Ferredoxin reducing substance

Figure 2. Schematic of photoinduced electron transport and phosphorylation reactions considered to occur in chloroplast lamellae [from Moreland and Hilton (2)]. Open arrows indicate light reactions solid arrows indicate dark reactions and the narrow dashed line represents the cyclic pathway. Abbreviations used PS I, photosystem I PS II, photosystem II Y, postulated electron donor for photosystem II Q, unknown primary electron acceptor for photosystem II PQ, plastoquinones cyt b, b-type cytochromes cyt f, cytochrome f PC, plastocyanin P700, reaction center chlorophyll of photosystem I FRS, ferredoxin-reducing substance Fd, ferredoxin Fp, ferredoxin-NADP oxidoreductase FeCy, ferricyanide asc, ascorbate and DPIP, 2,6-dichloropheno-lindophenol. The numbers la, lb, 2, 3, and 4 indicate postulated sites of action by...

P700 (a special chlorophyll a molecule) serves as the reaction center of photosystem I, and a bound form of ferredoxin (ferredoxin-reducing substance) may be the electron acceptor. Electrons flow subsequently to NADP through ferredoxin (a nonheme iron protein) and a flavoprotein. 

JH Wang (1970) Oxidative and photosynthetic phosphorylation mechanisms. Science 167 25-30 CF Yocum and A San Pietro (1969) Ferredoxin-reducing substance (FRS) from spinach. Biochem Biophys Res Common 36 614-620... 

Fig. 27. Electron flow in green plant photosynthesis. Vertical wavy arrows represent excitation of chlorophyll molecules by absorbed light. Reaction intermediates are as follows Z , unknown intermediate donating electrons to photocenter II Q , unknown intermediate accepting electrons from excited chlorophyll PQ, plasto-quinone, structurally similar to coenzyme Q or ubiquinone of Fig. 4 bss>, haa,, cytochrome components PC, plastocyanin, a copper-containing nonheme protein FR8, unknown ferredoxin reducing substance FD, ferredoxin FP, flavoprotein mediating reduction of NADP. ...

Ferredoxin reducing substance is as yet an unidentified redox system. 

In a sense Fd, rather than NADP+, is the direct recipient of electrons from PSI. For example, much of the reduced Fd is used to reduce NADP+, but some is used for other reductive reactions, too, such as the reduction of thioredoxin (Figure 17.23). Ferredoxin can also use electrons to reduce substances and may also be considered the final electron accepting molecule of PSI. 

This enzyme [EC 1.18.99.1], also known as hydrogenly-ase, catalyzes the reaction of H2 with two oxidized ferre-doxin to produce two H+ and two reduced ferredoxin. This enzyme is an iron-sulfur protein and requires nickel ions. It can use molecular hydrogen to reduce a variety of substances. See also Hydrogen Dehydrogenase Cytochrome C3 Hydrogenase... 

Flavin-dependent reductases use hydrogen atoms from NADH or NADPH to reduce many specific substances or classes of compounds. The FAD-containing ferredoxin ... 

The membrane-bound iron-sulfiir centers were discovered by Dick Malkin and Alan Bearden in 1971 in spinach chloroplasts using EPR spectroscopy. Since the EPR spectrum was found to resemble that of the iron-sulfur protein ferredoxin and since the soluble ferredoxin had already been removed from the chloroplast sample used in the measurement, the substance represented by the newly found EPR spectrum was initially called membrane-bound ferredoxin. And since the iron-sulfur center was also found to be photo-reducible at cryogenic temperature, it was therefore suggested that it was the primary electron acceptor of photosystem I. 

The absorption of light by P qq then leads to the series of electron transfer reactions of photosystem I. The substance to which the excited-state chlorophyll, P700, gives an electron is apparently a molecule of chlorophyll a this transfer of electrons is mediated by processes that take place in the reaction center. The next electron acceptor in the series is bound ferredoxin, an iron—sulfur protein occurring in the membrane in photosystem I. The bound ferredoxin passes its electron to a molecule of soluble ferredoxin. Soluble ferredoxin in turn reduces an FAD-containing enzyme called ferredoxin-NADP+ reductase. The FAD portion of the enzyme reduces NADP+ to NADPH (Figure 22.6). We can summarize the main features of the process in two equations, in which the notation ferredoxin refers to the soluble form of the protein. 

It was next found that substances with too low a reduction potential formed too little of the free radical form in the plant and when the results were recalculated on the basis of the amount of free radical formed, all members of the series were equitoxic (Homer, Mees and Tomlinson, 1960). High herbicidal activity is confined to analogues with between —300 and —500 mV. This makes it likely that they are reduced to free radicals by ferredoxin. 

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