Although there are differences between the electron transport chain (ETC) of mammals and yeast, mostly at complex I, O 2 cerevisiae is a useful tool in the study of mitochondrial function because mutations in mitochondrial DNA or deletion of nuclear genes encoding mitochondrial proteins are not lethal. − rather than a mixture of partially reduced species ( Turrens, 1997).Several studies showed that the complex I and III sites generate mostly O 2 This principally occurs at the semiquinone at the center o (Q o) electron carrier in complex III ( Trumpower, 2002 Andreyev et al., 2005). It is widely accepted that radicals derived from the partial reduction of ubiquinone in complex III are the main generators of ROS during respiration. The incomplete reduction of O 2 primarily occurs at two sites in the respiratory chain: complex I at NADH dehydrogenase ( Turrens and Boveris, 1980) and complex III at ubisemiquinone ( Boveris et al., 1976 Cadenas et al., 1977 Turrens et al., 1985). − and H 2O 2 to stronger oxidants, in particular the hydroxyl radical, which is the most oxidizing radical known to arise in biological systems ( Youngman, 1984 Buettner, 1993).The main threat to the cell is the subsequent transformation of O 2 SOD2 encodes a manganese-containing enzyme found in the mitochondrial matrix ( Bermingham-McDonogh et al., 1988 Gralla and Kosman, 1992 O'Brien et al., 2004). In Saccharomyces cerevisiae the SOD1 gene encodes a copper- and zinc-containing enzyme located in the cytoplasm and mitochondrial intermembrane space. These reactions are among the main sources of H 2O 2 in vivo and are either nonenzymatic or catalyzed by superoxide dismutases (SODs). − also produce H 2O 2 as a result of disproportionation reactions ( Gille and Sigler, 1995).This is produced by single-electron reduction of oxygen to a large extent by electron carriers of the respiratory chain in the mitochondria ( Lambert and Brand, 2004). The first intermediate in the sequential reduction of oxygen is often O 2 ), and their formation can result in damage to proteins, lipids, and nucleic acids ( Aung-Htut et al., 2012).−), hydrogen peroxide (H 2O 2), and the hydroxyl radical (OH.ROS commonly formed in vivo include the superoxide radical anion (O 2 The molecules and radicals formed by the incomplete reduction of oxygen are termed reactive oxygen species (ROS Halliwell and Gutteridge, 1989). This toxicity is mainly due to partially reduced forms of O 2 ( Gille and Sigler, 1995), since the O 2 molecule per se has low reactivity ( Halliwell and Gutteridge, 1990). Increased intracellular levels of oxygen, however, are potentially toxic. Most organisms rely on the role of oxygen as a terminal electron acceptor for efficient energy production in the form of ATP. These opposing, concentration-dependent roles of the superoxide radical comprise a form of hormesis and show one ROS having a hormetic effect on the toxicity of another. High levels of the superoxide radical are still toxic. ![]() This provides a direct link between complex III as the main source of ROS and its role in defense against ROS. Small increases in levels of mitochondrially produced superoxide radicals have a protective effect during H 2O 2-induced stress, and in response to H 2O 2, the wild-type strain increases superoxide radical production to activate this defense mechanism. Consistent with this correlation, overexpression of superoxide dismutase increases sensitivity to H 2O 2, and this phenotype is partially rescued by addition of small concentrations of menadione. Of interest, the same H 2O 2-sensitive mutant strains have the lowest superoxide radical levels, and strains with the highest resistance to H 2O 2 have the highest levels of superoxide radicals. Disruption of complex III renders cells sensitive to H 2O 2 but not to the superoxide radical generator menadione. Respiratory chain complex III and possibly cytochrome b function are essential for this increase. After H 2O 2 treatment, yeast cells significantly increase superoxide radical production. Reactive oxygen species (ROS) consist of potentially toxic, partly reduced oxygen species and free radicals.
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