Nitric oxide (NO), a multifunctional second messenger, is implicated in physiological functions in mammals that range from immune response and potentiation of synaptic transmission to dilation of blood vessels and muscle relaxation. NO is a highly active molecule that diffuses across cell membranes and cannot be stored inside the producing cell. Its signaling capacity must be controlled at the levels of biosynthesis and local availability. Indeed, NO production by NO synthases is under complex and tight control, being regulated at transcriptional and translational levels, through co- and posttranslational modifications, and by subcellular localization. NO is synthesized from L-arginine by a family of nitric oxide synthases (NOS). Three NOS isoforms have been characterized: neuronal NOS (nNOS, NOS1) primarily found in neuronal tissue and skeletal muscle; inducible NOS (iNOS, NOS2) originally isolated from macrophages and later discovered in many other cells types; and endothelial NOS (eNOS, NOS3) present in vascular endothelial cells, cardiac myocytes, and in blood platelets. The enzymatic activity of all three isoforms is dependent on calmodulin, which binds to nNOS and eNOS at elevated intracellular calcium levels, while it is tightly associated with iNOS even at basal calcium levels. As a result, the enzymatic activity of nNOS and eNOS is modulated by changes in intracellular calcium levels, leading to transient NO production, while iNOS continuously releases NO independent of fluctuations in intracellular calcium levels and is mainly regulated at the gene expression level (Pacher et al. 2007).
The NOS enzymes share a common basic structural organization and requirement for substrate cofactors for enzymatic activity. A central calmodulin-binding motif separates an oxygenase (NH2-terminal) domain from a reductase (COOH-terminal) domain. Binding sites for cofactors NADPH, FAD, and FMN are located within the reductase domain, while binding sites for tetrahydrobiopterin (BH4) and heme are located within the oxygenase domain. Once calmodulin binds, it facilitates electron transfer from the cofactors in the reductase domain to heme enabling nitric oxide production. Both nNOS and eNOS contain an additional insert (40-50 amino acids) in the middle of the FMN-binding subdomain that serves as autoinhibitory loop, destabilizing calmodulin binding at low calcium levels and inhibiting electron transfer from FMN to the heme in the absence of calmodulin. iNOS does not contain this insert.<p>Because NOS enzymatic activity is modulated by the presence of its substrates and cofactors within the cell, under certain conditions, NOS may generate superoxide instead of NO, a process referred to as uncoupling (uncoupling of NADPH oxidation and NO synthesis).<p>The molecular details of eNOS function are annotated here.
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Signaling by AKT is one of the key outcomes of receptor tyrosine kinase (RTK) activation. AKT is activated by the cellular second messenger PIP3, a phospholipid that is generated by PI3K. In ustimulated cells, PI3K class IA enzymes reside in the cytosol as inactive heterodimers composed of p85 regulatory subunit and p110 catalytic subunit. In this complex, p85 stabilizes p110 while inhibiting its catalytic activity. Upon binding of extracellular ligands to RTKs, receptors dimerize and undergo autophosphorylation. The regulatory subunit of PI3K, p85, is recruited to phosphorylated cytosolic RTK domains either directly or indirectly, through adaptor proteins, leading to a conformational change in the PI3K IA heterodimer that relieves inhibition of the p110 catalytic subunit. Activated PI3K IA phosphorylates PIP2, converting it to PIP3; this reaction is negatively regulated by PTEN phosphatase. PIP3 recruits AKT to the plasma membrane, allowing TORC2 to phosphorylate a conserved serine residue of AKT. Phosphorylation of this serine induces a conformation change in AKT, exposing a conserved threonine residue that is then phosphorylated by PDPK1 (PDK1). Phosphorylation of both the threonine and the serine residue is required to fully activate AKT. The active AKT then dissociates from PIP3 and phosphorylates a number of cytosolic and nuclear proteins that play important roles in cell survival and metabolism. For a recent review of AKT signaling, please refer to Manning and Cantley, 2007.
