/2e(-)) converter, for the transfer of reducing equivalent to the Q-pool.

GTP cyclohydrolase I phosphorylation and interaction with GTP cyclohydrolase feedback regulatory protein provide novel regulation of endothelial tetrahydrobiopterin and nitric oxide

RATIONALE

GTP cyclohydrolase I (GTPCH-1) is the rate-limiting enzyme involved in de novo biosynthesis of tetrahydrobiopterin (BH(4)), an essential cofactor for NO synthases and aromatic amino acid hydroxylases. GTPCH-1 undergoes negative feedback regulation by its end-product BH(4) via interaction with the GTP cyclohydrolase feedback regulatory protein (GFRP). Such a negative feedback mechanism should maintain cellular BH(4) levels within a very narrow range; however, we recently identified a phosphorylation site (S81) on human GTPCH-1 that markedly increases BH(4) production in response to laminar shear.

OBJECTIVE

We sought to define how S81 phosphorylation alters GTPCH-1 enzyme activity and how this is modulated by GFRP.

METHODS AND RESULTS

Using prokaryotically expressed proteins, we found that the GTPCH-1 phospho-mimetic mutant (S81D) has increased enzyme activity, reduced binding to GFRP and resistance to inhibition by GFRP compared to wild-type GTPCH-1. Using small interfering RNA or overexpressing plasmids, GFRP was shown to modulate phosphorylation of GTPCH-1, BH(4) levels, and NO production in human endothelial cells. Laminar, but not oscillatory shear stress, caused dissociation of GTPCH-1 and GFRP, promoting GTPCH-1 phosphorylation. We also found that both GTPCH-1 phosphorylation and GFRP downregulation prevents endothelial NO synthase uncoupling in response to oscillatory shear. Finally oscillatory shear was associated with impaired GTPCH-1 phosphorylation and reduced BH(4) levels in vivo.

CONCLUSIONS

These studies provide a new mechanism for regulation of endothelial GTPCH-1 by its phosphorylation and interplay with GFRP. This mechanism allows for escape from GFRP negative feedback and permits large amounts of BH(4) to be produced in response to laminar shear stress.

Inhibition of nitric oxide synthase by cobalamins and cobinamides

Cobalamins are important cofactors for methionine synthase and methylmalonyl-CoA mutase. Certain corrins also bind nitric oxide (NO), quenching its bioactivity. To determine if corrins would inhibit NO synthase (NOS), we measured their effects on -L-[(14)C]arginine-to-L-[(14)C]citrulline conversion by NOS1, NOS2, and NOS3. Hydroxocobalamin (OH-Cbl), cobinamide, and dicyanocobinamide (CN(2)-Cbi) potently inhibited all isoforms, whereas cyanocobalamin, methylcobalamin, and adenosylcobalamin had much less effect. OH-Cbl and CN(2)-Cbi prevented binding of the oxygen analog carbon monoxide (CO) to the reduced NOS1 and NOS2 heme active site. CN(2)-Cbi did not react directly with NO or CO. Spectral perturbation analysis showed that CN(2)-Cbi interacted directly with the purified NOS1 oxygenase domain. NOS inhibition by corrins was rapid and not reversed by dialysis with L-arginine or tetrahydrobiopterin. Molecular modeling indicated that corrins could access the unusually large heme- and substrate-binding pocket of NOS. Best fits were obtained in the "base-off" conformation of the lower axial dimethylbenzimidazole ligand. CN(2)-Cbi inhibited interferon-gamma-activated Raw264.7 mouse macrophage NO production. We show for the first time that certain corrins directly inhibit NOS, suggesting that these agents (or their derivatives) may have pharmacological utility. Endogenous cobalamins and cobinamides might play important roles in regulating NOS activity under normal and pathological conditions.

