Phosphoribulose kinase

P, the seven-carbon sugar serving as the transketolase substrate. Likewise, phosphoribulose kinase carries out the unique plant function of providing RuBP from Ru-5-P (reaction 15). The net conversion accounts for the fixation of six equivalents of carbon dioxide into one hexose at the expense of 18 ATP and 12 NADPH. 

Finally, ribose-5-phosphate is converted into ribulose 5-phosphate by phosphopentose isomerase while xylulose 5-phosphate is converted into ribulose 5-phosphate by phosphopentose epimerase. Ribulose 5-phosphate is converted into ribulose 1,5-bisphosphate through the action of phosphoribulose kinase (Figure 20,11). The sum of these reactions is... 

Other means of control also exist. For instance, phosphoribulose kinase and glyceraldehyde 3-phosphate dehydrogenase also are regulated hy NADPH directly. In the dark, these enzymes associate with a small protein called CP 12 to form a large complex in which the enzymes are inactivated. NADPH generated in the light reactions hinds to this complex, leading to the release of the enzymes. Thus, the activity of these enzymes depends first on reduction by thioredoxin and then on the NADPH-mediated release from CP 12. 

In the final step (reaction 15 in Table 22.1), ribulose-l,5-fo iphosphate is regenerated by the phosphorylation of ribulose-5-phosphate. This reaction requires ATP and is catalyzed by the enzyme phosphoribulose kinase. The reactions leading to the regeneration of ribulose-l,5-fe iphosphate summarized in Table 22.1 give a net equation obtained by adding all the reactions. 

In contrast, experiments with heterotropically grown C. pyrenoidosa (with cysteine as the sole sulfur source) showed that the organism, as expected, lacked sulfolipid but showed normal rates of O2 evolution (Sinensky, 1977). Chlorophyll synthesis was relatively unaffected, but CO2 fixation was very poor, apparently because of a reduction in phosphoribulose kinase activity. Sinensky (1977) suggested that sulfolipid might be involved in the enzyme s action, and since the purified enzyme showed no lipid requirement (Racker, 1957), possibly by acting as a chloroplast binding site for the kinase. 

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