Photosynthesis carboxylase/oxygenase

Fmax at light saturation and at the optimal temperature for photosynthesis varies with plant species but is usually from 2 to 10 mol m-3 s-1. We can also estimate Vmax from measurements of the maximum rates of CO2 fixation by isolated chloroplasts. These maximum rates—which are sustained for short periods and are for optimal conditions—can be 100 mmol of CO2 fixed (kg chlorophyll)-1 s-1 [360 pmol (mg chlorophyll)-1 hour-1 in another common unit], which is approximately 3 mol m-3 s-1 (1 kg chlorophyll is contained in about 0.035 m3 of chloroplasts in vivo). In vitro, the key enzyme for CO2 fixation, ribulose-l,5-bisphosphate carboxylase/oxygenase, can have rates equivalent to 200 mmol (kg chlorophyll)-1 s-1. The estimates of Vmax using isolated chloroplasts or enzymes usually are somewhat lower than its values determined for a leaf Measurements using leaves generally indicate that KqOz is 5 to 20 mmol m-3. For instance, Kcch can be 9 mmol m-3 at 25°C with a Q10 of 1.8 (Woodrow and Berry, 1988 Q10 is defined in Chapter 3, Section 3.3B). 

Figure 8-13. Schematic illustration of Rubisco (ribulose-l,5-bisphosphate carboxylase/oxygenase) acting as the branch point for photosynthesis and photorespiration. All three of the organelles involved, but only a few of the biochemical steps, are indicated. ( represents phosphate. Note that 3-phosphoglycerate and glycolate refer to the dissociated forms of 3-phosphoglyceric acid and glycolic acid, respectively.)...

Macintyre, H. L., Sharkey, T. D., and Geider, R. J. (1997). Activation and deactivation of ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco) in three marine microalgae. Photosynthesis Res. 51, 93-106. 

Figure 9 A model of the cyanobacterial carbon concentrating mechanism from Synechococcus PCC 7942, a freshwater species. Solid circles represent transporters located on the plasma membrane and interior to the cell wall. Boxes represent the catalyzing enzymes CA, Carbonic Anhydrase RUBISCO, Ribulose 1-5 bisphosphate Carboxylase Oxygenase. The carboxysome is the site of carbon fixation (dark reactions) and the thylakoid is the site of the light reactions of photosynthesis (after Badger et aL, 2002).

In plants and some other eukaryotes, pentoses are components of cell wall polysaccharides such as xylans and arabinogalactans. More interestingly, all green forms of life, i. e., those that perform photosynthesis, contain ribulose 1,5-bisphosphate carboxylase/oxygenase (often abbreviated as Rubisco) as the central enzyme involved in carbon dioxide fixation. Consequently, this enzyme has become of interest in numerous genetic engineering projects aimed at the improvement of photosynthesis in agriculturally important plants [4]. 

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]. 

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... 

Rubisco (ribuiose-1,S-bisphosphate carboxylase/ oxygenase) enzyme that binds carbon dioxide (C02) in photosynthesis, incorporating it into carbohydrate molecules. It can also bind oxygen, leading to photorespiration, saturated fat fat composed of fatty acids without any double bonds between carbon atoms, selection pressure likelihood that a disadvantageous trait will be eliminated from a population by natural selection. Traits that jeopardize reproduction are not passed on, and so disappear from the population mildly disadvantageous traits may be offset by hidden benefits. 

For the moment, we will focus on photoautotrophs that utilize rubisco. Rubisco is the official name of an enzyme for which the systematic name—ribulose-l,5-bisphosphate carboxylase oxygenase—is inconveniently long. As indicated by the first activity specified in the systematic name, this enzyme catalyzes the carboxylation of ribulose-1,5-bisphosphate, RuBP, a five-carbon molecule. A six-carbon product is formed as a transient intermediate, but the first stable products are two molecules of PGA, 3-phosphoglyceric acid, C3H7O7P. The carbon number of this compound gives the process its shorthand name, C3 photosynthesis. At physiological pH, the acidic functional groups on the reactants and products are ionized as shown below. 

In the dark reactions of photosynthesis, the fixation of carbon dioxide takes place when the key intermediate ribulose-1,5-fo iphosphate reacts with carbon dioxide to produce two molecules of 3-phosphoglycerate. This reaction is catalyzed by the enzyme ribulose-l,5-feq3hosphate carboxylase/oxygenase (rubisco), one of the most abundant proteins in nature. The remainder of the dark reaction is the regeneration of ribulose-1,5-fo iphosphate in the Galvin cycle. 

The ambient concentration of CO2 in seawater ( 10 pM) is low compared to the half saturation constant of RubisCO (ribulose 1,5-bisphosphate carboxylase/oxygenase) the enzyme that fixes the inorganic carbon into PG3 (3-phos-phoglycerate) in the first step of the Calvin cycle (the dark reaction of photosynthesis). Microalgae have thus evolved various carbon concentrating mechanisms (CCM) to augment the CO2 concentration at the site of fixation [55]. These mechanisms all involve interconversion between CO2 and HCOJ at some point. But at neutral pH the hydration/dehydration reaction of CO2/ HCOj" has a half life of 30 s, much too slow for a cellular process (for example, diffusion from one end of the cell to the other takes on the order of 10 ms), and requires catalysis. The enzyme CA is an extraordinary effective catalyst some CA catalyze the CO2/HCO3 reaction at a rate that nearly reaches the limit imposed by the diffusion of molecules [56]. 

Ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) is the key enzyme both of photosynthesis and photorespiration in plants. The dual function of Rubisco makes it an important target enzyme for genetic engineering in attempts to improve the productivity of crop plants. 

Rubisco (ribulose bisphosphate carboxylase-oxygenase) The enzyme that accomplishes carbon fixation in photosynthesis by adding CO2 to ribulose-1,5-bisphosphate. It can also add O2 in place of CO2, initiating photorespiration. 

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. 

He suggests that the relative activities of the carboxylase and oxygenase activities of rubisco actually have set, and now maintain, the ratio of C02 to 02 in the earth s atmosphere. Discuss the pros and cons of this hypothesis, in molecular terms and in global terms. How does the existence of C4 organisms bear on the hypothesis [Tolbert, N.E. (1994) The role of photosynthesis and photorespiration in regulating atmospheric C02 and 02. In Regulation of Atmospheric C02 and 02 by Photosynthetic Carbon Metabolism (Tolbert, N.E. Preiss, J., eds), pp. 8-33, Oxford University Press, New York.]... 

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