Identification and characterization of VPO1, a new animal heme-containing peroxidase

Animal heme-containing peroxidases play roles in innate immunity, hormone biosynthesis, and the pathogenesis of inflammatory diseases. Using the peroxidase-like domain of Duox1 as a query, we carried out homology searching of the National Center for Biotechnology Information database. Two novel heme-containing peroxidases were identified in humans and mice. One, termed VPO1 for vascular peroxidase 1, exhibits its highest tissue expression in heart and vascular wall. A second, VPO2, present in humans but not in mice, is 63% identical to VPO1 and is highly expressed in heart. The peroxidase homology region of VPO1 shows 42% identity to myeloperoxidase and 57% identity to the insect peroxidase peroxidasin. A molecular model of the VPO1 peroxidase region reveals a structure very similar to that of known peroxidases, including a conserved heme binding cavity, critical catalytic residues, and a calcium binding site. The absorbance spectra of VPO1 are similar to those of lactoperoxidase, and covalent attachment of the heme to VPO1 protein was demonstrated by chemiluminescent heme staining. VPO1 purified from heart or expressed in HEK cells is catalytically active, with a K(m) for H(2)O(2) of 1.5 mM. When co-expressed in cells, VPO1 can use H(2)O(2) produced by NADPH oxidase enzymes. VPO1 is likely to carry out peroxidative reactions previously attributed exclusively to myeloperoxidase in the vascular system.

Neuronal nitric oxide synthase: prototype for pulsed enzymology

The established paradigm in understanding and describing enzyme activity uses formalisms based on steady-state assumptions, including Michaelis-Menten and King-Altman approaches. These are appropriate for enzymes operating under steady-state conditions. Signal generating enzymes transfer information, rather than material. Because the information capacity of a signal channel depends on frequency, steady-state descriptions may not be appropriate. Recently, Stuehr and coworkers described a novel product inhibition mechanism for NO synthases. Simulations presented here suggest that at physiological temperatures neuronal nitric oxide synthase produces sharp pulses of NO, consistent with its signaling function. These temporal pulses greatly restrict the effective spatial range of NO signaling.

The role of the conserved COOH-terminal triad in alphaA-crystallin aggregation and functionality

PURPOSE

The sequentially variable COOH-terminal region of small heat shock protein superfamily members usually contains a conserved IXI/V feature where X is typically a proline. When present in solved sHsp crystal structures (e.g. MjHsp16.5 and wheat Hsp16.9), this short sequence forms an isolated beta strand apparently involved in the alignment of dimers into larger oligomers. Because it is a common feature of many sHsp family members, it is possible that this triad has a similar role in alphaA-crystallin. This study was undertaken to determine the contribution of this conserved triad to the quaternary structure and function of alphaA-crystallin.

METHODS

A series of site-directed mutants was generated in both wild type alphaA and in an alphaA deletion mutant lacking the NH2-terminal residues 1-50. After overexpression and purification, each protein's oligomer size was characterized by size-exclusion fast protein liquid chromatography (FPLC), thermal transition temperature by non-denaturing composite gel electrophoresis, and chaperone activity by the inhibition of DL-dithiothreitol (DTT)-induced insulin aggregation.

RESULTS

Using the alphaA-crystallin NH2-deletion mutant, the hydrophobic triad was changed from IPV to TPT, GPG, IGV, ITV, or GGG. All six D51 mutants associated into tetramers with small amounts of dimer and monomer also present. Chaperone-like activity was reduced but not eliminated in some of these triad mutants with GGG and ITV the most strongly affected. Similar modifications to wild type alphaA-crystallin (IPV to ITV, IGV, or GGG) restored oligomer sizes similar, but not identical to, native alphaA-crystallin, with additional small amounts of tetramer and dimer. Interestingly, equivalent mutants of wild type alphaA-crystallin did not have reduced chaperone-like activity but differed considerably in their thermal transition temperatures.

CONCLUSIONS

The conserved COOH-terminal triad does not appear to have a strong effect on the steady-state aggregation of wild type alphaA-crystallin or its 50-residue deletion mutant at 25 degrees C. However, it can exert a considerable effect on chaperone-like activity in the absence of the NH2-terminal 50-residue sequence extension and can influence the thermal transition temperature in its presence. These results suggest that the conserved triad in alphaA-crystallin contributes to the stability of higher order oligomers but is not essential for the formation of tetramers.

