ATP Synthesis by Photophosphorylation:- A Proton Gradient Couples Electron Flow and Phosphorylation
Several properties of photosynthetic electron transfer and photophosphorylation in chloroplasts indicate that a proton gradient plays the same role as in mitochondrial oxidative phosphorylation. (1) The reaction centers, electron carriers, and ATP-forming enzymes are located in a proton-impermeable membrane—the thylakoid membrane—which must be intact to support pho tophosphorylation. (2) Photophosphorylation can be uncoupled from electron flow by reagents that promote the passage of protons through the thylakoid membrane. (3) Photophosphorylation can be blocked by venturi cidin and similar agents that inhibit the formation of ATP from ADP and Pi by the mitochondrial ATP synthase (Table 19–4). (4) ATP synthesis is catalyzed by FoF1 complexes, located on the outer surface of the thylakoid membranes, that are very similar in structure and function to the FoF1 complexes of mitochondria.
Electron-transferring molecules in the chain of carriers connecting PSII and PSI are oriented asym metrically in the thylakoid membrane, so photoinduced electron flow results in the net movement of protons across the membrane, from the stromal side to the thylakoid lumen (Fig. 19–57). In 1966 André Jagendorf showed that a pH gradient across the thylakoid membrane (alkaline outside) could furnish the driving force to generate ATP. His early observations provided some of the most important experimental evidence in support of Mitchell’s chemiosmotic hypothesis.
Jagendorf incubated chloroplasts, in the dark, in a pH 4 buffer; the buffer slowly penetrated into the inner compartment of the thylakoids, lowering their internal pH. He added ADP and Pi to the dark suspension of chloroplasts and then suddenly raised the pH of the outer medium to 8, momentarily creating a large pH gradient across the membrane. As protons moved out of the thylakoids into the medium, ATP was generated from ADP and Pi. Because the formation of ATP occurred in the dark (with no input of energy from light), this experiment showed that a pH gradient across the membrane is a high-energy state that, as in mitochondrial oxidative phosphorylation, can mediate the trans duction of energy from electron transfer into the chemical energy of ATP.
FIGURE 19–57 Proton and electron circuits in thylakoids. Electrons (blue arrows) move from H2O through PSII, through the intermediate chain of carriers, through PSI, and finally to NADP+. Protons (red arrows) are pumped into the thylakoid lumen by the flow of electrons through the carriers linking PSII and PSI, and reenter the stroma through proton channels formed by the Fo (designated CFo) of ATP synthase. The F1 subunit (CF1) catalyzes synthesis of ATP.
