Calvin cycle, Rubisco

The metabolic capabilities of the chloroplasts of the mesophyll and bimdle sheath cells of C4 plants are quite different. The PSII-defident bundle sheath chloroplasts of C4 plants of group (i) were mentioned earlier. That apart, the chloroplasts of both cell types generate NADPH and ATP by non-cyclic photophosphorylation. However, only the bundle sheath chloroplasts of all three groups of C4 plants have a functional Calvin cycle. Rubisco and several other Calvin cycle enzymes are absent from the mesophyll chloroplasts. 

Eormadon of PEP by pyruvate Pi dikinase reini-dates the cycle. The CO9 liberated in the bundle slieadi cell is used to syndiesize hexose by die convendonal rubisco-Calvin cycle series of reacdons. 

J. Schwender, F. Goffman, J. B. Ohlrogge, and Y. Shachar Hill, Rubisco without the Calvin cycle improves the carbon efficiency of developing green seeds. Nature 432, 779 782 (2004). 

All the reactions of the Calvin cycle except those catalyzed by rubisco, sedoheptulose 1,7-bisphospha-tase, and ribulose 5-phosphate kinase also take place in animal tissues. Lacking these three enzymes, animals cannot carry out net conversion of C02 to glucose. 

Photosynthesis in vascular plants takes place in chloroplasts. In the C02-assimilating reactions (the Calvin cycle), ATP and NADPH are used to reduce C02 to triose phosphates. These reactions occur in three stages the fixation reaction itself, catalyzed by rubisco reduction of the resulting 3-phosphoglycerate to glyceraldehyde 3-phosphate and regeneration of ribulose 1,5-bisphosphate from triose phosphates. 

FIGURE 20-20 Oxygenase activity of rubisco. Rubisco can incorporate 02 rather than C02 into ribulose 1,5-bisphosphate. The unstable intermediate thus formed splits into 2-phosphoglycolate (recycled as described in Fig. 20-21) and 3-phosphoglycerate, which can reenter the Calvin cycle. 

In C4 plants, the carbon-assimilation pathway minimizes photorespiration C02 is first fixed in mesophyll cells into a four-carbon compound, which passes into bundle-sheath cells and releases C02 in high concentrations. The released C02 is fixed by rubisco, and the remaining reactions of the Calvin cycle occur as in C3 plants. 

Calvin cycle 752 plastids 752 chloroplast 752 amyloplast 752 carbon-fixation reaction 753 ribulose 1,5-bisphosphate 753 3-phosphoglycerate 753 pentose phosphate pathway 753 reductive pentose phosphate pathway 753 C3 plants 754 ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) 754 rubisco activase 757... 

The dark reactions (carbon-fixation reactions) use the ATP and NADPH produced by the light reactions to fix carbon dioxide as carbohydrate sucrose and starch. The reactions form a cycle (the Calvin cycle) in which the enzyme ribulose bisphosphate carboxylase (rubisco), located in the stroma, condenses a C02 molecule with ribulose 1,5-bisphosphate to produce two molecules of 3-phosphoglycerate. Other reactions then regenerate the ribulose... 

The rubisco reaction forms part of a cycle of reactions, called the Calvin cycle, that leads to the regeneration of ribulose 1,5-bisphosphate (ready to fix another C02) and the net production of glyceraldehyde 3-phosphate for the synthesis... 

Answer In maize, C02 is fixed by the C4 pathway elucidated by Hatch and Slack. Phospho-enolpyruvate is rapidly carboxylated to oxaloacetate, some of which undergoes transamination to aspartate but most of which is reduced to malate in the mesophyll cells. Only after subsequent decarboxylation of labeled malate does 14C02 enter the Calvin cycle for conversion to glucose. The rate of entry into the cycle is limited by the rate of the rubisco-catalyzed reaction. 

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... 

The Calvin cycle or reductive pentose phosphate cycle occurs in all green plants and many microorganisms. Carboxylation is catalyzed by ribulose-bis-phosphate carboxylase-oxygenase (Rubisco). Rubisco also functions as an oxygenase during photorespiration, but its affinity for O2 is 20-80 times lower than for CO2. As with most enzymes, Rubisco has a preference for lighter stable isotopes and CO2 fixation results in the depletion of C (5 C) ranging from —10%o to —2Q%c. 

Fig. 13.13 The dark reactions involve three distinct parts carbon fixation (Rubisco), gluconeogenesis, and RuBP regeneration (Calvin cycle).

CO2 are fixed into sugar (glucose) and mediated by the enzyme rubisco (ribulose-l-5-biphosphate carboxylase). It occurs in the stroma of chloroplasts. The Calvin cycle is also known as the dark reaction, as opposed to the first-stage light reactions. 

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... 

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