Ribulose 5-phosphate photosynthesis

Ribulose phosphate kinase is active only when a cystine disulfide on the enzyme is reduced to two cysteines. An electron carrier, thioredoxin, carries out this reduction, and is then itself reduced by electrons from NADPH. Because the action of Photosystems I and II forms NADPH, this reduction ensures that ribulose bisphosphate is made only when enough light exists to support Photosynthesis. In other words, the light and dark reactions are coupled. 

The end result of Eq. (13)-(22) is the conversion of five molecules of glyceraldehyde-3-phosphate to three molecules of ribulose-5-phosphate (see Fig. 6). Two of these molecules formed by Eq. (21) and (22) are labeled in carbon atom position 3, while the third one, from Eq. (19) and (20), is labeled in positions 1, 2, and 3. The resultant average labeling of ribulose phosphate is heavy in position 3 and lighter in positions 1 and 2. When the ribulose molecules, labeled after a few seconds photosynthesis with C 02, were degraded (Bassham et al., 1954), this pattern of... 

Ribulose is the ketose corresponding to ribose. In general, the ketoses are designated by the ending -ulose (unless they have trivial names, such as fructose). Xylulose is an epimer to ribulose at C-3. Ribulose phosphate and diphosphate play a role in biologic intereonversions of the sugars and in photosynthesis (Chapt. XVI-4). 

Phosphoric acid esters of the ketopentose D-ribulose (2) are intermediates in the pentose phosphate pathway (see p.l52) and in photosynthesis (see p.l28). The most widely distributed of the ketohexoses is D-fructose. In free form, it is present in fruit juices and in honey. Bound fructose is found in sucrose (B) and plant polysaccharides (e.g., inulin). 

Photosynthesis in vascular plants takes place in chloroplasts. In the C02-assimilating reactions (the Calvin cycle), ATP and NADPH are used to reduce C02 to triose phosphates. These reactions occur in three stages the fixation reaction itself, catalyzed by rubisco reduction of the resulting 3-phosphoglycerate to glyceraldehyde 3-phosphate and regeneration of ribulose 1,5-bisphosphate from triose phosphates. 

Phosphate and bicarbonate ions are important substrates for many enzymatic processes and as such have regulatory functions. Bicarbonate controls the key enzyme of photosynthesis, ribulose bisphosphate carboxylase, by carbamate formation (Fig. 13-12). Chloride ions activate amylases and may affect the action of "G proteins" that mediate hormone actions. Other observed effects of ions are too numerous to mention. 

Photosynthesis. The formation of carbohydrates in green plants by the process of photosynthesis is described in ihc entry on Photosynthesis. The synthetic mechanism involves the addition of carbon dioxide to ribulose-1,5-diphosphate and the subsequent formation of two molecules of 3-phosphoglyccric acid which are reduced to glyceraldehyde-3-phosphate. The triose phosphates are utilized to again from ribulose-5-phosphates by enzymes of the pentose phosphate cycle Phosphorylation or ribulose-5-phosphate with ATP regenerates ribulose-1.5-diphosphate to accept another molecule of carbon dioxide. See also Phosphorylation (Photosynthetlc). 

For cadmium, Weigel (1985 a, b) concluded that, in vitro, this metal inhibits photosynthesis mainly by interaction with several sites in the Calvin cycle and not by interaction with photochemical reactions located on the thylakoid membrane. In vitro studies showed a 90% inhibition of phosphoribulokinase (EC 2.7.1.18) by cadmium ions (Hurwitz et al., 1956). This element also inhibited light activation of the Calvin cycle enzymes glyceraldehyde-3-phosphate kinase (EC 1.2.1.13) and ribulose-5-phos-phate kinase (EC 2.7.1.19) in mesophyll protoplasts of Valerianella locusta (Weigel,... 

Answer The reductive pentose phosphate pathway regenerates ribulose 1,5-bisphosphate from triose phosphates produced during photosynthesis, in a series of reactions involving sugars of three, four, five, six, and seven carbons and the enzymes transaldolase and transketo-lase. The oxidative pentose phosphate pathway plays a different metabolic role it provides NADPH for reductive biosynthesis and pentose phosphates for nucleotide synthesis. 