Tetrahydrobiopterin (BH4) is an essential co-factor for the aromatic amino acid hydroxylases and glycerol ether monooxygenase and it regulates nitric oxide synthase (NOS) activity. Inherited BH4 deficiency leads to hyperphenylalaninemia, and dopamine and neurotransmitter deficiency in the brain. BH4 maintains enzymatic coupling to L-arginine oxidation to produce NO. Oxidation of BH4 to BH2 results in NOS uncoupling, resulting in superoxide (O2.-) formation rather than NO. Superoxide rapidly reacts with NO to produce peroxynitrite which can further uncouple NOS. These reactive oxygen species (superoxide and peroxynitrite) can contribute to increased oxidative stress in the endothelium leading to atherosclerosis and hypertension (Thony et al. 2000, Crabtree and Channon 2011,Schulz et al. 2008, Schmidt and Alp 2007). The synthesis, recycling and effects of BH4 are shown here. Three enzymes are required for the de novo biosynthesis of BH4 and two enzymes for the recycling of BH4.
The cofactor tetrahydrobiopterin (BH4) ensures endothelial nitric oxide synthase (eNOS) couples electron transfer to L-arginine oxidation (Berka et al. 2004). During catalysis, electrons derived from NADPH transfer to the flavins FAD and FMN in the reductase domain of eNOS and then on to the ferric heme in the oxygenase domain of eNOS. BH4 can donate an electron to intermediates in this electron transfer and is oxidised in the process, forming the BH3 radical. This radical can be reduced back to BH4 by iron, completing the cycle and forming ferrous iron again. Heme reduction enables O2 binding and L-arginine oxidation to occur within the oxygenase domain (Stuehr et al. 2009).
The oxidation product of BH4, 7,8-dihydrobiopterin (BH2), can compete with BH4 for binding to eNOS. This can lead to the uncoupling of eNOS and can result in the formation of reactive oxygen species (Vasquez-Vivar et al. 2002).
BH2 may compete with BH4 to bind eNOS, uncoupling eNOS leading to the formation of superoxide rather than nitric oxide. BH2, the oxidised form of BH4, cannot contribute electrons to heme in the reductase domain of eNOS, thereby uncoupling it from arginine oxidation and producing superoxide from oxygen instead (Vasquez-Vivar et al. 2002).
In the first of two salvage steps to maintain BH4 levels in the cell, sepiapterin is taken up by the cell and reduced by sepiapterin reductase (SRP) to form BH2 (Sawabe et al. 2008).
Superoxide (O2.-) formed from an uncoupled eNOS action, together with nitric oxide (NO) formed from a coupled eNOS action, readily react together to fom peroxynitrite (ONOO-) (Jourd'heuil et al. 2001, Reiter et al. 2000).
HSP90 serves as a scaffold to promote productive interaction between AKT1 and eNOS. Due to the proximity of these proteins once complexed with HSP90, AKT1 phosphorylates eNOS at Ser1177. When Ser1177 is phosphorylated, the level of NO production is elevated two- to three-fold above basal level.
Nitric oxide (NO) is produced from L-arginine by the family of nitric oxide synthases (NOS) enzymes, forming the free radical NO and citrulline as byproduct. The cofactor tetrahydrobiopterin (BH4) is an essential requirement for the delivery of an electron to the intermediate in the catalytic cycle of NOS.
Once depalmitoylated, it's proposed that eNOS is displaced from the plasma membrane and redistributed to other intracellular membranes, including the Golgi, where re-palmitoylation occurs. The mechanism of transport from the plasma membrane is still unknown.
NOSIP (eNOS interacting protein) binds to the carboxyl-terminal region of the eNOS oxygenase domain. Note that the eNOS binding sites for caveolin and NOSIP overlap.
DHHC-21 is a Golgi-localized acyl transferase that palmitoylates eNOS, which targets eNOS to plasmalemmal caveolae. Localization to this microdomain is likely to optimize eNOS activation and the extracellular release of nitric oxide.
Increases in intracellular calcium and calmodulin stimulate depalmitoylation of eNOS by acyl protein thioesterase 1, which displaces eNOS from the membrane. This might be a mechanism to downregulate NO production following intense stimuli.
NOSTRIN translocates eNOS from the plasma membrane to intracellular vesicular structures. NOSTRIN internalization of eNOS is proposed to occur via vesicle fission and caveolar transport through cooperation with dynamin and N-WASP.
