There is a delicate balance between

There is a delicate balance between ROS generation and scavenging by the protective antioxidant defenses in the cell. Antioxidant systems present in the d-glucose include enzymes like superoxide dismutase (SOD), catalases, glutathione peroxidases (GPxs) and peroxiredoxins (PRxs) (Fig. 3) [38]. SODs are responsible for catalyzing the rapid dismutation of highly reactive O2− anions to H2O2: SOD1 converts cytosolic O2−, mitochondrial matrix-associated SOD2 converts O2− generated by mitochondrial ETC, while SOD3 is secreted extracellularly and is responsible for converting extracellular O2− produced by the NADPH oxidases [43]. The antioxidant enzymes like catalases, GPxs and PRxs convert H2O2 to H2O and O2. The GPx family proteins reduce H2O2 by oxidizing glutathione, which is then reduced back by glutathione reductase using NADPH as an electron donor, thus normalizing the levels of reduced glutathione in the cells [44]. PRx family enzymes contain a redox-sensitive cysteine residue in their active site, which is inactivated by H2O2-mediated oxidation. Thioredoxin acts as an electron donor to reduce and activate PRx, thus completing the catalytic cycle [45]. Historically, ROS were thought to be deleterious to the cell causing oxidative stress by indiscriminately damaging proteins, lipids, and nucleic acids. This holds true for the highly reactive O2− and OH radicals, as they can cause irreversible oxidative damage due to their strong oxidizing potential and lipid insolubility, thereby contributing to cellular dysfunction and various pathologies. However, it is becoming more appreciated now that ROS can also mediate important signaling functions, referred to as redox-signaling [46]. For example, H2O2 is a perfect candidate to function as second messenger or as signaling molecule due to its relatively long half-life, high stability, and its ability to diffuse across membranes. Indeed oxidation of critical cysteine residues in redox-sensitive proteins is the most studied mechanism by which H2O2 functions as signaling molecule [47]. The cellular targets of H2O2 that undergo this reversible cysteine oxidation encompass a vast range of biological processes. Examples include the phosphatases PTEN and PTP1B, kinases like MAPK and redox-reactive transcription factors like YAP1 in yeast and FOXO4 in mammals [37]. H2O2 oxidizes the thiol side-chain of cysteine to form reactive sulfenic acid (-SOH) that can form intra- and inter-molecular disulphide (SS) bonds or cyclic sulfenamide (SN) structures, or can undergo hyperoxidation to form sulfinic (-SO2H) or sulfonic (-SO3H) acids [48]. These reversible modifications may lead to changes in protein structure, function or activation state. Along with cysteine thiols, H2O2 can oxidize several other amino acids like methionine, lysine, arginine, proline, histidine and tyrosine [49]. Multiple mechanisms have been proposed for understanding how target proteins are selected for oxidation by H2O2. One mechanism proposes colocalization of ROS sources and targets, so that redox signaling events are triggered close to the source of ROS generation [37]. For example, NOX proteins are often seen colocalized with putative targets like phosphatases or kinases at the plasma membrane, thereby influencing receptor tyrosine kinase signaling [50]. Another mechanism termed as “redox relay” proposes that H2O2 oxidizes the scavenging enzymes like PRx or GPx, which subsequently transfer the oxidation to target proteins [51]. This kind of relay process is seen for H2O2-mediated oxidation of apoptosis signaling kinase 1 (ASK1) and downstream phosphorylation of its MAPK substrate p38, which is dependent on formation of a ASK1-PRx1 disulphide intermediate [52]. Yet another mechanism termed as “floodgate model” proposes that transient inactivation of scavenging enzymes by hyperoxidation or posttranslational modifications causes accumulation of H2O2 allowing oxidation of target proteins [53]. Thus, by localized alterations in the redox buffering capacity, the cell can control ROS flux for selective signaling events [54]. Furthermore, depending on the location of a Cys residue within a protein, not all Cys residues are equally susceptible to H2O2-mediated oxidation [55], [56], further increasing specificity. Another form of physiological ROS regulation involves transport of H2O2 across cell membranes via aquaporins, which are integral membrane proteins involved in transport of water and small-molecule metabolites [57]. Aquaporins enhance the membrane permeability of H2O2, and are useful for transporting the extracellular H2O2 produced by NADPH oxidases across the plasma membrane to mediate intracellular signaling cascades [58].