Direct measurement by laser flash photolysis of intraprotein electron transfer in a rat neuronal nitric oxide synthase

Intraprotein interdomain electron transfer (IET) from flavin mononucleotide (FMN) to heme is essential in nitric oxide (NO) synthesis by NO synthase (NOS). Our previous laser flash photolysis studies have provided a direct determination of the kinetics of IET between the FMN and heme domains in truncated oxyFMN constructs of rat neuronal NOS (nNOS) and murine inducible NOS (iNOS), in which only the oxygenase and FMN domains along with the calmodulin (CaM) binding site are present [Feng, C. J.; Tollin, G.; Holliday, M. A.; Thomas, C.; Salerno, J. C.; Enemark, J. H.; Ghosh, D. K. Biochemistry 2006, 45, 6354-6362. Feng, C. J.; Thomas, C.; Holliday, M. A.; Tollin, G.; Salerno, J. C.; Ghosh, D. K.; Enemark, J. H. J. Am. Chem. Soc. 2006, 128, 3808-3811]. Here, we report the kinetics of IET between the FMN and heme domains in a rat nNOS holoenzyme in the presence and absence of added CaM using laser flash photolysis of CO dissociation in comparative studies on partially reduced NOS and a single domain NOS oxygenase construct. The IET rate constant in the presence of CaM is 36 s-1, whereas no IET was observed in the absence of CaM. The kinetics reported here are about an order of magnitude slower than the kinetics in a rat nNOS oxyFMN construct with added CaM (262 s-1). We attribute the slower IET between FMN and heme in the holoenzyme to the additional step of dissociation of the FMN domain from the reductase complex before reassociation with the oxygenase domain to form the electron-transfer competent output state complex. This work provides the first direct measurement of CaM-controlled electron transfer between catalytically significant redox couples of FMN and heme in a nNOS holoenzyme.

iNOS regulation by calcium/calmodulin-dependent protein kinase II in vascular smooth muscle

Nitric oxide synthase (NOS) expression is regulated transcriptionally in response to cytokine induction and posttranslationally by palmitoylation and trafficking into perinuclear aggresome-like structures. We investigated the effects of multifunctional calcium/calmodulin-dependent protein kinase II protein kinase (CaMKII) on inducible NOS (iNOS) trafficking in cultured rat aortic vascular smooth muscle cells (VSMCs). Immunofluorescence and confocal microscopy demonstrated colocalization of iNOS and CaMKIIdelta(2) with a perinuclear distribution and concentration in aggresome-like structures identified by colocalization with gamma-tubulin. Furthermore, CaMKIIdelta(2) coimmunoprecipitated with iNOS in a CaMKII activity-dependent manner. Addition of Ca(2+)-mobilizing stimuli expected to activate CaMKII; a purinergic agonist (UTP) or calcium ionophore (ionomycin) caused a general redistribution of iNOS from cytosolic to membrane and nuclear fractions. Similarly, adenoviral expression of a constitutively active CaMKIIdelta(2) mutant altered iNOS localization, shifting iNOS from the cytosolic fraction. Suppression of CaMKIIdelta(2) using an adenovirus expressing a short hairpin, small interfering RNA increased nuclear iNOS localization in resting cells but inhibited ionomycin-induced translocation of iNOS to the nucleus. Following addition of these chronic and acute CaMKII modulators, there were fewer aggresome-like structures containing iNOS. All of the treatments that chronically affected CaMKII activity or expression significantly inhibited iNOS-specific activity following cytokine induction. The results suggest that CaMKIIdelta(2) may be an important regulator of iNOS trafficking and activity in VSMCs.

The role of a conserved serine residue within hydrogen bonding distance of FAD in redox properties and the modulation of catalysis by Ca2+/calmodulin of constitutive nitric-oxide synthases

