Redox potentials Ferredoxin, NADPH

C. If ferredoxin has a redox potential of —0.580 V and the activity of NADPH is 3% of that of NADP+, what is the difference in redox potential between the two couples ... 

Therefore, the difference in redox potential between the NADP+-NADPH couple and ferredoxin is... 

The herbicidal effect of paraquat is attributable to the formation of superoxide anion (02 ). Superoxide anion is very toxic compound and is formed by the reaction of oxygen with paraquat radical (paraquat ). Plants, algae, and cyanobacteria have ferredoxin-NADP reductase to form NADPH for the reduction of carbon dioxide (see below). The chemolithoautotrophs also have NAD(P) (NAD and NADP) reductase to form NAD(P)H for the reduction of carbon dioxide. Paraquat [mid-point redox potential at pH 7.0 (Emj 0) = -0.43 V] radical is produced when paraquat is reduced by the catalysis of ferredoxin-NAD(P) reductase or NAD(P) reductase, which catalyzes the reduction of many compounds with of around -0.4 V. Although the aerobic organisms (and even many anaerobic organisms) have superoxide dismutase (SOD) which detoxifies superoxide anion in cooperation with catalase [ascorbate peroxidase in the case of plants (Asada, 1999)], the anion accumulates in the organisms when it is over-produced beyond the capacity of SOD. 

A high NADPH/NADP ratio rather than the proton-motive force is responsible for generation of a low redox potential for ferredoxin reduction (cf. Haaker et al. 1980). As NADPH-dependent nitrogenase activity (via FNR) was not light-dependent (Table 1), DBMIB inhibition with this donor in the light was surprising. In the light, FNR can evidently not directly... 

It can be seen from the normal potentials E° (see p. 18) of the most important redox systems involved in the light reactions why two excitation processes are needed in order to transfer electrons from H2O to NADP"". After excitation in PS II, E° rises from around -IV back to positive values in plastocyanin (PC)—i. e., the energy of the electrons has to be increased again in PS I. If there is no NADP" available, photosynthetic electron transport can still be used for ATP synthesis. During cyclic photophosphorylation, electrons return from ferredoxin (Fd) via the plastoquinone pool to the b/f complex. This type of electron transport does not produce any NADPH, but does lead to the formation of an gradient and thus to ATP synthesis. 

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