Ribulose-1, -diphosphate carboxylase carbon dioxide

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. 

Ribulose 1,5-Diphosphate Carboxylase A Regulatory Enzsnne in the Photosynthetic Assimilation of Carbon Dioxide... 

Scheme 11.1. A cartoon representation of the addition of carbon dioxide (CO2) to the five-carbon ketosugar diphosphate, ribulose-l,5-bisphosphate, in an imagined aldol-type process in the presence of (which might further be imagined to coordinate with the enediol as in pinacol formation, Chapter 8) and the enzyme ribulose-l,5-bisphosphate carboxylase (EC 4.1.1.39). Since 2-carboxy-3-ketoribitol-l,5-bisphosphate is not isolable, the sense of addition re or si) cannot be established. Retroaldol-type fragmentation is presumed to occur to yield, after suitable proton tautomeric adjustments, 2 equivalents of 3-phosphoglycerate (Cl + C5 = 2 C3).

Buchanan, B. B. and SchOrmann, P. (1973) Ribulose 1,5-diphosphate carboxylase a regulatory enzyme in the photosynthetic assimilation of carbon dioxide. In Current Topics in Cellular Regulation (Horecker, B. L. and Stadtman, E. R., eds.), pp. 1-20. Academic Press, New York. 

Malic acid is a very active intermediary product of grape metabolism. The vine contains the L-(—) malic isomer. The vine assimilates carbon dioxide in the air by a C3 mechanism (Ruflher et al., 1983). In this manner, during the dark phase of photosynthesis, the leaves and young green grapes fix CO2 on ribulose 1,5-diphosphate to produce phospho-glyceric acid, which condenses to form hexoses and may also become dehydrated into phosphoenol pyruvic acid. CO2, catalyzed by PEP carboxylase, is fixed on this acid to form oxaloacetic acid, which is, in turn, reduced into malic acid. 

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