The crystal structure of the neuronal nitric-oxide synthase (nNOS) NADPH/FAD binding domain indicated that Ser-1176 is within hydrogen bonding distance of Asp-1393 and the O4 atom of FAD and is also near the N5 atom of FAD (3.7 A). This serine residue is conserved in most of the ferredoxin-NADP+ reductase family of proteins and is important in electron transfer. In the present study, the homologous serines of both nNOS (Ser-1176) and endothelial nitric-oxide synthase (eNOS) (Ser-942) were mutated to threonine and alanine. Both substitutions yielded proteins that exhibited decreased rates of electron transfer through the flavin domains, in the presence and absence of Ca2+/CaM, as measured by reduction of potassium ferricyanide and cytochrome c. Rapid kinetics measurements of flavin reduction of all the mutants also showed a decrease in the rate of flavin reduction, in the absence and presence of Ca2+/CaM, as compared with the wild type proteins. The serine to alanine substitution caused both nNOS and eNOS to synthesize NO more slowly; however, the threonine mutants gave equal or slightly higher rates of NO production compared with the wild type enzymes. The midpoint redox potential measurements of all the redox centers revealed that wild type and threonine mutants of both nNOS and eNOS are very similar. However, the redox potentials of the FMN/FMNH* couple for alanine substitutions of both nNOS and eNOS are >100 mV higher than those of wild type proteins and are positive. These data presented here suggest that hydrogen bonding of the hydroxyl group of serine or threonine with the isoalloxazine ring of FAD and with the amino acids in its immediate milieu, particularly nNOS Asp-1393, affects the redox potentials of various flavin states, influencing the rate of electron transfer.

Intraprotein electron transfer in a two-domain construct of neuronal nitric oxide synthase: the output state in nitric oxide formation

Intersubunit intraprotein electron transfer (IET) from flavin mononucleotide (FMN) to heme is essential in nitric oxide (NO) synthesis by NO synthase (NOS). Previous crystal structures and functional studies primarily concerned an enzyme conformation, which serves as the input state for reduction of FMN by electrons from NADPH and flavin adenine dinucleotide (FAD) in the reductase domain. To favor the formation of the output state for the subsequent IET from FMN to heme in the oxygenase domain, a novel truncated two-domain oxyFMN construct of rat neuronal NOS (nNOS), in which only the FMN and heme domains were present, was designed and expressed. The kinetics of IET between the FMN and heme domains in the nNOS oxyFMN construct in the presence and absence of added calmodulin (CaM) were directly determined using laser flash photolysis of CO dissociation in comparative studies on partially reduced oxyFMN and single-domain heme oxygenase constructs. The IET rate constant in the presence of CaM (262 s(-)(1)) was increased approximately 10-fold compared to that in the absence of CaM (22 s(-)(1)). The effect of CaM on interdomain interactions was further evidenced by electron paramagnetic resonance (EPR) spectra. This work provides the first direct evidence of the CaM control of electron transfer (ET) between FMN and heme domains through facilitation of the FMN/heme interactions in the output state. Therefore, CaM controls IET between heme and FMN domains by a conformational gated mechanism. This is essential in coupling ET in the reductase domain in NOS with NO synthesis in the oxygenase domain.

Direct measurement by laser flash photolysis of intramolecular electron transfer in a two-domain construct of murine inducible nitric oxide synthase

Intersubunit intramolecular electron transfer (IET) from FMN to heme is essential in the delivery of electrons required for O2 activation in the heme domain and the subsequent nitric oxide (NO) synthesis by NO synthase (NOS). Previous crystal structures and functional studies primarily concerned an enzyme conformation that serves as the input state for reduction of FMN by electrons from NADPH and FAD in the reductase domain. To favor formation of the output state for the subsequent IET from FMN to heme in the oxygenase domain, a novel truncated two-domain oxyFMN construct murine inducible nitric oxide synthase (iNOS), in which only the FMN and heme domains were present, was designed and expressed. The kinetics of the IET between the FMN and heme domains in this construct was directly determined using laser flash photolysis of CO dissociation in comparative studies on partially reduced oxyFMN and single domain heme oxygenase constructs.

A single-stage polymerase-based protocol for the introduction of deletions and insertions without subcloning

A single-stage polymerase-based procedure is described that allows extensive modifications of DNA. The version described here uses the QuikChange Site-Directed Mutagenesis System kit supplied by Stratagene. The original protocol is replaced by a single-stage method in which linear production of complementary strands is accomplished in separate single primer reactions. This has proved effective in introducing insertions and deletions into large gene/vector combinations without subcloning.

