"Oxygen Sensing" by Na,K-ATPase: These Miraculous Thiols

Abstract

Control over the Na,K-ATPase function plays a central role in adaptation of the organisms to hypoxic and anoxic conditions. As the enzyme itself does not possess O2 binding sites its "oxygen-sensitivity" is mediated by a variety of redox-sensitive modifications including S-glutathionylation, S-nitrosylation, and redox-sensitive phosphorylation. This is an overview of the current knowledge on the plethora of molecular mechanisms tuning the activity of the ATP-consuming Na,K-ATPase to the cellular metabolic activity. Recent findings suggest that oxygen-derived free radicals and H2O2, NO, and oxidized glutathione are the signaling messengers that make the Na,K-ATPase "oxygen-sensitive." This very ancient signaling pathway targeting thiols of all three subunits of the Na,K-ATPase as well as redox-sensitive kinases sustains the enzyme activity at the "optimal" level avoiding terminal ATP depletion and maintaining the transmembrane ion gradients in cells of anoxia-tolerant species. We acknowledge the complexity of the underlying processes as we characterize the sources of reactive oxygen and nitrogen species production in hypoxic cells, and identify their targets, the reactive thiol groups which, upon modification, impact the enzyme activity. Structured accordingly, this review presents a summary on (i) the sources of free radical production in hypoxic cells, (ii) localization of regulatory thiols within the Na,K-ATPase and the role reversible thiol modifications play in responses of the enzyme to a variety of stimuli (hypoxia, receptors' activation) (iii) redox-sensitive regulatory phosphorylation, and (iv) the role of fine modulation of the Na,K-ATPase function in survival success under hypoxic conditions. The co-authors attempted to cover all the contradictions and standing hypotheses in the field and propose the possible future developments in this dynamic area of research, the importance of which is hard to overestimate. Better understanding of the processes underlying successful adaptation strategies will make it possible to harness them and use for treatment of patients with stroke and myocardial infarction, sleep apnoea and high altitude pulmonary oedema, and those undergoing surgical interventions associated with the interruption of blood perfusion.

Keywords: S-glutathionylation; S-nitrosylation; Sodium-Potassium-Exchanging ATPase; hypoxia; redox regulation; thiols.

Figure 1

Schematic representation of reactions in which reactive oxygen and nitrogen species are formed. Shown in blue are the half-life for each species and in brackets the concentration range for each species in biological systems, and in green the enzymes catalyzing the corresponding reactions: dual oxidizes (DOUXes), superoxide dismutases (SOD). Under the arrows are the rate constants for the reactions shown. Myoglobin (Mb) and hemoglobin (Hb) are shown as sinks for NO.

Figure 2

Schematic representation of localization S-glutathionylated cysteine residues in α (in blue), β (in yellow), and phospholemman (FXYD, in cyan) subunits (A). Regulatory S-glutathionylation sites are shown in red. Basal S-glutathionylation is shown in green. ATP binding site is highlighted in red. Blue rectangle depicts ouabain binding site. Stars highlight the Cys residues with differences in pK between the α1 and α2 isoform (for details see Table 1). (B) Shows schematically the cavities with trapped S-glutathionylated Cys residues inaccessible for de-glutathionylation without detergents and representing “redox memory.”

Figure 3

Schematic representation of the multiple pathways involved in regulatory phosphorylation of the Na,K-ATPase under hypoxic conditions. The following abbreviations were used in the scheme: cyclic adenosine and guanosine monophosphates (cGMP and cAMP), protein kinases C, A, and G (PKC, PKA, PKG), cardiotonic steroids (CTS), Src kinase family (SFK), AMP-activated protein kinase (AMPK), phospholipase C (PLC), diacylglycerol (DAG), inositol 3-phosphate (IP3), ubiquitin (Ub), NKA (Na,K-ATPase), adaptor protein 2 (AP-2).

Figure 4

Schematic representation of O₂-induced regulation of activity and abundance of Na,K-ATPase in majority of cells and tissues (for details see the Section The role of acute regulation of the Na,K-ATPase in adaptation or irreparable damage at low O₂ of levels of the review).