Photosynthesis rubisco ribulose bisphosphate

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. 

This set of reactions is very detrimental to the efficiency of photosynthesis. Oxygen is added to carbon, CO2 is lost, energy is consumed, and ribulose bisphosphate is destroyed. For a plant to be able to increase the discrimination of Rubisco for CO2 would obviously be advantageous, but that hasn t happened, either naturally or through the efforts of scientists. An increased concentration of CO2 in the atmosphere may lead to increased photosynthesis and decreased photorespiration, but high CO2 concentrations would also contribute to global warming (and the increased photosynthetic carbon fixation would not likely reduce the amount of CO2 in any event). 

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.)...

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]. 

Starch and sucrose, key substrates for the development of dental caries, are exclusively synthesized by plants. They are made in plant leaves by a process called photosynthesis, which utilizes sunlight as the energy source. This chapter outlines the light and dark reactions of photosynthesis and compares the light reaction with mitochondrial electron transport (Sect. 1). The key dark reaction, the production of phosphoglycerate by the enzyme ribulose bisphosphate carboxylase (rubisco), is described along with the production of fructose, sucrose, and starch (Sect. 2). The chapter concludes with a detailed discussion of the roles of starch and sucrose in plant metabolism (Sect. 3). 

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. 

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]. 

The control of pH within the stroma of chloroplasts is clearly a significant factor in photosynthesis. The [CO2] available to ribulose bisphosphate carboxylase (Rubisco) kinetically determines its carbon dioxide fixing activity. In C3 plants, [CO2] is controlled in the equilibrium reaction catalysed by stromal carbonic anhydrase ... 

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. 

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. 

Among the proteins found in the chloroplast stroma are the enzymes of the Calvin cycle, which functions in fixing carbon dioxide into carbohydrates during photosynthesis (Chapter 8). The large (L) subunit of ribulose 1,5-bisphosphate carboxylase (rubisco) is encoded by chloroplast DNA and synthesized on chloroplast ribosomes in the stromal space. The small (S) subunit of rubisco and all the other Calvin cycle enzymes are encoded by nuclear genes and transported to chloroplasts after their synthesis in the cytosol. The precursor forms of these stromal proteins contain an N-terminal stromal-import sequence (see Table 16-1). 

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. 

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. 

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