Ribulose bisphosphate, carboxylation

The key reaction of the Calvin-Benson cycle of C02 reduction is the carboxylation of ribulose bisphos-phate to form two molecules of 3-phosphoglycerate (Eq. 13-48). The properties of ribulose bisphosphate carboxylase (rubisco, Figs. 13-10 to 13-12), which catalyzes this reaction, are discussed in Chapter 13. It... 

In addition to serving as a substrate, C02 activates ribulose-bisphosphate carboxylase by binding to the e-amino group of a lysyl residue in the large subunit to form a carbamate (lysine—NH—C02 ). This process requires Mg2+, which binds to the carboxyl group of the carbamate. The Mg2+ in turn forms part of the binding site for a second molecule of C02, which acts as the substrate in the carboxylase reaction. The reactive molecule of C02 probably adds to the enolate of ribulose-1,5-bisphosphate to form 2-carboxy-3-ketoarabinitol-l,5-bisphosphate as an intermediate, as shown in figure 15.26. 

Photorespiration results from the oxygenation reaction catalyzed by ribulose bisphosphate carboxylase/ oxygenase. 2-Phosphoglycolate generated by the reaction moves from the chloroplast to the cytosol, where other enzymes break it down to C02, H20, and Pj. The oxygenation reaction, like carboxylation, does not require light directly. It occurs mainly during illumination, however, because the formation of the substrate, ribulose-1,5-bisphosphate, requires ATP and NADPH (see fig. 15.25). 

Nature circumvents thermodynamics by coupling unfavorable equilibria to favorable equilibria. This occurs, for example, in the carboxylation of pyruvic acid by malic enzyme, where the oxaloacetate that is formed initially is immediately reduced to malate. The same strategy is used in isocitrate dehydrogenase. In the carboxylation of ribulose bisphosphate, a thermodynamically unfavorable carboxylation is made irreversible by cleavage of the six-carbon product to form two three-carbon products (vide infra). 

Ribulose-l,5-bisphosphate carboxylase/oxygenase (RuBP carboxylase) catalyzes the carboxylation of ribulose bisphosphate to give 3-phosphoglyceric acid [Eq. (14)] (7/. 72). 

Carboxylation is, in the first place, a thermodynamic problem. Nature solves this problem by associating the carboxylation with some exergonic process. This may be reduction of a product (as in the case of malic enzyme and isocitrate dehydrogenase), hydrolysis of ATP (as in the case of pyruvate carboxylase), hydrolysis of phosphoenolpyruvate (as in the case of phosphoenolpyruvate carboxylase), or cleavage of a carbon-carbon bond (as in the case of ribulose-bisphosphate carboxylase). 

Roeske CA, O Leary MH (1985) Carbon isotope effect on carboxylation of ribulose bisphosphate catalyzed by ribrrlosebisphosphate carboxylase fromRhodospirillum rubum. Biochem 24 1603-1607... 

The chloroplast genome is similar to that of mitochondria, reflecting its similar bacterial origin. However, the circular chromosome is larger than its mitochondrial counterpart, encoding 30 membrane proteins that are involved in photosynthesis. It also encodes the four components of the bacterial ribosomal system (23S, 16S, 5S, and 4.5S), 20 ribosomal proteins, and 30 tRNAs. Notable among the proteins is one of the subunits of ribulose bisphosphate carboxylase (Rubisco). This enzyme catalyzes the carboxylation of ribulose 1,5-bisphosphate from COj and is responsible for carbon fixation in plants. It is the most abimdant protein on earth. 

Ozone fumigation decreased the initial slope of the A/c curve which is dependent on the efficiency of ribulose bisphosphate carboxylase. Carboxylation efficiency declined to 20% of the control value after 16 hours at 400 nmol mol O3 (Fig. 2). Conversely, regeneration of the primary CO2 acceptor, i.e. RubP, was not severely affected by O3 because at elevated levels of c the CO2 saturated rate of photosynthesis was inhibited by no more than 30%. The capacity for regeneration of RubP relates to the maximum rate of coupled electron transport in vivo providing it is not restricted by the balance of chloroplast sugar phosphate export and inorganic phosphate import. 

The fundamental basis of photosynthetic carbon metabolism is the incorporation of carbon dioxide by ribulose-bisphosphate carboxylase (rubisco). This leads to the synthesis of three-carbon sugars which are either exported from the chloroplast or metabolized to regenerate the acceptor ribulose bisphosphate. Rubisco is a bifunctional enzyme in that, in parallel to carboxylation, it catalyzes an oxygenation reaction that leads to phospho-glycolate. This is the starting point for photorespiratory metabolism, which will be discussed below (Section 1.6.2). In C4 plants, the conventional C3 pattern of the photosynthetic carbon reduction Calvin cycle is confined to the bundle sheath cells. The surrounding mesophyll cells act as an ancillary carbon dioxide pump, fixing carbon dioxide via phosphoenolpyruvate carboxylase into C4 acids. These are transported to the bundle sheath for decarboxylation.In this way, photorespiration is limited because of the elevated carbon dioxide levels. 

Tapia, O., Andres, J. and Safont, V. S. Enzyme catalysis and transition structures in vacuo. Transition structures for the enolization, carboxylation and oxygenation reactions in ribulose- 1,5-bisphosphate carboxylase/oxygenase enzyme (Rubisco), J.Chem.Soc.Faraday Trans., 90 (1994), 2365-2374... 

FUNCTION RUBISCO CATALYSES TWO REACTIONS THE CARBOXYLATION OF D-RIBULOSE 1,5-BISPHOSPHATE, THE PRIMARY EVENT IN PHOTOSYNTHETIC CARBON DIOXIDE FIXATION, AS WELL AS THE OXIDATIVE FRAGMENTATION OF THE PENTOSE SUBSTRATE IN THE PHOTORESPIRATION PROCESS. BOTH REACTIONS OCCUR SIMULTANEOUSLY AND IN COMPETITION AT THE SAME ACTIVE SITE. 

Lorimer, G. H., The carboxylation and oxygenation of ribulose-1,5-bisphosphate The primary events in photosynthesis and photorespiration. Ann. Rev. Plant Physiol. 32 349, 1981. Mechanisms and biochemical significance of the carboxylase and oxygenase reactions. 

Carboxylation of ribulose-1,5-bisphosphate by ribulose 1,5-bisphosphate carboxylase-oxygenase (Rubisco) with the formation of two molecules of glycerate-3-P. 

The crux of the pathway (Fig. 1) is the carboxylation of ribulose 1,5-bisphos-phate (Rbu-1,5-P2) at the C-2 carbon, giving rise to a short-lived six-carbon intermediate which is cleaved to produce two molecules of 3-phosphoglycerate (3-PGA) (Eqn. 2). This reaction is catalysed by ribulose-1,5-bisphosphate carboxylase oxygenase (rubisco), one of the most abundant proteins on earth. 

As stated earlier, the rate-limiting step in the Calvin cycle is the carboxylation of ribulose 1,5-bisphosphate to form two molecules of 3-phosphogly cerate. The activity of rubisco increases markedly on illumination. The addition of CO2 to... 

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