FIG. 4.

ROS production by the mitochondrial electron-transport chain. The mitochondrial intermembrane space is toward the top of the figure. The path of electron transfer is indicated by the broken arrows. Electrons derived from the TCA cycle enter the chain at either complex I or II. They then transfer to the coenzyme Q pool (Q) that carries them to complex II. I, NADH dehydrogenase; II, succinate dehydrogenase; III, bc1 complex; IV, cytochrome oxidase; V, F₁.F_O−ATP synthase. Superoxide is shown being produced at complex I. It also can be produced at complex III under some circumstances (e.g., antimycin A treatment; 58).