INSULT: a novel mutagenesis method generating high yields of closed circular mutant DNA with one primer per mutant

INSULT, a novel method for the creation of insertions, deletions, and point mutations without subcloning, requires only one new primer per mutant, and produces circular plasmids, obviating the need for special "ultracompetent" cells. The method includes cycles of linear amplification with a thermophilic polymerase, and nick repair after each cycle with a thermophilic ligase. After production of multiple single-stranded copies of circular mutation-bearing plasmid DNA, addition of a "generic" primer followed by one or more polymerase reaction cycles generates double-stranded circular DNA bearing the desired mutation.

Nitric-oxide Synthase Output State

Mammalian nitric-oxide synthases are large modular enzymes that evolved from independently expressed ancestors. Calmodulin-controlled isoforms are signal generators; calmodulin activates electron transfer from NADPH through three reductase domains to an oxygenase domain. Structures of the reductase unit and its homologs show FMN and FAD in contact but too isolated from the protein surface to permit exit of reducing equivalents. To study states in which FMN/heme electron transfer is feasible, we designed and produced constructs including only oxygenase and FMN binding domains, eliminating strong internal reductase complex interactions. Constructs for all mammalian isoforms were expressed and purified as dimers. All synthesize NO with peroxide as the electron donor at rates comparable with corresponding oxygenase constructs. All bind cofactors nearly stoichiometrically and have native catalytic sites by spectroscopic criteria. Modest differences in electrochemistry versus independently expressed heme and FMN binding domains suggest interdomain interactions. These interactions can be convincingly demonstrated via calmodulin-induced shifts in high spin ferriheme EPR spectra and through mutual broadening of heme and FMNH. radical signals in inducible nitric-oxide synthase constructs. Blue neutral FMN semiquinone can be readily observed; potentials of one electron couple (in inducible nitric-oxide synthase oxygenase FMN, FMN oxidized/semiquinone couple = +70 mV, FMN semiquinone/hydroquinone couple = -180 mV, and heme = -180 mV) indicate that FMN is capable of serving as a one electron heme reductant. The construct will serve as the basis for future studies of the output state for NADPH derived reducing equivalents.

Binding and activation of nitric oxide synthase isozymes by calmodulin EF hand pairs

Calmodulin (CaM) is a cytosolic Ca(2+) signal-transducing protein that binds and activates many different cellular enzymes with physiological relevance, including the nitric oxide synthase (NOS) isozymes. CaM consists of two globular domains joined by a central linker; each domain contains an EF hand pair. Four different mutant CaM proteins were used to investigate the role of the two CaM EF hand pairs in the binding and activation of the mammalian inducible NOS (iNOS) and the constitutive NOS (cNOS) enzymes, endothelial NOS (eNOS) and neuronal NOS (nNOS). The role of the CaM EF hand pairs in different aspects of NOS enzymatic function was monitored using three assays that monitor electron transfer within a NOS homodimer. Gel filtration studies were used to determine the effect of Ca(2+) on the dimerization of iNOS when coexpressed with CaM and the mutant CaM proteins. Gel mobility shift assays were performed to determine binding stoichiometries of CaM proteins to synthetic NOS CaM-binding domain peptides. Our results show that the N-terminal EF hand pair of CaM contains important binding and activating elements for iNOS, whereas the N-terminal EF hand pair in conjunction with the central linker region is required for cNOS enzyme binding and activation. The iNOS enzyme must be coexpressed with wild-type CaM in vitro because of its propensity to aggregate when residues of the highly hydrophobic CaM-binding domain are exposed to an aqueous environment. A possible role for iNOS aggregation in vivo is also discussed.

NADPH analog binding to constitutive nitric oxide activates electron transfer and NO synthesis

We report here that NADPH analogs such as 2'5'ADP, ATP, and 2'AMP paradoxically activate constitutive calcium/calmodulin regulated nitric oxide synthases (cNOS), including the endothelial isoform (eNOS) and the neuronal isoform (nNOS). These activators compete with NADPH by filling the binding site of the adenine moiety of NADPH, but do not occupy the entire NADPH binding domain. Effects of these analogs on cNOS's include increasing the electron transfer rate to external acceptors, as assessed by cytochrome c reductase activity in the absence of calmodulin. In addition, NO synthase activity in the presence of calmodulin (with or without added calcium) was increased by the addition of NADPH analogs. In contrast, the same NADPH analogs inhibit iNOS, the calcium insensitive inducible isoform, which lacks control elements found in constitutive isoforms. Because ATP and ADP are among the effective activators of cNOS isoforms, these effects may be physiologically relevant.

