Ribulose diphosphate carboxylase/oxygenase

I. Storroe, and B. R. McFadden, Ribulose diphosphate carboxylase/oxygenase in toluene-permeabilized Rhodospirillum rubrum, Biochem. J. 1983, 212, 45-54. 

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... 

In those organisms that perform photosynthesis, further phosphorylation of D-ribulose 5-phos-phate into D-ribulose 1,5-diphosphate by phosphoribulokinase represents an important prerequisite in CO2 fixation. As has already been mentioned, the key and extensively studied enzyme involved in this reaction is D-ribulose 1,5-bisphosphate carboxylase/oxygenase. The subfamily divergence in this multigene family has been studied in certain plants of Triticeae and other families [13]. The occurrence of this enzyme in anoxic Archaea is very interesting, since it had to evolve in the absence of molecular oxygen [14]. 

Branden, R. Ribulose-1,5-diphosphate Carboxylase and Oxygenase from Green Plants are Two Different Enzymes. Biochem. Biophys. Res. Commun. 81, 539 (1978). 

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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