NADP* reduction

Water Is the Ultimate e Donor for Photosynthetic NADP Reduction... 

The immediate electron acceptor for P700 is a special molecule of chlorophyll. This unique Chi a (Aq) rapidly passes the electron to a specialized quinone (Aj), which in turn passes the e to the first in a series of membrane-bound ferredoxins (Fd, Chapter 21). This Fd series ends with a soluble form of ferredoxin, Fd, which serves as the immediate electron donor to the fiavo-protein (Fp) that catalyzes NADP reduction, namely, ferredoxin NADP reductase. 

All the O atoms evolved as Og come from water none comes from carbon dioxide. But 12 O atoms are evolved as 6 Og, and only 6 O atoms appear as 6 HgO in the equation. Also, 6 COg have 12 O atoms, yet there are only 6 O atoms in CgHigOg. How can yon account for these discrepancies Hint Consider the partial reactions of photosynthesis ATP synthesis, NADP reduction, photolysis of water, and the overall reaction for hexose synthesis in the Calvin-Benson cycle.)... 

Figure I.IAJ shows that after addition of NADP, reductions were more efficient. Gonversion of acetophenone was complete after a reaction time of 20 min. ADH activity in E. coli BL21(DE3)/pAW-3 is threefold higher than in E. coli BL21(DE3)/ pAW-4 cells. Enantioselective reductions of various ketones are more efficient using E. coli BL21(DE3)/pAW-3 than with pAW-4. All ketones were reduced completely in a stereoselective manner alcohols were formed with >99% ee and de.

By regulating the partitioning of electrons between NADP+ reduction and cyclic photophosphorylation, a plant adjusts the ratio of ATP to NADPH produced in the light-dependent reactions to match its needs for these products in the carbon-assimilation reactions and other biosynthetic processes. As we shall see in Chapter 20, the carbon-assimilation reactions require ATP and NADPH in the ratio 3 2. 

The oxygen formed clearly comes from H20 and not from C02, because photosynthesis in the presence of water labeled with lgO produces oxygen labeled with 180, whereas carbon dioxide labeled with 180 does not give oxygen labeled with 180. Notice that the oxidation of the water produces two electrons, and that the formation of NADPH from NADP requires two electrons. These reactions occur at different locations within the chloroplasts and in the process of transferring electrons from the water oxidation site to the NADP reduction site, adenosine diphosphate (ADP) is converted to adenosine triphosphate (ATP see Section 15-5F for discussion of the importance of such phosphorylations). Thus electron transport between the two photoprocesses is coupled to phosphorylation. This process is called photophosphorylation (Figure 20-7). 

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). 

The first indication of the existence of an energy-linked NADP reduction was brought forward by Krebs (SO) who found that NADPH-dependent dehydrogenase reactions in pigeon heart homogenate were, sensitive to dinitrophenol. This finding formed the basis for the postulate... 

At variance with Hill s scheme [3], which has been discussed above in its recent developments, a three-light reaction scheme has been proposed by Arnon and coworkers [4,59]. According to this scheme, Fd and subsequently NADP would be reduced by PS II directly, and PS II would perform two different photoacts with two acceptors Fd and Q (Qa ) [4]. The role of PS I would be limited to the performance of cyclic photophosphorylation, catalysed by Fd as the electron carrier. Recent experiments showing that PS Il-enriched, inside-out thylakoid vesicles are capable of low rates of NADP reduction upon addition of Fd, FNR and plasto-cyanin [67] have been designed to investigate the view that only PS II is required to transfer electrons from water to NADP. However, the presence of PS I in the preparations, though in low proportions, was not ruled out, and the cause of the absolute requirement for PC, which is known to be oxidized by P-700 [29], was unexplained. 

Most experimental results indicate that the ATP/2 e ratio observed with isolated chloroplasts (or washed thylakoids) ranges between 1 and 1.3 (see review by Ort and Melandri [5]). Only a few reports of higher ratios have appeared in the literature [86,87] related to class 1 chloroplasts in spite of the correction applied [87] it is difficult to rule out completely the possibility that some cyclic electron transport occurring together with NADP reduction might have contributed to ATP synthesis under the conditions of the experiments. 

Interconversion of cortisone and cortisol catalyzed by llfSHSDHl is fully reversible in vitro. The physiologic direction in vivo is cortisone reduction, which is driven by a high [NADPH]/[NADP+] ratio in the ER lumen. The high ratio can be maintained only by local NADP+ reduction catalyzed by certain luminal dehydrogenases, such as the H6PDH (15, 16, 24), because the permeability of the ER membrane to pyridine nucleotides is negligible (17, 18). 

CH3CH=CHC0SACP + NADPH + H+ CH3CH2CH2COSACP + NADP+ reduction... 

The pathway is cytoplasmic and results in the production of CO2, but no cfatty acid synthesis. Hence, an increase in activity of the PPP would be expected to result in an increase in the value of the RQ. The two enzymes of the FPF directly involved in NADP reduction are glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, as shown under Thianrdrt in Chapter 9. 

The thylakoids and stroma are the sites of the so-called light and dark reactions of photosynthesis, respectively. This compartmentalization of photosynthetic functions was recognized by Park and Pon when they broke open the chloroplasts, separated the contents into thylakoid and stroma fractions and examined their properties. The specific activities of the thylakoids include photochemical reactions, electron transport, oxygen evolution, ATP synthesis and NADP reduction, while the stroma contains enzymes for CO2 fixation driven by ATP and NADPH and other biochemical reactions in the dark. Our understanding and appreciation of the detailed structure and organization of the thylakoid membranes has increased tremendously in recent years. Further discussion of thylakoid structure will be continued in section VII on page 26. 

The hexane-extracted particles that still retained one Q per P700 retained 81 % of the NADP -reduction activity ofthe unextracted control. In contrast, particles extracted with hexane-0.3% methanol had all the OQ removed and the NADP -reduction activity was completely lost. The NADP -reduction activity of the hexane/methanol-extracted particles could be reactivated by readdition of exogenous but only when the hexane extract was also added back. Exogenous OQ alone, even at a rather high concentration, could not reactivate NADP photoreduction and the hexane extract alone was also not effective, presumably because of its low Q content. The nature ofthe component in the hexane extract that contributes to reconstitution is as yet unknown. As the hexane extract contains OQ,chlorophylls,carotenoids,lipidsand other nonpolar molecules, some critical component is probably needed to ensure the correct membrane structure for binding ferredoxin and/or Fd-NADP -reductase. This conclusion is supported by the fact that activity of prior terminal acceptors such as the iron-sulfur centers do not require the hexane extract. 

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. 

Using a transcriptome database (see p. 280) that contains the relative abundance of messenger ribonucleic acid (mRNA) for every hour of the parasite developmental cycle, Bozdech and Ginsburg (2005) were unable to find the genes for transaldolase and NAD kinase that are essential to the PPP. During early stages of parasite development the activity of the PPP involves only the oxidative arm and is geared for NADP reduction and ribose-5-phosphate production, whereas later in the cycle the... 

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