NH2-terminal stabilization of small heat shock protein structure: a comparison of two NH2-terminal deletion mutants of alphaA-crystallin

PURPOSE

To assess the role of the NH2-terminal in alphaA-crystallin folding and chaperone-like activity.

METHODS

Two NH2-terminal deletion mutants of alphaA-crystallin were generated by standard mutagenesis methods, one with and one without a leader sequence in place of the first 50 residues. Aggregate size of each before and after thermal stress was assessed by FPLC, and chaperone-like activity was assessed using DTT-induced insulin denaturation.

RESULTS

Both mutants assemble primarily into tetramers, and both exhibit similar levels of chaperone-like activity, but are less protective than recombinant alphaA-crystallin. After a cycle of heat stress to 70 degrees C, tetramers of the mutant without the leader sequence dissociate into dimers and monomers and show severely reduced chaperone-like activity. In contrast, the mutant with the leader sequence retains its tetrameric form and its chaperone-like activity.

CONCLUSIONS

The NH2-terminal region is an important determinant of alpha-crystallin aggregate size, but is not required for folding of the alpha-crystallin domain, since the aggregate size and chaperone-like activity of the two mutants at room temperature are essentially the same. The leader sequence appears to increase the thermal stability of the alpha-crystallin domain and/or to contribute to the reformation of the active form after cooling, suggesting that the native NH2-terminal also plays a role in alpha-crystallin's resistance to environmental stress.

Conformation-driven and semiquinone-gated proton-pump mechanism in the NADH-ubiquinone oxidoreductase (complex I)

A novel mechanism for proton/electron transfer is proposed for NADH-quinone oxidoreductase (complex I) based on the following findings: (1) EPR signals of the protein-bound fast-relaxing semiquinone anion radicals (abbreviated as Q(Nf)-) are observable only in the presence of proton-transmembrane electrochemical potential; (2) Iron-sulfur cluster N2 and Q(Nf)- are directly spin-coupled; and (3) The projection of the interspin vector extends only 5A along the membrane normal [Yano, T., Dunham, W.R. and Ohnishi, T. (2005) Biochemistry, 44, 1744-1754]. We propose that the proton pump is operated by redox-driven conformational changes of the quinone binding protein. In the input state, semiquinone is reduced to quinol, acquiring two protons from the N (matrix) side of the mitochondrial inner membrane and an electron from the low potential (NADH) side of the respiratory chain. A conformational change brings the protons into position for release at the P (inter-membrane space) side of the membrane via a proton-well. Concomitantly, an electron is donated to the quinone pool at the high potential side of the coupling site. The system then returns to the original state to repeat the cycle. This hypothesis provides a useful frame work for further investigation of the mechanism of proton translocation in complex I.

Ligand, cofactor, and residue vibrations in the catalytic site of endothelial nitric oxide synthase

A study of bovine endothelial nitric oxide synthase by Fourier transform infrared (FTIR) spectroscopy in the 1000-2500 cm(-)(1) range is reported. Binding of CO to the reduced enzyme gives two heme(II)-CO nu(C)(-)(O) stretches (1927 and 1904 cm(-)(1)) which appear to be in rapid equilibrium. Photolysis of this heme(II)-CO compound is accompanied by perturbation of the local fine structure around the catalytic site giving vibrational changes of protein backbone, substrate, amino acid residues, and cofactors, to which heme, substrate arginine, and catalytic site residues contribute. Possible assignments of vibrations to heme, substrate arginine, and catalytic site residues are discussed. The discussion of assignments is informed by known structures, absorbance frequencies, and extinction coefficients of residues and cofactors, analysis of H(2)O-D(2)O exchange effects, analysis of substrate (14)N-(15)N (guanidinium)-arginine exchange effects, and comparison with the nNOS isoform (which differs in the replacement of asparagine 368 with an aspartate within the substrate binding site). The FTIR data can be modeled on the known structure of the catalytic site and indicate the extent of modulation of vibrational modes upon photolysis of the CO compound.