NOSIP promotes translocation of eNOS from the plasma membrane to intracellular sites, thereby uncoupling eNOS from plasma membrane caveolae and inhibiting NO synthesis. eNOS appears to be shifted to intracellular sites that colocalize with Golgi and/or cytoskeletal marker proteins.
Palymitoylated, myristoylated eNOS forms a dimer and is transported from the Golgi to the plasma membrane. Transport is thought to be mediated by intracellular vesicles, but the details remain unknown.
Caveolin-1 is the primary negative regulatory protein for eNOS. Caveolin-1 binding to eNOS compromises its ability to bind Calmodulin (CaM), thereby inhibiting enzyme activity. The major binding region of caveolin-1 for eNOS is within amino acids 60-101 and to a lesser extent, amino acids 135-178.
eNOS interacts with the SH3 domain of NOSTRIN (positions 434-506). Caveolin-1 also binds directly to NOSTRIN (residues 323-434), thus allowing formation of a ternary complex.
Vertebrates possess multiple respiratory globins that differ in structure, function, and tissue distribution. Three different globins have been described so far: hemoglobin facilitates oxygen transport in blood, myoglobin mediates oxygen transport and storage in the muscle and neuroglobin has a yet unidentified function in nerve cells. A fourth globin has been identified in mouse, human and zebrafish. It is ubiquitously expressed in human tissue and therefore called cytoglobin (CYGB) (Burmester et al. 2002, Trent & Hargrove 2002). Unlike the specific expression patterns of Hb and Mb, CYGB is found in vascular smooth muscle, fibroblasts and cardiomyocytes. CYGB functions as a homodimer (Hamdane et al. 2003) and is localised to the cytosol of these cells where its O2 loading and unloading ability within a narrow O2 tension range makes it an ideal protein for O2 storage, especially during hypoxia (Fago et al. 2004).
Vertebrates possess multiple respiratory globins that differ in structure, function, and tissue distribution. Three different globins have been described so far: haemoglobin facilitates oxygen transport in blood, myoglobin mediates oxygen transport and storage in the muscle and neuroglobin has a yet unidentified function in nerve cells. A fourth globin has been identified in mouse, human and zebrafish. It is ubiquitously expressed in human tissue and therefore called cytoglobin (CYGB) (Trent & Hargrove 2002). Unlike the specific expression patterns of Hb and Mb, CYGB is found in vascular smooth muscle, fibroblasts and cardiomyocytes. CYGB functions as a homodimer (Hamdane et al. 2003) and is localised to the cytosol. As well as oxygen binding capability, CYGB possesses nitric oxide dioxygenase activity (Halligan et al. 2009), a common feature amongst the globin family (Smagghe et al. 2008). CYGB consumes NO through the dioxygenase pathway, which regulates cell respiration and proliferation (Smagghe et al. 2008). O2 binds to the ferric form of CYGB (CYGB-Fe2+:O2). During NO dioxygenation, CYGB is reduced to the ferrous form (CYGB-Fe3+) (Gardner 2005).
N(G),N(G)-dimethylarginine dimethylaminohydrolases 1 and 2 (DDAH1 and 2) play a role in the regulation of nitric oxide generation. They can hydrolyse an endogenous inhibitor of nitric oxide synthase (NOS), N(omega),N(omega)-dimethyl-L-arginine (ADMA) to dimethylamine (DMA) and L-citrulline (L-Cit) (Forbes et al. 2008, Wang et al. 2009, Cillero-Pastor et al. 2012).
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(BH4) synthesis, recycling, salvage
and regulationThese reactive oxygen species (superoxide and peroxynitrite) can contribute to increased oxidative stress in the endothelium leading to atherosclerosis and hypertension (Thony et al. 2000, Crabtree and Channon 2011,Schulz et al. 2008, Schmidt and Alp 2007). The synthesis, recycling and effects of BH4 are shown here. Three enzymes are required for the de novo biosynthesis of BH4 and two enzymes for the recycling of BH4.
myristoylated eNOS
dimerAnnotated Interactions
myristoylated eNOS
dimermyristoylated eNOS
dimermyristoylated eNOS
dimermyristoylated eNOS
dimer