Photosystem NADP+ reduction

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). 

Answer Plants have two photosystems. Photosystem I absorbs light maximally at 700 nm and catalyzes cyclic photophosphorylation and NADP+ reduction (see Fig. 19-56). Photosystem II absorbs light maximally at 680 nm, splits H20 to 02 and H+, and donates electrons and H+ to PSI. Therefore, light of 680 nm is better in promoting 02 production, but maximum photosynthetic rates are observed only when plants are illuminated with light of both wavelengths. 

Answer No NADPH is produced. Artificial electron acceptors can remove electrons from the photosynthetic system and stimulate 02 production. Ferricyanide competes with the cytochrome b6f complex for electrons and removes them from the system. Consequently, P700 (of photosystem I) does not receive any electrons that can be activated for NADP+ reduction. However, 02 is evolved because all components of photosystem II are oxidized (see Fig. 19-56). 

Although the NADP -reduction activity ofthe PS-I particles was relatively low, the reconstitution results are significant in that the requirement for OQ in NADP -photoreduction has been unambiguously demonstrated by these experiments. These authors also used transient optical spectroscopy to probe the electron-transport kinetics in the absence and presence of kinetic data and the NADP -photoreduction results are consistent with phylloquinone, i.e., vitamin K, being an intermediary electron acceptor of photosystem I. 

Binding of [ H]2-acetoxymethyl-1,4-naphthoquinone is unspecific, i. e. there exists a linear correlation between free and bound inhibitor (Fig. 1). 50 % inhibition of uncoupled photosynthetic NADP-reduction by 2-acetoxymethyl-1,4-naphthoquinone was achieved at 22.5 uM (pI Q-value 4.65) and 50 % inhibition of uncoupled DCPIP-re-duction at photosystem II at 0.18 mM (pI Q-value 3.7). 

The reaction-center proteins for Photosystems I and II are labeled I and II, respectively. Key Z, the watersplitting enzyme which contains Mn P680 and Qu the primary donor and acceptor species in the reaction-center protein of Photosystem II Qi and Qt, probably plastoquinone molecules PQ, 6-8 plastoquinone molecules that mediate electron and proton transfer across the membrane from outside to inside Fe-S (an iron-sulfur protein), cytochrome f, and PC (plastocyanin), electron carrier proteins between Photosystems II and I P700 and Au the primary donor and acceptor species of the Photosystem I reaction-center protein At, Fe-S a and FeSB, membrane-bound secondary acceptors which are probably Fe-S centers Fd, soluble ferredoxin Fe-S protein and fp, is the flavoprotein that functions as the enzyme that carries out the reduction of NADP+ to NADPH. 

The light reactions in photosynthesis bring about two strongly endergonic reactions—the reduction of NADP to NADPH+H and ATP synthesis (see p. 122). The chemical energy needed for this is produced from radiant energy by two photosystems. 

Photosystems I and II operate in concert. Their interaction is described in the Z scheme (shown in outline in Figure 18). In photosystem II, the primary oxidant is able to remove electrons from water. These electrons are transported to photosystem I via plastoquinone and plastocyanin to replace PSI electrons that have been used in the reduction of iron-sulfur proteins and transferred via NADP to 0O2. Electron flow between PSII and PSI is accompanied by the synthesis of Atp 367 These oxidizing and reducing aspects of photosynthesis can be separated and other substrates incorporated. 

Through a series of oxidation-reduction reactions driven by two light reactions operating in series and involving several hundred chlorophyll molecules, electrons flow from water to NADP. Participating in the overall reaction is a water-splitting complex that includes a mangano-protein and chloride ions. An unidentified chlorophyll a molecule serves as the reaction center of photosystem II, with Q as the primary electron acceptor. Involved sequentially on the electron transport chain are plasto-... 

For each photon absorbed by any of the accessory pigments or Chi s whose excitations are funneled into a reaction center, one electron can be removed from its trap chi. Because four electrons are involved per 02 derived from water, the evolution of this molecule of 02 requires the absorption of four photons by Photosystem II or the light-harvesting antennae feeding into it (see Eq. 5.8 and Fig. 5-15). An additional four photons whose excitations arrive at the trap chi of Photosystem I are required for the reduction of the two molecules of NADP+ to NADPH necessary for the subsequent reduction of one C02 molecule (Eq. 5.9 Figs. 5-1 and 5-15). Hence eight photons are needed for the evolution of one molecule of 02 and the fixation of one molecule of C02. (In Chapter 6, Section 6.3D, we will consider how many photons are used to provide the ATP s required per C02 fixed.) The series representation (Fig. 5-15) proposed by Hill and Fay... 

The reduction of each CO2 to the level of a hexose requires 2 moles of NADPH. The reduction of NADP+ is a two-electron process. Hence, the formation of 2 moles of NADPH requires the pumping of four photons hy photosystem... 

One photosystem for the reduction of NADP" to NADPH consists of N,N -dimethyl-4,4 -bipyridinium (methylviologen, MV +) as an electron mediator,... 

Anabaena, is a 36 kDa basic protein having a noncovalently bound flavin (FAD) cofactor. Fd is a smaller (11 kDa) acidic [2Fe-2S] protein that is present in all photosynthetic organisms, and acts as a shuttle between larger proteins (in this case the iron-sulfur subunit of photosystem I and FNR), which are often anchored in membranes and have restricted mobility. Note that Fd is a one-electron carrier and NADP" " requires the simultaneous addition of two electrons for its reduction. 

Plastoquinone in turn is a reductant for excited P700 of photosystem PS I, which operates similarly to the system PS II and has a reduction potential sufficient for an electron transfer to the iron-sulfur complex of ferredoxin and finally to NADP , producing NADP -H,. 

In vivo, most electrons from reduced ferredoxin are passed onto nicotinamide adenine dinucleotide phosphate cation (NADP ), via ferredoxin-NADP reductase, to generate the NADPH needed to drive carbon dioxide fixation by the Calvin cycle. Thus electrons from photosystem I can pass through at least three routes (Figure 1), of which route C is preferred (II). However, if the supply of NADP were limited, for example, because of a poor supply of carbon dioxide causing a slow turnover of the Calvin cycle, the electron flow rate along pathway C would be expected to be decreased and more 02" should be made by route B and, to a lesser extent, by route A (15-17), Some oxygen reduction takes place even when carbon dioxide is present in ample amounts (18). 

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