N-terminal control of small heat shock protein oligomerization: changes in aggregate size and chaperone-like function

The small heat shock protein superfamily is composed of proteins from throughout the phylogenetic spectrum that are induced upon environmental stress. Their structural stability under stress derives in large part from the central region of the proteins, which forms two beta sheets held together by hydrophobic interactions and appears to be present in all superfamily members. The length, sequence, and amino acid composition of the N- and C-terminals, in contrast, are quite variable. The role of the N-terminal has been hypothesized to control species-specific assembly of subunits into higher level structures. To test this, a set of constructs was designed and expressed: the N-terminal sequences preceding the start of the core regions of alphaA-crystallin and HSP 16.5 from Methanococcus jannaschii were swapped; the N-terminal of each protein was removed, and replaced with a brief N-terminal extension sequence; and two nonsense N-terminal sequences of approximately the same length and hydropathicity as the original replaced the alphaA-crystallin N-terminal. All constructs, plus the original recombinant sequences, could be overexpressed except for the 16.5 N-terminal extension, and all showed chaperone-like activity except for the hybrid with the 16.5 C-terminal. Size and properties of the replacement N-terminal place limits on aggregate size. Additional restrictions are imposed by the structure of the dimer.

Structural diversity in the small heat shock protein superfamily: control of aggregation by the N-terminal region

The small heat shock protein superfamily, extending over all kingdoms, is characterized by a common core domain with variable N- and C-terminal extensions. The relatively hydrophobic N-terminus plays a critical role in promoting and controlling high-order aggregation, accounting for the high degree of structural variability within the superfamily. The effects of N-terminal volume on aggregation were studied using chimeric and truncated proteins. Proteins lacking the N-terminal region did not aggregate above the tetramers, whereas larger N-termini resulted in large aggregates, consistent with the N-termini packing inside the aggregates. Variation in an extended internal loop differentiates typical prokaryotic and plant superfamily members from their animal counterparts; this implies different geometry in the dimeric building block of high-order aggregates.

The function of the small insertion in the hinge subdomain in the control of constitutive mammalian nitric-oxide synthases

Control of nitric oxide (NO) synthesis in the constitutive nitric-oxide synthases (NOS) by calcium/calmodulin is exerted through the regulation of electron transfer from NADPH through the reductase domains. This process has been shown previously to involve the calmodulin binding site, the autoinhibitory insertion in the FMN binding domain, and the C-terminal tail. Smaller sequence elements also appear to correlate with control. Although some of these elements appear well positioned to function in control, they are poorly conserved; their role in control is neither well established nor defined by available information. In this study mutations have been induced in the small insertion of the hinge subdomain, which has been shown recently to form a beta hairpin in structural studies of the neuronal NOS reductase domains adjacent to the calmodulin site and the autoinhibitory element. Modification of the small insertion in neuronal NOS tends to increase cytochrome c reduction but not NO synthetic activity; some modifications or deletions in the corresponding region in endothelial NOS modestly increase activity under some conditions. Unexpectedly, some minor changes in the sequence introduce a loss in the content of heme relative to flavin cofactors. Taken together, these results suggest that the small insertion protects the calmodulin binding site and that it may be a modulator of NOS activity.

Differential activation of nitric-oxide synthase isozymes by calmodulin-troponin C chimeras

