The sites and topology of mitochondrial superoxide production

Abstract

Mitochondrial superoxide production is an important source of reactive oxygen species in cells, and may cause or contribute to ageing and the diseases of ageing. Seven major sites of superoxide production in mammalian mitochondria are known and widely accepted. In descending order of maximum capacity they are the ubiquinone-binding sites in complex I (site IQ) and complex III (site IIIQo), glycerol 3-phosphate dehydrogenase, the flavin in complex I (site IF), the electron transferring flavoprotein:Q oxidoreductase (ETFQOR) of fatty acid beta-oxidation, and pyruvate and 2-oxoglutarate dehydrogenases. None of these sites is fully characterized and for some we only have sketchy information. The topology of the sites is important because it determines whether or not a site will produce superoxide in the mitochondrial matrix and be able to damage mitochondrial DNA. All sites produce superoxide in the matrix; site IIIQo and glycerol 3-phosphate dehydrogenase also produce superoxide to the intermembrane space. The relative contribution of each site to mitochondrial reactive oxygen species generation in the absence of electron transport inhibitors is unknown in isolated mitochondria, in cells or in vivo, and may vary considerably with species, tissue, substrate, energy demand and oxygen tension.

Figure 1. The sites and topology of mitochondrial superoxide production

The seven identified sites of superoxide production in mitochondria are shown in the upper diagram, with an indication of the location of their substrate-binding sites and the topology of their superoxide production. PDH, pyruvate dehydrogenase; OGDH, 2-oxoglutarate dehydrogenase; site IF, the FMN-containing NADH binding site of complex I; site IQ, ubiquinone reduction site of complex I; ETFQOR, electron transferring flavoprotein ubiquinone oxidoreductase – the entry point of electrons from flavin-linked beta-oxidation of fatty acids via ETF (electron transferring flavoprotein) to ubiquinone in the electron transport chain; GPDH, glycerol 3-phosphate dehydrogenase; site IIIQo, the outer quinone-binding site of the Q-cycle in complex III. PDH and OGDH may also produce hydrogen peroxide directly. The lower panel shows representative values of the maximum superoxide production rate from each site, and from SDH (succinate dehydrogenase; complex II) and IV (complex IV). Rates are for rat skeletal muscle mitochondria, from Lambert et al., 2007; Lambert and Brand, 2004a, ; Lambert et al., 2008a; Lambert et al., 2008b; St-Pierre et al., 2002 and Jason Treberg, Casey Quinlan and Martin D Brand (unpublished observations), and are greatest from sites IQ and IIIQo, followed by GPDH, then sites IF, EFTQOR, PDH and OGDH. There is insignificant superoxide production from complex II and complex IV (or from pool Q and cytochrome c, not shown).