Meta-Biol

• Pathways

eNOS activity is regulated by numerous post-translational modifications including phosphorylation and acylation, which also modulate its interactions with other proteins and its subcellular localization.

In general, following myristoylation and palmitoylation, eNOS localizes to caveolae in the plasma membrane, where in resting cells, it is bound to caveolin and remains inactive. Several agonists that raise intracellular calcium concentrations promote calmodulin binding to eNOS and the dissociation of caveolin from the enzyme, leading to an activated eNOS-calmodulin complex.

Phosphorylation plays a significant role in regulating eNOS activity, especially the phosphorylation of Ser1177, located within the reductase domain, which increases enzyme activity by enhancing reductase activity and calcium sensitivity. In unstimulated, cultured endothelial cells, Ser1177 is rapidly phosphorylated following a variety of stimuli: fluid shear stress, insulin, estrogen, VEGF, or bradykinin. The kinases involved in this process depend upon the stimuli applied. For instance, shear stress phosphorylates Ser1177 by activating Akt and PKA; insulin activates both Akt and the AMP-activated protein kinase (AMPK); estrogen and VEGF mainly phosphorylate eNOS via Akt; whereas the bradykinin-induced phosphorylation of Ser1177 is mediated by CaMKII. When Ser1177 is phosphorylated, NO production is increased several-fold above basal levels.

The phosphorylation of a threonine residue (Thr 495), located in the CaM binding domain, is associated with a decrease in eNOS activity. When this residue is dephosphorylated, substantially more CaM binds to eNOS and elevates enzyme activity. Stimuli associated with dephosphorylation of Thr495 (e.g., bradykinin, histamine, and Ca2+ ionophores) also increase Ca2+ levels resulting in the phosphorylation of Ser1177.

Additional phosphorylation sites, such as Ser114 and Ser633, and tyrosine phosphorylation have all been detected, but their functional relevance remains unclear. It is speculated that the tyrosine phosphorylation of eNOS is unlikely to affect enzyme activity directly, but more likely to impact the protein-protein interactions with associated scaffolding and regulatory proteins.

Nitric oxide (NO), a multifunctional second messenger, is implicated in physiological processes 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 is controlled at the levels of biosynthesis and local availability. Its 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 cell 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 NH2-terminal oxygenase domain from a COOH-terminal reductase 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.

In this Reactome pathway module, details of eNOS activation and regulation are annotated. Originally identified as endothelium-derived relaxing factor, eNOS derived NO is a critical signaling molecule in vascular homeostasis. It regulates blood pressure and vascular tone, and is involved in vascular smooth muscle cell proliferation, platelet aggregation, and leukocyte adhesion. Loss of endothelium derived NO is a key feature of endothelial dysfunction, implicated in the pathogenesis of hypertension and atherosclerosis. The endothelial isoform eNOS is unique among the nitric oxide synthase (NOS) family in that it is co-translationally modified at its amino terminus by myristoylation and is further acylated by palmitoylation (two residues next to the myristoylation site). These modifications target eNOS to the plasma membrane caveolae and lipid rafts.

Factors that stimulate eNOS activation and nitric oxide (NO) production include fluid shear stress generated by blood flow, vascular endothelial growth factor (VEGF), bradykinin, estrogen, insulin, and angiopoietin. The activity of eNOS is further regulated by numerous post-translational modifications, including protein-protein interactions, phosphorylation, and subcellular localization.

Following activation, eNOS shuttles between caveolae and other subcellular compartments such as the noncaveolar plasma membrane portions, Golgi apparatus, and perinuclear structures. This subcellular distribution is variable depending upon cell type and mode of activation.

Subcellular localization of eNOS has a profound effect on its ability to produce NO as the availability of its substrates and cofactors will vary with location. eNOS is primarily particulate, and depending on the cell type, eNOS can be found in several membrane compartments: plasma membrane caveolae, lipid rafts, and intracellular membranes such as the Golgi complex.

Metabolic processes in human cells generate energy through the oxidation of molecules consumed in the diet and mediate the synthesis of diverse essential molecules not taken in the diet as well as the inactivation and elimination of toxic ones generated endogenously or present in the extracellular environment. The processes of energy metabolism can be classified into two groups according to whether they involve carbohydrate-derived or lipid-derived molecules, and within each group it is useful to distinguish processes that mediate the breakdown and oxidation of these molecules to yield energy from ones that mediate their synthesis and storage as internal energy reserves. Synthetic reactions are conveniently grouped by the chemical nature of the end products, such as nucleotides, amino acids and related molecules, and porphyrins. Detoxification reactions (biological oxidations) are likewise conveniently classified by the chemical nature of the toxin.

At the same time, all of these processes are tightly integrated. Intermediates in reactions of energy generation are starting materials for biosyntheses of amino acids and other compounds, broad-specificity oxidoreductase enzymes can be involved in both detoxification reactions and biosyntheses, and hormone-mediated signaling processes function to coordinate the operation of energy-generating and energy-storing reactions and to couple these to other biosynthetic processes.