Rubisco ribulose-1,5-diphosphate

FIGURE 17. Mechanism for the Rubisco-catalyzed carboxylation and cleavage of ribulose 1,5-diphosphate... 

In subaerial C3 plants substrate and reactant (s and r, respectively, in Fig. 5.56) for photosynthesis are both gaseous (atmospheric) C02, which flows through the Calvin cycle (the dark reactions of photosynthesis see Box 1.10) to yield simple carbohydrates (p), which are in turn the source of various metabolic intermediates. The source of the intracellular (kinetic) isotopic fractionation during C fixation is the enzyme rubisco (D-ribulose 1,5-diphosphate carboxylase/oxygenase). There is also an isotopic fractionation resulting from the passage of C02 into the cell. Passive diffusion of C02, at a rate , favours 12C, but the fractionation is small... 

DSC, reported that SCN, another chaotropic anion, stabilizes BSA in the same conditions. It is known that SCN binds strongly to BSA [191]. We observed [195] that Ca2+ and Mg2+ decrease QD and Tm of ribulose 1,5-diphosphate carboxylase (Rubisco) to a limited extent but in a definite and steady way as their concentration increases over the ionic strength range 0-0.3 mol/L Na+ and NH+4 had an opposite effect. In the case of Rubisco, specificity of the effect of Ca2+ and Mg2 + is supported by the fact that these cations are known to bind to the protein and to affect its enzymatic activity by inducing structural changes. Specific effects of Na+ and Ca2+ are also observed by DSC on the heat stability of a-lactalbumin [190] they result in a linear increase of QD with the cation/protein molar ratio up to 1, followed by a plateau (Fig. 7). 

Three of the reactions in the Calvin cycle are irreversible and serve to drive the whole cycle in a clockwise direction. The first step in the cycle can be regarded as the action of carbon dioxide on ribulose-l,5-diphosphate to produce two molecules of 3-phosphoglyceric acid which is then converted into 1,3-diphosphoglyceric acid by the action of ATP and the appropriate enzyme. The important first step is catalysed by the phosphoprotein D-ribulose-1,5 bis phosphate carboxylase oxygenase (RUBISCO) which is probably the most abundant enzyme in the world. 

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