Thylakoid membranes photophosphorylation

The proton gradient drives ATP synthesis via an ATP synthase located in the thylakoid membrane (photophosphorylation). Since the electron transport involves a linear array of electron carriers, the system is called noncyclic 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. 

FIGURE 22.21 The mechanism of photophosphorylation. Photosynthetic electron transport establishes a proton gradient that is tapped by the CFiCFo ATP synthase to drive ATP synthesis. Critical to this mechanism is the fact that the membrane-bound components of light-induced electron transport and ATP synthesis are asymmetrical with respect to the thylakoid membrane so that vectorial discharge and uptake of ensue, generating the proton-motive force. 

CFi was originally isolated as a coupling factor, that is, a protein which when removed from the thylakoid membrane leaves a membrane unable to catalyse photophosphorylation, and which when reconstituted into it restores this ability. The... 

The latter process was shown to require ATP, but the source of this ATP was unclear and a matter of considerable dispute. The breakthrough came in 1954 when Arnon and his colleagues demonstrated light-induced ATP synthesis in isolated chloroplasts. The same year Frenkel described photophosphorylation in cell-free preparations from bacteria. Photophosphorylation in both chloroplasts and bacteria was found to be associated with membranes, in the former case with the thylakoid membrane and in the latter with structures derived from the plasma membrane, called chromatophores. In the following years work in a number of laboratories, including those of Arnon, Avron, Chance, Duysens, Hill, Jagendorf, Kamen, Kok, San Pietro, Trebst, Witt and others, resulted in the identification and characterization of various catalytic components of photosynthetic electron transport. Chloroplasts and bacteria were also shown to contain ATPases similar to the F,-ATPase of mitochondria. 

The electron transport chain of photophosphorylation has many similarities to that found in mitochondria many of the chemical players are the same, and much of the gist will seem familiar but reversed. We will discuss the complexes found in the thylakoid membranes in turn. 

Photophosphorylation due to electron flow (blue) and proton flow (red) in chloroplast thylakoid membranes... 

Table I. (A) Photoreduction of NADP and coupled photophosphorylation using an artificial electron donor to PS I isolated by DCMU treatment of chloroplasts (B) lists the NADPH and ATP yields in the presence of DCIP, DAD or TMPD as the electron donor (C) Chemical structure and electrochemical behavior of TMPD and DAD. Data in (B) from Hauska, Oettmeier, Reimer and Trebst (1974) Studies of artificial electron donors for photosystem I across the thylakoid membrane. Z Naturforsch 30c 39.

Ammonia and simple aliphatic amines were among the first uncouplers reported for photophosphorylation by Krogmann, Jagendorf and Avron and by Good respectively. Ammonia and simple aliphatic amines in their lipid-soluble, unprotonated forms can freely permeate the thylakoid membrane. Once inside, they can take up protons and be converted to the corresponding ammonium ions [Fig. 10 (B)]. To maintain electrical neutrality, the accumulation of ammonium ions results in the influx of anions. The accumulation of ammonium salts inside consequently results in an osmotic influx of water and swelling of the thylakoids. 

The thylakoid lumen is the part of the chloroplast enclosed by the thylakoid membrane (Figure 17.15). During photosynthesis, electrons are pumped into the thylakoid lumen from the stroma, forming a proton gradient. Movement of the protons out of the thylakoid lumen through the CFO-CFl complex back to the stroma provides the driving force for photophosphorylation, the process of making ATP in photosynthesis. A similar mechanism is responsible for ATP synthesis in oxidative phosphorylation. 

The atebrin/phosphorylation ratio (Table II, last column) relates inhibition of photophosphorylation to dissipation of the postulated energized state (A pH) of the thylakoid membrane. A low ratio suggests that the two responses are correlated. However, the high ratios obtained for ioxynil and propanil suggest that these two herbicides act as electron transport inhibitors rather than as uncouplers. 

In previous papers, we proved that the photophosphorylation could be proceeded in the combination of the deficient thylakoid membranes from spinach chloroplasts with the crista membranes from mitochondria (1. 2), and that the electron transport of such combined system was linked together (2. 3) the possible electron pathways... 

SUMMARY Antibodies, by reacting with some C-tenninal residues of CFn6, accessible and thus exposed in situ on the thylakoid membrane, inhibit the PMS mediated cyclic photophosphorylation. 

We report here the isolation and the properties of Fj (AFj) from a C3ranobacterium, Anacystis nidulans (Meyer s strain). The purified AFi preparation could restore photophosphorylation when added to AFj-deficient thylakoid membranes from A.nidulans and CFi-deficient ones from spinach. 

The synthesis of ATP during photosynthesis, called photophosphorylation, is presently accounted for in terms of the chemiosmotic hypothesis (Section 13.6). The proton gradient is established across the thylakoid membrane by the electron-driven translocation of protons from the stroma into the intrathylakoid space. Proton translocation may be effected at three stages during... 

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