Figure 8-13. Schematic illustration of Rubisco (ribulose-l,5-bisphosphate carboxylase/oxygenase) acting as the branch point for photosynthesis and photorespiration. All three of the organelles involved, but only a few of the biochemical steps, are indicated. ( represents phosphate. Note that 3-phosphoglycerate and glycolate refer to the dissociated forms of 3-phosphoglyceric acid and glycolic acid, respectively.)...

Of the two D-tetroses, only 4-phospho-D-erythrose is found in any quantity, as an intermediate in the photo synthetic reactions. Of the four pentoses, only D-ribose and D-xylose occur to any extent. The phospho- z/J6>-D-pentoses, D-ribose-5-phosphate and D-xylose-5-phosphate, are found in the photo synthetic reactions. Three phospho- t6>-D-pentoses are also found as intermediates in the photosynthetic reactions D-ribulose-5-phosphate, D-xylulose-5-phosphate, and D-ribulose-l,5-bis-phosphate, the latter carbohydrate being directly involved in the fixing of C02in photosynthesis. 

The pentose phosphate pathway also catalyzes the interconversion of three-, four-, five-, six-, and seven-carbon sugars in a series of non-oxidative reactions. All these reactions occur in the cytosol, and in plants part of the pentose phosphate pathway also participates in the formation of hexoses from CO2 in photosynthesis. Thus, D-ribulose 5-phosphate can be directly converted into D-ribose 5-phosphate by phosphopentose isomerase, or to D-xylulose 5-phosphate by phosphopentose epimerase. D-Xylulose 5-phosphate can then be combined with D-ribose 5-phosphate to give rise to sedoheptulose 7-phosphate and glyceraldehyde-3-phosphate. This reaction is a transfer of a two-carbon unit catalyzed by transketolase. Both products of this reaction can be further converted into erythrose 4-phosphate and fructose 6-phosphate. The four-carbon sugar phosphate erythrose 4-phosphate can then enter into another transketolase-catalyzed reaction with the D-xylulose 5-phosphate to form glyceraldehyde 3-phosphate and fructose 6-phosphate, both of which can finally enter glycolysis. 

Figure 20.12 Calvin cycle, The diagram shows the reactions necessary with the correct stoichiometry to convert three molecules of CO2 into one molecule of dihydroxyacetone phosphate (DHAP). The cycle is not as simple as presented in Figure 20.1 rather, it entails many reactions that lead ultimately to the synthesis of glucose and the regeneration of ribulose 1,5-bisphosphate. [After J. R. Bowyer and R. C. Leegood. Photosynthesis, in Plant Biochemistry, P. M. Dey and J. B, Harborne, Eds. (Academic Press, 1997), p. 85.]...

C4 plants possess two types of photosynthesizing cells in their leaves mesophyll cells and bundle sheath cells. (In C3 plants, photosynthesis occurs in mesophyll cells.) Most mesophyll cells in both plant types are positioned so that they are in direct contact with air when the leaf s stomata are open. In C4 plants, C02 is captured in specialized mesophyll cells that incorporate it into oxaloacetate (Figure 13B). Phosphoenolpymvate carboxylase (PEP carboxylase) catalyzes this reaction. Oxaloacetate is then reduced to malate. Once formed, malate diffuses into bundle sheath cells. (As their name implies, bundle sheath cells form a layer around vascular bundles, which contain phloem and xylem vessels.) Within bundle sheath cells, malate is decarboxylated to pyruvate in a reaction that reduces NADP+ to NADPH. The pyruvate product of this latter reaction diffuses back to a mesophyll cell, where it can be reconverted to PEP. Although this reaction is driven by the hydrolysis of one molecule of ATP, there is a net cost of two ATP molecules. An additional ATP molecule is required to convert the AMP product to ADP so that it can be rephos-phorylated during photosynthesis. This circuitous process delivers CO, and NADPH to the chloroplasts of bundle sheath cells, where ribulose-1,5-bisphosphate carboxylase and the other enzymes of the Calvin cycle use them to synthesize triose phosphates. 

Jacob, J. and Lawlor, D.W. (1992) Dependence of photosynthesis of sunflower and maize leaves on phosphate supply, ribulose-1,5-bisphos-phate pool size. Plant Physiology 98, 801-807. 

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