Chapter 18: Oxidative Phosphorylation

The text elucidates the principles of the electron-transport chain, where electrons flow from high-transfer-potential carriers like NADH and FADH2 to molecular oxygen, a process driven by standard reduction potentials. This electron transfer occurs through four major protein complexes: NADH-Q oxidoreductase (Complex I), Succinate-Q reductase (Complex II), Q-cytochrome c oxidoreductase (Complex III), and Cytochrome c oxidase (Complex IV). The summary details how Complexes I, III, and IV function as proton pumps, generating a transmembrane proton gradient known as the proton-motive force, as described by Peter Mitchell’s chemiosmotic hypothesis. Key electron carriers such as the lipid-soluble ubiquinone (Coenzyme Q) and the protein cytochrome c are explained in terms of their roles in shuttling electrons between these large complexes. The chapter further explores the sophisticated mechanism of ATP synthase (Complex V), a molecular motor composed of a proton-conducting F0 unit and a catalytic F1 unit. It describes how the flow of protons back into the matrix drives the rotation of the gamma subunit, which in turn induces conformational changes (the binding-change mechanism involving Open, Loose, and Tight states) in the beta subunits to synthesize and release ATP. Additionally, the text covers the transport systems required to move metabolites across the impermeable inner membrane, including the ATP-ADP translocase and shuttle systems like the glycerol 3-phosphate shuttle and the malate-aspartate shuttle, which transport electrons from cytoplasmic NADH. The regulation of cellular respiration via respiratory control, dependent on ADP availability, is analyzed, alongside the phenomenon of uncoupling proteins (like UCP-1 in brown adipose tissue) that generate heat instead of ATP. Finally, the summary addresses the medical implications of oxidative phosphorylation, including the toxic effects of inhibitors like cyanide and carbon monoxide, the cellular defense against reactive oxygen species (ROS) via enzymes like superoxide dismutase, and the role of mitochondria in initiating apoptosis through cytochrome c release.