RuDP carboxylase

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. 

However, when CO2 plus carbonic anhydrase were added together, they did not obtain a rate comparable to HCO3 alone, but 50% of the rate. Hence, a discrepancy was noted. In comparable experiments with other carboxylases including RudP carboxylase, the predicted data more nearly agreed with theory. 

Other carboxylases which are known to function in CAM plants, such as RuDP carboxylase and PEP carboxykinase, use CO2 as the active carboxylating species (Cooper et al., 1968, 1969). 

Fig. 3.4. The double carboxylation hypothesis of Bradbeer et al. (1958). The first carboxylation through RudP carboxylase results in one labeled PGA in the C-1 position and one unlabeled. After conversion to PEP, a second carboxylation through PEP carboxylase yields equal label in the C-4 of malate giving the ratios of 2 C-4 label to one C-1 label so frequently...

The above interpretation is supported by the finding of Osmond and Bjork-man (1975) that both dark CO2 fixation and the initial burst of light CO2 fixation were not affected by different O2 concentrations. In contrast, CO2 fixation at the end of the day was high with low oxygen concentration and vice versa (see Fig. 5.12). The conclusion is that only at the end of the light period does RudP carboxylase/oxygenase contribute substantially to fixation of exogenous CO2. 

Fig. 3.10. Salient features of the proposed photorespiration pathway in green plants. Because of the oxygenase activity of RudP carboxylase, glycolate is produced at the expense of PGA. The glycolate is subsequently oxidized with concomitant oxygen consumption through glycolate oxidase. The glyoxylate thus produced is aminated to glycine two of which can be converted to serine and CO2. Hence, oxygen is consumed and CO2 released in the...

Badger et al. (1975) reported that the RudP carboxylase isolated from Kalan-choe daigremontiana is not unlike that of C3 plants (or C4 plants) in that it shows oxygenase activity and is inhibited by O2. 

We can conclude that CAM plants do have photorespiration in that glycolate is produced in the light, RudP carboxylase has oxygenase activity, there is a postillumination CO2 burst, and O2 inhibits photosynthesis. Dissimilarities between these photorespiratory features and photorespiration in C3 plants are most likely due to the gas exchange phenomena centered around malate metabolism. 

In a survey of some 35 different CAM species, Dittrich et al. (1973) reported data giving a mean activity for RudP carboxylase of 3.3 pmol min" mg chl" ... 

Badger et al. (1975) studied RudP carboxylase and its associated oxygenase activity,... 

RuDP carbocylase and the Calvin cycle rather than via the C4 pathway. In contrast, the lack of oxygen inhibition of dark CO2 fixation suggests that RuDP carboxylase is not involved in harvesting CO2 from the atmosphere during the night. 

The finding by Buchanan et a/. that RuDP carboxylase activity is stimulated by F6P and inhibited by FDP suggested that these two intermediates of the carbon cycle could regulate carboxylase activity during photosynthesis. Activation of RuDP carboxylase by F6P would solve a serious problem, namely the low affinity of RuDP carboxylase for CO2. This difficulty is particularly noteworthy because photosynthesis in nature operates at low partial pressures of CO2. To overcome this limitation the enzyme would require a concentration of CO2 about 100-fold greater than that normally present in air. 

RuDP carboxylase is present at very high concentration in leaves (about half of the soluble protein in chloroplasts) and catalyzes the carboxylation of RuDP to 2 PGA with CO2 rather than COgH" as the active carbon species. As shown by work in the laboratories... 

Phosphoribulokinase, the other unique enzyme of the reductive pentose cycle together with RuDP carboxylase, is also located exclusively in the chloroplasts and is likewise subject to activation by light, but apparently by a diflerent and more direct mechanism. Kinetic studies by Gibbs and collaborators have shown that Ru5P kinase is activated 2-to 4-fold by illumination of intact chloroplast preparations, with a half-time of less than 15 seconds. The photoactivated state of the enzyme decays in the dark with a half-time of about 8 minutes. Since dark incubation of broken chloroplasts with dithiothreitol causes... 

RuDP carboxylase has been found in Thiobacillus thioparus (310), Thiobaeillus denitrificans (18,354), Hydrogenorrwnas ruhlandii (M. Santer and W. Vishniac, unpublished observation), and another Hydrogenomonaa species. In the ThidbadUi exposure to leads to rapid labeling of the carboxyl group of 3-PGA, and the distribution of radioactivity in the phosphate esters of Thiobacillus denitrificans... 

The COj fixation brought about by illuminated intact chloroplasts is sensitive to low concentrations (10 to 10 M) of p-chloro-mercuribenzoate (12), while phosphorylation is much less sensitive and photolysis is unaffected by these concentrations of the pmson. Sensitivity to p-chloromercuiibenzoate was reported by Fager (108) for the formation of phosphoglycerate by CO2 fixation in cell-free spinach preparations. This sensitivity can now be understood since both phosphoribulokinase and RuDP carboxylase activities depend on sulfhydryl groups. 

Further study of the effects of light-dark transitions on the levels of RuDP suggested to Bassham that the affinity of RuDP carboxylase for RuDP was lower in the dark than in the light. This may be one aspect of control mechanisms which co-ordinate the catabolic and anabolic reactions of the cell. The regulation of the flow of carbon between the two by way of the several compounds which are intermediate in both chloroplastic and non-chloroplastic reaction sequences and which can traverse the chloroplast membrane may also be involved. Doubtless many subtle interactions between the photosynthetic and non-photosynthetic reactions of the green cell await discovery. 

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