**1. Oxidative stress and protein glutathionylation**

Reactive oxygen species (ROS) are generated at a very high rate throughout our lives as part of normal aerobic life. In particular, the mitochondria due to its high metabolic capacity are the primary site of endogenous ROS production. With each run of the mitochondrial electron transport chain, 1–2% of free radicals are predicted to escape the mitochondria [1]. With time,

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ROS accumulation can be detrimental, as they can cause oxidative damage to proteins, lipids, DNA, and other crucial biological molecules [2]. Among these macromolecules, proteins are very sensitive to oxidative modification. Cysteine residues are particularly reactive with ROS due to the presence of thiol (-SH) group, which can be oxidized to sulfenic (SOH), sulfinic (SO2 H), sulfonic acids (SO3 H), or formed disulfide bonds (S-S). Protein glutathionylation, defined as the reversible formation of a mixed disulfide (PSSG) between protein thiols (P-SH) and glutathione (GSH), appears to be the most important mode of thiol oxidation. GSH, normally an antioxidant molecule that scavenges free radicals, oxidizes to form glutathione mixed disulfide (GSSG). As the GSSG/GSH ratio increases, GSSG naturally adds to other proteins, causing protein glutathionylation. In most cases, the addition of GSSG to proteins renders them inactive, but for specific proteins such as heat shock proteins or peroxiredoxin 6, it may serve as an activation mechanism or a cell signaling event [3, 4]. In small amounts, PSSG may alert the cell that oxidative stress is present and may lead to certain signaling cascades to restore a healthy cellular redox state. Nonetheless, excessive PSSG can be lethal because of severe protein inactivation and damage [5]. Therefore, maintenance of a redox state can translate to increased cell survival and protection.

has two main subsets in humans: the cytosolic glutaredoxin 1 (Grx1) and the mitochondrial glutaredoxin 2 (Grx2) [7]. Grx1 has garnered much interest due to its similarity with Trx1 in promoting cytosolic protection, yet Grx2 with its primary mitochondrial localization may hold a crucial role in preventing cell death. This is especially important considering that mitochondria are the primary sites of ATP and ROS production and are critically involved in balancing pro- and anti-apoptotic signals [1, 4]. However, to date, only a few studies have been published to highlight Grx2's potential roles in humans. This chapter hopes to emphasize Grx2 as an imperative and crucial antioxidant enzyme, encourage more research studying Grx2's possible roles and protein targets, and provide support for Grx2 activating drugs to

The Important Functions of GSH-Dependent Enzyme Glutaredoxin 2 (Grx2)

http://dx.doi.org/10.5772/intechopen.78653

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Grx2 was first discovered simultaneously by two research groups leading by Holmgren and Gladeshev in the same year 2001. Both teams characterized Grx2 as a 18 kDa oxidoreductase enzymes capable of reversing PSSG and working as electron donors for ribonucleotide reductase [9]. There are several alternative spliced forms in different types of organisms, but in humans specifically, there are two main subsets: the cytosolic Grx1 and the mitochondrial Grx2. Grx1 has also been implicated in the mitochondrial inner membrane space but in miniscule amounts. Grx2 is rather unique, in that depending on cell type and organism, may be present in different cellular components. Grx2 has three alternatively spliced forms: Grx2a, Grx2b, and Grx2c. Grx2a is the mitochondrial isoform, whereas Grx2b and Grx2c mainly reside in the cytoplasm and nucleus. Grx2a is ubiquitous in every cell, except in the testes [6, 10, 11]. In spermatids, Grx2c is prominently present in the cytoplasm, yet Grx2a is absent (found to be less than 1% in the mice testes) [12, 13]. In embryos, Grx2 expression tends to surpass other dominant antioxidant enzymes such as Trx1, Trx2, and Grx1. During the gestation of E11 embryos, more than 50% of Grx2 mRNA transcripts compose of Grx2a [13]. This is not remarkable when considering that aerobic respiration and mitochondria formation takes precedent in this stage of gestation. Surprisingly, Grx2b and Grx2c are rarely found in normal human cells, but its presence has been discovered in HeLa cells and certain cancer cell lines. In patients with hepatocellular carcinoma and underlying metabolic syndrome that increases a person's risk for diabetes, stroke, and heart disease, Grx2 expression was particularly high [14]. In non-small cell lung cancer and adenocarcinoma, proliferative effects are correlated with higher cytoplasmic and nuclear Grx2 levels [15]. Along with several studies supporting Grx2 is highly expressed in cancer cells, Grx2 may have a distinct and special function in

cancer cells, possibly in enhancing cancer cell survival and proliferation [15–17].

Patterns of Grx2 expression vary from species to species. Unsurprisingly, Grx2's localization in mice showed that Grx2a was present in all tissue types. However, it was also identified that Grx2b and Grx2c mRNA transcripts were found at various expression levels in several tissues with higher concentrations in the heart, liver, kidney, and eyes [12, 13]. Another study looking at porcine ocular tissues showed that Grx2 is abundantly present in the ciliary body and

treat oxidative stress-induced diseases.

**3.1. Grx2 localization and expression**

**3. Glutaredoxin 2 (Grx2)**
