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Fig. 1 | Clinical and Translational Medicine

Fig. 1

From: Emerging therapeutic roles for NAD+ metabolism in mitochondrial and age-related disorders

Fig. 1

Schematic illustration of mammalian oxidative phosphorylation system. The mammalian OXPHOS comprises of five multimeric enzyme complexes (CI–V). Electrons from reducing equivalents i.e. NADH and FADH2 enter mitochondrial electron transport chain (ETC) and reduce complex I and complex II, respectively. An inner membrane electron carrier, coenzyme Q10 or ubiquinone accepts an electron from either complex I or complex II and donates it to complex III. Cytochrome c, another electron carrier in the intermembrane space accepts an electron from complex III and donates it to complex IV, which in turn reduces molecular O2 to H2O. During the electron flow, complex I, III and IV simultaneously pump protons from the matrix towards intermembrane space generating an electrochemical gradient or membrane potential (Ψm) across the inner mitochondrial membrane. The energy in this gradient is harnessed by complex V to generate ATP from ADP and inorganic phosphate (Pi), a phenomenon termed as OXPHOS. Approximately 1–2 % electrons leak from the ETC and reduce O2 to superoxide radical (O •−2 ) thereby producing reactive oxygen species (ROS)

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