Phosphorylation photophosphorylation

The thylakoid membrane is asymmetrically organized, or sided, like the mitochondrial membrane. It also shares the property of being a barrier to the passive diffusion of H ions. Photosynthetic electron transport thus establishes an electrochemical gradient, or proton-motive force, across the thylakoid membrane with the interior, or lumen, side accumulating H ions relative to the stroma of the chloroplast. Like oxidative phosphorylation, the mechanism of photophosphorylation is chemiosmotic. 

Arnon, D. I., 1984. The discovery of photosyntlietic phosphorylation. Trends in Biochemical Sciences 9 258-262. A historical account of photophosphorylation by its discoverer. 

Boyer PD, Chance B, Ernester L, et al. 1977. Oxidative phosphorylation and photophosphorylation. Annu Rev Biochem 46 955-1026. 

FIGURE 11-40 Reversibility of F-type ATPases. An ATP-driven proton transporter also can catalyze ATP synthesis (red arrows) as protons flow down their electrochemical gradient. This is the central reaction in the processes of oxidative phosphorylation and photophosphorylation, both described in detail in Chapter 19. 

The reaction catalyzed by F-type ATPases is reversible, so a proton gradient can supply the energy to drive the reverse reaction, ATP synthesis (Fig. 11-40). When functioning in this direction, the F-type ATPases are more appropriately named ATP synthases. ATP synthases are central to ATP production in mitochondria during oxidative phosphorylation and in chloroplasts during photophosphorylation, as well as in eubacteria and archaebacteria. The proton gradient needed to drive ATP synthesis is produced by other types of proton pumps powered by substrate oxidation or sunlight. As noted above, we return to a detailed description of these processes in Chapter 19. 

ATP is the primary high-energy phosphate compound produced by catabolism, in the processes of glycolysis, oxidative phosphorylation, and, in photosynthetic cells, photophosphorylation. Several enzymes then cany phosphoryl groups from ATP to the other nucleotides. Nucleoside diphosphate kinase, found in all cells, catalyzes the reaction... 

We examine the function of flavoproteins as electron carriers in Chapter 19, when we consider their roles in oxidative phosphorylation (in mitochondria) and photophosphorylation (in chloroplasts), and we describe the photolyase reactions in Chapter 25. 

In eukaryotes, oxidative phosphorylation occurs in mitochondria, photophosphorylation in chloroplasts. Oxidative phosphorylation involves the reduction of 02 to H20 with electrons donated by NADH and FADH2 it occurs equally well in light or darkness. Photophosphorylation involves the oxidation of H20 to 02, with NADP+ as ultimate electron acceptor it is absolutely dependent on the energy of light. Despite their differences, these two highly efficient energy-converting processes have fundamentally similar mechanisms. 

Oxidative phosphorylation and photophosphorylation are mechanistically similar in three respects. (1) Both... 

TABLE 19-4 Agents That Interfere with Oxidative Phosphorylation or Photophosphorylation... 

Chemiosmotic theory provides the intellectual framework for understanding many biological energy transductions, including oxidative phosphorylation and photophosphorylation. 

FIGURE 19-19 Two chemical uncouplers of oxidative phosphorylation. Both DNP and FCCP have a dissociable proton and are very hydrophobic. They carry protons across the inner mitochondrial membrane, dissipating the proton gradient. Both also uncouple photophosphorylation (see Fig. 19-57). 

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