Electron noncyclic flow

Using an alternative path of light-induced electron flow, plants can vary the ratio of NADPH to ATP formed in the light this path is called cyclic electron flow to differentiate it from the normally unidirectional or noncyclic electron flow from H20 to NADP+, as discussed thus far. Cyclic electron flow (Fig. 19-49) involves only PSI. Electrons passing from P700 to ferredoxin do not continue to NADP+, but move back through the cytochrome bef complex to plastocyanin. The path of... 

A close relationship exists (correlation coefficient = 0.98) between light-induced proton translocation coupled to noncyclic electron flow, with either NADP1 or ferricyanide as Hill reagent, and the percentage of cytochrome b-559 in its HP form. 

Fig. 2. Diagrammatic representation of the energy —transducing steps involved in the photosynthetic noncyclic electron flow.

Another class of energy storage compounds consists of redox couples such as NADP+-NADPH (Table 6-1). The reduced form, NADPH, is produced by noncyclic electron flow in chloroplasts (Chapter 5, Section 5.5C). Photosynthesis in bacteria makes use of a different redox couple, NAD+-NADH. The reduced member of this latter couple also causes an... 

From ferredoxin to the next component in the noncyclic electron flow sequence, NADP+, electrons go from —0.42 to —0.32 V (midpoint redox potentials of the couples Fig. 6-4). Again, moving toward higher redox potentials is energetically downhill for electrons, so this step leading to the reduction of the pyridine nucleotide follows spontaneously from the reduced ferredoxin —a step catalyzed by the enzyme ferredoxin-NADP+ ox-idoreductase (Table 5-3 and Fig. 6-4). 

In many organisms, a cyclic process takes place, in which the reduced electron acceptor transfers its electron through a series of carriers back to the oxidized donor. Energy conservation is achieved by coupling proton translocation across a membrane to the electron flow. This type of cyclic electron flow occurs in eukaryotes under some conditions and in many anoxygenic photosynthetic bacteria. No NADPFl is produced, only ATP. This process occurs when cells may require additional ATP, or when there is no NADP+ to reduce to NADPFl. In other organisms, noncyclic electron flow takes... 

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)... 

Rieske protain/centar an iron-sulphur protein first isolat from Complex III of the mitochondrial electron transport chain, in which it occurs with cytochromes b and C) [J.S, Rieske el al. Biochem. Biophys Res Commun. IS (1%4) 338-344], but which has now been found in the equivalent cytochrome be complexes in the bacterial plasma membrane and the chloroplast thylakoid membrane. The latter, known as the cytochrome bff complex, partidpates in cyclic and noncyclic electron flow in the light phase of photosynthesis (see Photosynthesis). All Rieske proteins are one-electron redox systems with a standanl redox potential in the + 0.2 to + 0.3V range and have a (2Fe-2S] center, a single membrane-spanning a-helix, and a characteristic electron spin resonance (ESR) spectrum. The chloroplastidic R.p/c, with a M, of - 20,000, is smaller than that of the mitochondnon. It is encoded in the nucleus, synthesized in the cytoplasm and translocated to the chloroplast, where it is inserted into the thylakoid membrane. Within the thylakoid membrane its [2Fe-2S] redox centre (near to its C-terminus) can readily pass electrons to cytochrome /, a c-type cytochrome that projects from the luminal surface cytochrome / then passes electrons to plastocyanin (see) dissolved in the aqueous milieu of the thylakoid lumen. 

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