Figure 1.

Oxidative phosphorylation process and oxidative stress balance. NADH, a product of the citric acid cycle, is oxidized to NAD⁺ at complex I, transferring an electron to ubiquinone (Q). At complex II, FADH₂ is oxidized to FAD⁺, transferring another electron to Q. When Q arrives at complex III, it transfers its electrons to cytochrome C (CytoC). At complex IV, the electrons carried by CytoC are drawn to diatomic oxygen (O₂) and together with hydrogen, they form H₂O. In the electron transport chain process, complexes I, III and IV pump protons from the mitochondrial matrix to the intermembrane space, creating an electrochemical proton gradient and a membrane potential (∆ψ) on the inner membrane of the mitochondria. Complex V (ATP synthase) uses the proton gradient to allow protons back into the mitochondrial matrix, synthesizing ATP in the process. Complexes I and III are the major sites of free radical production; if they release superoxide (O₂•−) to the mitochondrial matrix, mnSOD will transform it to H₂O₂. Then, depending on cell specificity, H₂O₂ will be transformed to O₂ and H₂O by catalase, or to H₂O by glutathione reductase/glutathione peroxidase (GR/GPx) or thioredoxin reductase/peroxiredoxin (TrxR/Prx) activity, in the presence of respiration products (i.e., NADPH).