Free Radical Production by the Mitochondrion
The electron transport chain located in the mitochondrial inner membrane catalyzes the complete oxidation of NADH to water. This reaction is carried out by a complex system which involves the sequential participation of more than thirty redox-active components including flavoproteins (dehydrogenases), iron-sulfur proteins (dehydrogenases, Rieske protein, etc.), hemecontaining proteins (cytochromes), and a lipid (ubiquinone). Some of these components carry only one electron at a time (cytochromes of the b and c type), while others transport two (flavoproteins) and four (cytochrome oxidase) electrons in a single step. As a consequence, there are several points in the chain in which a two-electron carrier must reduce a one-electron carrier, producing free radical intermediates. One of them, ubisemiquinone, is present in relatively large amounts in the respiring respiratory chain, and may be detected by electron spin resonance.
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