Ribulose-1-5-diphosphate RuDP

Fig. 33. Experiment to determine the CO2 acceptor. Removal of CO2 leads to an increase in the concentration of ribulose-1,5-diphosphate (RudP) and a decrease in that of 3-phosphogylceric acid (PGA) (from Baron 1967).

Binding of C02 takes place in aqueous medium by the carboxylation reaction of ribulose-diphosphate (RuDP) with the formation of 3-phospho-glycerine acid (PGA) - table 5. Water molecule and radical C=0 at the distances of molecular interaction have quite similar values of PE-parameters for forming the general structural grouping of dimeric composite type. Total PE-parameter of water molecule and radical C=0 hearly equals PE-parameter of C02 and therefore the molecules of C02 and H20 join RuBP with the formation of two radicals COOH b PGA (table 5). In ferment RuDP- carboxylase, Mg atoms and 0" ions (5.4867 eV and 4.755 eV) play an active role, their PE-parameters similar to PE-parameter of radical COOH. 

The neutral FDPase and SDPase activities, which were present in the crude spinach extracts, were precipitated at lower ammonium sulfate concentration and could thus be separated from the specific alkaline FDPase. These activities appeared to be associated with the chloroplast fraction and did not require the presence of a divalent cation for activity. In crude extracts only the alkaline FDPase activity was inhibited by antiserum prepared by immunizing rabbits with the purified alkaline FDPase. The neutral FDPase was also active with ribulose diphosphate (RuDP) (98). 

Note that six ribulose diphosphates (RuDP) combine with six COj to yield twelve 3-phosphoglycerates (3PG). Two of these give rise to glucose the other ten eventually regenerate six RuDP to continue the fixation process. 

Fig. 5.9. Diagram summarising the path of carbon assimilation in chloroplasts (see also eqn. 49). The assimilation can be divided into three phases. Phase I The phosphorylation of ribulose monophosphate (Ru-5-P) to ribulose diphosphate (RuDP) which then accepts a molecule of COj and is cleaved to 2 molecules of PGA (eqn. 49 (g) and (a)). Phase II PGA is reduced to triose phosphate (eqn. 49 (b)). Phase III Triose phosphate acts as the precursor of both Ru-5-P and of hexose and starch. The reactions are driven by ATP and reduced NAD. The assimilation of 1 mole of COj requires 3 moles of ATP and 2 moles of reduced NADP. (After D. I. Amon, in Biological Structure and Function, edited by T. W. Goodwin and O. Lindberg, Academic Press, New York, 1961.)...

By contrast, the Calvin cycle in photosynthesis illustrates a positive feed-back mechanism. The concentration of Calvin cycle intermediates persisting in photosynthetic cells in the dark is very low. When photosynthesis commences the availability of ribulose diphosphate (RuDP) may limit the rate of COj assimilation. However, the formation of phosphoglyceric acid (PGA) immediately raises the level of other intermediates in the cycle, including that of RuDP, and this in turn speeds up the rate of CO assimilation. Similarly, when cells... 

These studies were continued by Wilson and Calvin (1955), who left the light on but suddenly lowered the tension of CO2 to 0.003%. Since PGA is a product of the carboxylation reaction, it was expected that its concentration would fall. This is exactly what happened. At the same time, the concentration of ribulose diphosphate rose rapidly and then fell. This behavior is to be expected if ribulose diphosphate is a substrate for the carboxylation reaction, since lowering the CO2 pressure will stop the reaction which uses up RuDP. Thus, the carboxylation of ribulose diphosphate to give PGA as a first step in the carbon reduction pathway was discovered [Eq. (8)]. 

PG = 3-phosphoglyceric acid G3P = glyceraldehyde 3-phosphatc DHAP = dihydroxyacetone phosphate FDP = fructose 1,6-diphosphate F6P = fructose 6-phosphate G6P = glucose 6-phosphate E4P = erythrose 4-phosphate X5P = xylulose 5-phosphate SDP = sedoheptulose 1,7-diphosphaie S7P = sedoheptulose 7-phosphate R5P = ribose 5-phosphate Ru5P = ribulose 5-phosphate RuDP = ribulose 1,5-diphosphate... 

After a time, several other sugar phosphates were identified. Most important among these were the seven-carbon compounds, sedoheptulose-7-phosphate (IX) and sedoheptulose-l,7-diphosphate (SDP) (X), and the five-carbon compounds, ribulose-l,5-diphos-phate (RuDP) (II) and ribose-5-phosphate (XI), xylulose-5-phosphate (XII), and ribulose-5-phosphate (I) (Benson, et al., 1952). The roles of these compounds in the path of carbon in photosynthesis became more clear after they had been degraded to locate the position of radiocarbon atoms within the individual molecules (Bassham et al., 1954). 

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