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. 2011 Jul 11;194(1):7–15. doi: 10.1083/jcb.201102095

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Figure 1. — Reactive oxygen species generation and disposal in the mitochondria. Primary sources of ROS occur from the transfer of electrons (e⁻) to molecular oxygen at either Complex I or III. Superoxide produced at Complex I is thought to form only within the matrix, whereas at Complex III superoxide is released both into the matrix and the inner mitochondrial space (IMS). In addition to the cytochrome chain, ROS can be formed by enzymatic action of numerous enzymes including monoamine oxidase (MAO) and cytochrome b₅ reductase (Cb₅R) located on the outer mitochondrial membrane (OMM), as well as glycerol-3-phosphate dehydrogenase (GPDH) and in some cell types, various cytochrome P450 enzymes located in the inner mitochondrial membrane (IMM). There are also several matrix enzymes and complexes (box) including aconitase, pyruvate dehydrogenase (PDH), and α-ketoglutarate dehydrogenase (αKGDH) that can generate superoxide. Although one-electron reactions predominate, two-electron reactions leading to direct hydrogen peroxide production can occur as when, for instance, cytochrome c (Cyt C) and p66shc interact within the IMS. Once generated, superoxide is dismutated spontaneously or enzymatically by manganese superoxide dismutase (MnSOD). The hydrogen peroxide that is formed is further catabolized by the action of enzymes such as catalase (CAT), glutathione peroxidase (GPx), and peroxiredoxin 3 (Prx3). For further details see the text, as well as other recent reviews (Lin and Beal, 2006; Brand, 2010). CoQ, Coenzyme Q.