ATP SYNTHASE (COMPLEX V) It is a protein assembly in the inner mitochondrial membrane. It is sometimes referred to as the 5th Complex (Figs 19.14 and 19.15). Proton pumping ATP synthase (otherwise called F1-Fo ATPase) is a multisubunit transmembrane protein. It has two functional units, named as F1 and Fo. It looks like a lollipop since the membrane-embedded Fo component and F1 are connected by a protein stalk. Fo Unit: The "o" is pronounced as "oh"; and not as "zero". The "o" stands for oligomycin, as Fo is inhibited by oligomycin. Fo unit spans inner mitochondrial membrane. It serves as a proton channel, through which protons enter into mitochondria (Fig.19.14). Fo unit has 4 polypeptide chains and is connected to F1. Fo is water-insoluble whereas F1 is a water-soluble peripheral membrane protein. F1 Unit: It projects into the matrix. It catalyses the ATP synthesis (Fig. 19.14). F1 unit has 9 polypeptide chains, (3 alpha, 3 beta, 1 gamma, 1 sigma, 1 epsilon). The alpha chains have binding sites for ATP and ADP and beta chains have catalytic sites. ATP synthesis requires Mg++ ions. Mechanism of ATP synthesis: Translocation of protons carried out by the Fo catalyses the formation of the phospho-anhydride bond of ATP by F1. Coupling of the dissipation of proton gradient with ATP synthesis (oxidative phosphorylation) is through the interaction of F1 and Fo.Ref: DM Vasudevan Textbook of Medical Biochemistry, 6th edition, page no: 232