In ETC, Oxidative phosphylaion (ATP formation) is regulated by:

A, B, C i.e. (NADH Co-Q reductase, Cytochrome C oxidase, Co-Q-Cytochrome C reductase) Mitchell's chemiosmotic hypothesis links the respiratory (i.e. electron transpo) chain to ATP production (i.e. oxidative phosphorylation)Q. It explains how the free energy generated by the transpo of electrons from NADH to molecular oxygen (in respiratory chain) is used to produce ATP from ADP + Fi (from proton pumps). For each NADH oxidized (pair of electron), complex I (NADH -Q- oxido reductase) and complex III (ubiquinone /Qcytochrome C- oxidoreductase) pump (translocate) 4 protons (H+) each, whereas complex IV (Cytochrome C- oxidase) translocates only 2 protons (H+). In ETC. oxidative phosphorylation (ATP formation) occurs at these sites (ie complex I, III & IV)