Oxidative Phosphorylation

Oxidative phosphorylation consists of two components: the Electron Transport Chain (ETC) and Chemiosmosis. The ETC comprises four major protein complexes (I-IV) and two mobile carriers (Ubiquinone and Cytochrome c). Electrons from NADH enter at Complex I, while electrons from FADH2 enter at Complex II. As electrons move through the complexes toward oxygen (the final electron acceptor), the energy released is used to pump protons from the mitochondrial matrix into the intermembrane space, creating an electrochemical gradient (the proton motive force). Complex IV reduces oxygen to water. In chemiosmosis, protons flow back into the matrix through ATP synthase (Complex V). This flow causes conformational changes in the enzyme that catalyze the synthesis of ATP from ADP and inorganic phosphate (Pi). The efficiency of this process can be altered by uncoupling proteins (like UCP1 in brown fat), which allow protons to leak back without making ATP, generating heat instead. This process is sensitive to various poisons (e.g., Cyanide, Carbon Monoxide) which inhibit specific complexes, leading to rapid cellular energy failure and death.

The transfer of electrons is a series of redox reactions. Complex I (NADH:ubiquinone oxidoreductase) transfers electrons from NADH to ubiquinone (CoQ) and pumps 4 protons. Complex II (succinate dehydrogenase) transfers electrons from FADH2 to CoQ but pumps no protons. CoQ carries electrons to Complex III (cytochrome bc1 complex), which pumps 4 protons and passes electrons to Cytochrome c. Complex IV (cytochrome c oxidase) transfers electrons to oxygen, pumping 2 protons and forming water. The Chemiosmotic Theory, proposed by Peter Mitchell, explains that the resulting H+ gradient represents stored potential energy. ATP synthase acts as a molecular motor; the F0 subunit provides a channel for protons, and the F1 subunit uses the rotational energy to synthesize ATP. This process is tightly coupled; if the proton gradient is not dissipated (e.g., by ATP synthesis), electron transport slows down. Oligomycin inhibits ATP synthase directly. Chemical uncouplers like 2,4-dinitrophenol (DNP) destroy the gradient, causing rapid oxygen consumption and hyperthermia without ATP production.

Cyanide and Carbon Monoxide (CO) poisoning are critical clinical scenarios; they bind to the iron in Cytochrome c oxidase (Complex IV), halting the ETC and causing 'histotoxic hypoxia' where oxygen is present in the blood but cannot be used by cells. This leads to profound lactic acidosis. Mitochondrial diseases (e.g., MELAS, LHON) involve defects in these complexes, often affecting high-energy tissues like the brain and muscles. Brown adipose tissue uses UCP1 (thermogenin) for non-shivering thermogenesis in neonates. DNP, sometimes used illicitly as a weight-loss drug, is highly lethal due to uncontrolled hyperthermia from uncoupling.

In cases of suspected ETC inhibition (e.g., CO or Cyanide), investigations include: Arterial Blood Gas (showing metabolic acidosis with high lactate), Carboxyhaemoglobin levels (for CO), and venous oxygen saturation (which may be abnormally high because cells aren't extracting oxygen). Muscle biopsy and genetic testing are used for primary mitochondrial disorders.

Acute management of ETC toxins: for CO, 100% oxygen (or hyperbaric oxygen); for Cyanide, hydroxocobalamin or sodium thiosulphate. For mitochondrial diseases, management is supportive and may include the 'mitochondrial cocktail' (CoQ10, vitamins), though evidence is limited. Management of DNP toxicity is aggressive cooling and supportive care; there is no specific antidote.