Proton–ATPase of Chloroplasts

This chapter discusses proton–ATPase of chloroplasts. It was demonstrated that upon reconstitution of the proton ATPase into phospholipid vesicles, the enzyme system can utilize an artificial proton gradient for ATP formation. In a study described in the chapter, proton–ATPases were identified and isolated from several energy transducing membranes. They are composed of two distinct structures: (1) a catalytic sector that is hydrophilic in nature and (2) a membrane sector that is hydrophobic in nature. The chloroplast proton–ATPase complex possesses similar structures and thus, they are applicable for the rest of the energy transducing membranes. Chemical reactions must be involved in the translation of proton motive force into the chemical energy in ATP. There are strong indications that for every electron transferred from water to a Hill acceptor, two protons are pumped inside the thylakoid membrane. The measured ΔpH contained sufficient energy to drive phosphorylation against its maximum potential, provided three protons are used by the ATPase complex per one ATP formed. Therefore, there is a possibility that the P: 2e ratio for noncyclic photophosphorylation is about 1.3. The mechanism by which three protons are involved in a single chemical event is difficult to visualize, and it might be that the third proton imposes a proper conformation in one of the catalytic steps. However, it is still unknown whether phosphate is covalently bound to the enzyme during the phosphorylation activity of proton ATPases