The interactions of neuronal nitric-oxide synthase (nNOS) with calmodulin (CaM) and mutant forms of CaM, including CaM-troponin C chimeras, have been previously reported, but there has been no comparable investigation of CaM interactions with the other constitutively expressed NOS (cNOS), endothelial NOS (eNOS), or the inducible isoform (iNOS). The present study was designed to evaluate the role of the four CaM EF hands in the activation of eNOS and iNOS. To assess the role of CaM regions on aspects of enzymatic function, three distinct activities associated with NOS were measured: NADPH oxidation, cytochrome c reduction, and nitric oxide (*NO) generation as assessed by the oxyhemoglobin capture assay. CaM activates the cNOS enzymes by a mechanism other than stimulating electron transfer into the oxygenase domain. Interactions with the reductase moiety are dominant in cNOS activation, and EF hand 1 is critical for activation of both nNOS and eNOS. Although the activation patterns for nNOS and eNOS are clearly related, effects of the chimeras on all the reactions are not equivalent. We propose that cytochrome c reduction is a measure of the release of the FMN domain from the reductase complex. In contrast, cytochrome c reduction by iNOS is readily activated by each of the chimeras examined here and may be constitutive. Each of the chimeras were co-expressed with the human iNOS enzyme in Escherichia coli and subsequently purified. Domains 2 and 3 of CaM contain important elements required for the Ca2+/CaM independence of *NO production by the iNOS enzyme. The disparity between cytochrome c reduction and *NO production at low calcium can be attributed to poor association of heme and FMN domains when the bound CaM constructs are depleted of Ca2+. In general cNOSs are much more difficult to activate than iNOS, which can be attributed to their extra sequence elements, which are adjacent to the CaM-binding site and associated with CaM control.

Thermodynamics of Oxidation-Reduction Reactions in Mammalian Nitric-oxide Synthase Isoforms

The three mammalian nitric-oxide synthases produce NO from arginine in a reaction requiring 3 electrons per NO, which are supplied to the catalytic center from NADPH through reductase domains incorporating FAD and FMN cofactors. The isoforms share a common reaction mechanism and requirements for reducing equivalents but differ in regulation; the endothelial and neuronal isoforms are controlled by calcium/calmodulin modulation of the electron transfer system, while the inducible isoform binds calmodulin at all physiological Ca(2+) concentrations and is always on. The thermodynamics of electron transfer through the flavin domains in all three isoforms are basically similar. The major flavin states are FMN, FMNH., FMNH(2), FAD, FADH., and FADH(2). The FMN/FMNH. couple is high potential ( approximately 100 mV) in all three isoforms and is unlikely to be catalytically competent; the other three flavin couples form a nearly isopotential group clustered around -250 mV. Reduction of the flavins by the pyridine nucleotide couple at -325 mV is thus moderately thermodynamically favorable. The ferri/ferroheme couple in all three isoforms is approximately -270 mV in the presence of saturating arginine. Ca(2+)/calmodulin has no effect on the potentials of any of the couples in endothelial nitric-oxide synthase (eNOS) or neuronal nitric-oxide synthase (nNOS). The pH dependence of the flavin couples suggests the presence of ionizable groups coupled to the flavin redox/protonation states.

Competition and binding of arginine, imidazole, and aminoguanidine to endothelial nitric oxide synthase: aminoguanidine is a poor model for substrate, intermediate, and arginine analog inhibitor binding

The oxygenase domains of nitric oxide synthases are unusual in that they contain at least three ligand binding sites; these correspond to the axial heme ligand position, the substrate binding site, and the pterin binding site. Ligands can occupy portions of a site or extend into regions of adjacent sites. Depending on the size, shape, and binding mode of ligands to these positions, cooperative and anticooperative interactions mediated conformationally and by binding domain overlap can be observed. In the present study we describe competition between arginine and imidazole at the axial heme ligand position; a second imidazole, which occupies part of the arginine site in some crystal structures, is too weak to contribute to the equilibria. All spectroscopic titrations using imidazole competition depend on displacement of the heme axial imidazole ligand, which drives the ferriheme low spin. Aminoguanidine, a partial arginine analog, has multiple binding modes. It is somewhat competitive with arginine; a ternary complex forms, but the K(d) for arginine increases from 1 to 15 microM in the presence of saturating aminoguanidine. Aminoguanidine competition with imidazole is very weak, amounting to approximately a factor of two increase in K(d). This implies that aminoguanidine has multiple binding modes and is not well described as an arginine analog. The major binding mode occupies part of the binding site but does not extend into the imidazole axial ligand binding domain and probably corresponds to the crystal structure. The other binding mode is not significantly overlapped with the arginine site.