Photophosphorylation coupling sites

Gould,J.M. and Izawa,S. (1973) Studies on the energy coupling sites of photophosphorylation, Biochim. Biophys. Acta 314, 211-223 Graber,P., Burmeister,M., and Hortsch,M. (1981) Regulation of the membrane permeability of spinach chloroplasts by binding of adenine nucleotides, FEBS Lett. 136, 25-31... 

The oxygen formed clearly comes from H20 and not from C02, because photosynthesis in the presence of water labeled with lgO produces oxygen labeled with 180, whereas carbon dioxide labeled with 180 does not give oxygen labeled with 180. Notice that the oxidation of the water produces two electrons, and that the formation of NADPH from NADP requires two electrons. These reactions occur at different locations within the chloroplasts and in the process of transferring electrons from the water oxidation site to the NADP reduction site, adenosine diphosphate (ADP) is converted to adenosine triphosphate (ATP see Section 15-5F for discussion of the importance of such phosphorylations). Thus electron transport between the two photoprocesses is coupled to phosphorylation. This process is called photophosphorylation (Figure 20-7). 

Inhibitory Uncouplers. Inhibitory uncouplers inhibit the reactions affected by both electron transport inhibitors and uncouplers. Hence, they inhibit basal, methylamine-uncoupled, and coupled electron transport with ferricyanide as electron acceptor and water as the electron donor, much like electron transport inhibitors. Coupled noncyclic photophosphorylation is inhibited and the phosphorylation reaction is slightly more sensitive than the reduction of ferricyanide. Cyclic photophosphorylation is also inhibited. NADP reduction, when photosystem II is circumvented with ascorbate + DPIP, is not inhibited however, the associated phosphorylation is inhibited. Inhibitory uncouplers act at both sites 1 and 2 (Figure 2). 

Various pathotypes also produce a cyclic tetrapep-tide known as tentoxin (Fig. 2) which induces chlorosis in many plants (e.g. lettuce, potato, cucumber, spinach), but not in Nicotiana, tomato, cabbage or radish. It binds to chloroplast coupling factor 1 (one toxin-binding site per aP subunit complex), inhibiting photophosphorylation and the Ca -dependent ATP-ase. Species specificity is due to different binding affinities for coupling factor 1 (1.3-20 x lO M for 50% inhibition in sensitive species, and 20-fold higher for insensitive species). 

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