Electrons are passed along the electron transport chain (ETC) from two donors, NADH and FADH2, produced by glycolysis, the TCA cycle and fatty-acid oxidation. They move through four membrane complexes — I (NADH dehydrogenase), II (succinate dehydrogenase), III (cytochrome c reductase) and IV (cytochrome c oxidase) — pumping protons into the intermembrane space. The final electron acceptor O2 becomes water, and Complex V (ATP synthase) uses the resulting proton gradient to convert ADP and Pi into ATP.

Complexes and enzymes. The chain comprises five complexes with differing subunit counts (Complex I has 45 subunits, II has 4, III has 11, IV has 13 and V has 17). Assay Genie also offers mitochondrial Complex I and III activity assay kits to measure how drugs, toxicants and environmental conditions affect their activity.

Electron carriers and cofactors. Mobile carriers cytochrome c and coenzyme Q (ubiquinone/CoQ10) shuttle electrons between complexes, while NADH and FADH2 deposit electrons and are recycled to NAD+ and FAD, sustaining the proton gradient that drives ATP synthesis.

Immunometabolism. OXPHOS is increasingly recognised beyond energy production: immune cells modulate OXPHOS versus glycolysis to meet the demands of activation and infection, and obesity-associated mitochondrial dysfunction impairs this response. Our validated kits let researchers interrogate OXPHOS to generate robust, reproducible data.