Ribulose-bisphosphate carboxylase photosynthesis

Phosphate and bicarbonate ions are important substrates for many enzymatic processes and as such have regulatory functions. Bicarbonate controls the key enzyme of photosynthesis, ribulose bisphosphate carboxylase, by carbamate formation (Fig. 13-12). Chloride ions activate amylases and may affect the action of "G proteins" that mediate hormone actions. Other observed effects of ions are too numerous to mention. 

While this efficiency is impressive, it also is rarely achieved. The difficulty is in the protein that carries out the first step of photosynthesis. Molecular oxygen, 02, competes with C02 for the active site of ribulose bisphosphate carboxylase, leading to an oxidation and loss of the ribulose bisphosphate acceptor. This competition is apparently intrinsic to the enzyme, because attempts to increase the discrimination for C02 by genetic engineering have resulted in a less-active enzyme, which fixes C02 very poorly. 

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 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 ... 

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. 

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. 

C3 plants Plants that assimilate CO2 (during photosynthesis) fixed directly by the enzyme ribulose bisphosphate carboxylase. They are known as C3 plants because the first stable compound is the three-carbon 3 phosphoglyceric acid. Photosynthesis in C3 plants is inhibited by oxygen. 

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.)...

Most plants reduce CO2 to carbohydrate according to the well-known Calvin-Benson or C3 pathway, where the initial product of photosynthesis is the 3C compound phosphoglycerate. Fixation of CO 2 to phosphoglycerate occurs with the assistance of the enzyme ribulose bisphosphate (RuBP) carboxylase, which discriminates heavily against C02 (11). Consequently, plants with C3 photosynthesis have 6 values that average -27.0 (12). Plants with the Hatch-Slack or Ci,... 

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]. 

Light Control of Photosynthesis Control of Ribulose-1.5-Bisphosphate Carboxylase... 

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