Photophosphorylation noncyclic

Photosynthetic electron transport, which pumps into the thylakoid lumen, can occur in two modes, both of which lead to the establishment of a transmembrane proton-motive force. Thus, both modes are coupled to ATP synthesis and are considered alternative mechanisms of photophosphorylation even though they are distinguished by differences in their electron transfer pathways. The two modes are cyclic and noncyclic photophosphorylation. 

Noncyclic photophosphorylation has been the focus of our discussion and is represented by the scheme in Figure 22.21, where electrons activated by quanta at PSII and PSI flow from HgO to NAJDP, with concomitant establishment of the proton-motive force driving ATP synthesis. Note that in noncyclic photophosphorylation, Og is evolved and NADP is reduced. 

Proton translocations accompany these cyclic electron transfer events, so ATP synthesis can be achieved. In cyclic photophosphorylation, ATP is the sole product of energy conversion. No NADPFI is generated, and, because PSII is not involved, no oxygen is evolved. The maximal rate of cyclic photophosphorylation is less than 5% of the rate of noncyclic photophosphorylation. Cyclic photophosphorylation depends only on PSI. 

The biological functions of chloroplast ferredoxins are to mediate electron transport in the photosynthetic reaction. These ferredoxins receive electrons from light-excited chlorophyll, and reduce NADP in the presence of ferredoxin-NADPH reductase (23). Another function of chloroplast ferredoxins is the formation oT" ATP in oxygen-evolving noncyclic photophosphorylation (24). With respect to the photoreduction of NADP, it is known that microbial ferredoxins from C. pasteurianum (16) are capable of replacing the spinach ferredoxin, indicating the functional similarities of ferredoxins from completely different sources. The functions of chloroplast ferredoxins in photosynthesis and the properties of these ferredoxin proteins have been reviewed in detail by Orme-Johnson (2), Buchanan and Arnon (3), Bishop (25), and Yocum et al. ( ). 

ATP synthesis is not the only energy-conserving reaction of photosynthesis in plants the NADPH formed in the final electron transfer is (like its close analog NADH) also energetically rich. The overall equation for noncyclic photophosphorylation (a term explained below) is... 

Figure E9.1 illustrates the photosynthetic process as it occurs in higher plants. This is called noncyclic photophosphorylation to distinguish it from cyclic photophosphorylation in photosynthetic bacteria. Cyclic photophosphorylation requires only photosystem I and a second series of electron carriers to return electrons to the electron-deficient chlorophyll. The dashed line in Figure E9.1 indicates the flow of electrons in cyclic photophosphorylation. ATP is produced during the cyclic process just as in the noncyclic process, but NADPH is not.

Shimabukuro et al. (1973) identified 2-chloro-4,6-diamino-.v-triazinc (G-28273), which represented complete dealkylation of the triazine ring, as an organosoluble metabolite in sorghum. This metabolite did not inhibit the Hill reaction and cyclic and noncyclic photophosphorylation in isolated pea chloroplasts. 

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. 

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). 

PS I appears to promote cyclic photophosphorylation and proceeds best in light of wavelengths greater than 700 nm, whereas PS II promotes noncyclic photophosphorylation and proceeds best in light of wavelengths shorter than... 

The electron transport chain process of photoreactions I and II is noncyclic photophosphorylation. Cyclic photophosphorylation, which may proceed in the case of oxygen deficiency and can be considered as a shortcircuiting of electron transport, presumably does not play a role in the normal photosynthesis energy storing of the cells. 

The process just described is termed noncyclic photophosphorylation, because ATP is synthesized without the cycling of electrons around a closed path. The dashed vertical line in Fig. 27 represents a short-circuiting pathway of cyclic photophosphorylation, in which electrons can be recycled from ferredoxin to cytochrome /, with the synthesis of ATP along the way. The driving force for this process comes only from photocenter I, no water is used, and no NADPH is synthesized. 

Fig. 30. Proposed scheme of cyclic and noncyclic photophosphorylation in Chroma-tium. Same diagram conventions as Fig. 29. X is an unidentified electron acceptor, probably a ferredoxin. Double-headed arrow represents free interchange of electrons between cytochromes c 5s and Csm. Adapted from information in references 319, 377, and 379.

The process depicted in Figure 17.12 of transferrring electrons from photosystem II (PSII) to photosystem I (PSI) and from water to PSII is called noncyclic electron flow. The generation of ATP by this process is called noncyclic photophosphorylation. An alternative pathway for the light reactions, called cyclic electron flow, utilizes the components of photosystem I, plastocyanin, and the cytochrome b6f complex. (Figure 17.17)... 

The proton gradient is created by the operation of the electron transport chain that links the two photosystems in noncyclic photophosphorylation. 

Cyclic photophosphorylation can take place when the plant needs ATP but does not have a great need for NADPH. Noncyclic photophosphorylation can take place when the plant needs both. 

Isolated chloroplasts incorporated shikimate into the aromatic amino acids and prenylquinones although in low amounts (Bickelet al., 1978 Buchholz r al., 1979). Since the plant enzyme is NADP-linked, generation of reducing power for the chloroplast enzyme could be coupled to noncyclic photophosphorylation. 

AUen JF Cyclic, pseudocyclic and noncyclic photophosphorylation new links in the chain. Trends Plant Sci 8(1) 15—19, 2003. 

Fig. 3. Scheme of photoelectron transport in photosynthesis. The path of photoelectron flow associated with noncyclic photophosphorylation through the two postulated light reactions mediated by Pigment Systems 1 and 2 is indicated by the heavy lines. Redox potentials of electron-carrying cofactors along this path is indicated by scale on the left. Further explanation in text. 

Arnon (1958) reported a stoichiometric relation between the simultaneous photoelectron transport of electrons from water to NADP+ and photophosphorylation. This combination of reactions which is called noncyclic photophosphorylation may be expressed by the following equation ... 

Losada et al. (1961) reported the separation of the two light reactions in noncyclic photophosphorylation and NADP+ reduction in green plants. They blocked System 2, which oxidizes water, by adding the inhibitor DCMU and then adding a dye as an electron donor. Under these conditions they were able to demonstrate the photochemical reduction of NADP+ and the simultaneous noncyclic photophosphorylation. 

Fe-Fd from blue-green bacteria, green algae and higher plants are highly homologous (96 to 98 amino acid residues with 4-6 cysteines). Four of the six cysteines are found in all plant Fd in positions 39,44,47 and 77, and are covalently bound to the Fe-S center. A proposed model of 2Fe-Fd is shown in the Rg. to Iron-sulfur proteins (see). 2Fe-Fd serve as electron-transfer catalysts in both cyclic (1) and noncyclic photophosphorylation (2) ... 

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