Substrate for ATP synthase is

1. ADP and Pi bind to L binding site 2. LT conversion is by energy-driven conformational change that catalyses the formation of ATP 3. T state reves to O state when ATP is released. 4. L state is regenerated for fuher ADP binding. For the complete rotation of F1 head through the 3 states, 10 protons are translocated. Protons entering the system, cause conformational changes in the F1 paicle. Initially the ADP and Pi are loosely bound to the catalytic site on F1. As the Fo accepts protons, the affinity for ADP is increased (step 1, Fig.19.16). Fuher conformational change induces catalytic activity, and ATP is synthesized (step 2, Fig.19.16). This moves protons to the matrix side. As the ATPs are released, the original conformation of the enzyme is assumed (step 3, Fig.19.16). Then ADP is again bound and the cycle repeats (step 4, Fig.19.16). The energy surplus produced by the proton gradient is stored as chemical energy in ATP. The energy-requiring step is not at the synthesis of ATP, but energy is required for the conformational changes. Regulation of ATP Synthesis The availability of ADP regulates the process. When ATP level is low and ADP level is high, oxidative phosphorylation proceeds at a rapid rate. This is called respiratory control or acceptor control. The major source of NADH and FADH2 is the citric acid cycle, the rate of which is regulated by the energy charge of the cell.Ref: DM Vasudevan Textbook of Medical Biochemistry, 6th edition, page no: 233