Photosynthesis carboxylase

An intriguing stress-induced alteration in gene expression occurs in a succulent plant, Mesembryanthemum crystallinum, which switches its primary photosynthetic CO2 fixation pathway from C3 type to CAM (Crassulacean acid metabolism) type upon salt or drought stress (Winter, 1974 Chapter 8). Ostrem et al. (1987) have shown that the pathway switching involves an increase in the level of mRNA encoding phosphoenol-pyruvate carboxylase, a key enzyme in CAM 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. 

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. 

Cretin, C., Keryer, E., Tagu, D., Lepiniec, L., Vidal, J. Gadal, P. (1990). Complete cDNA sequence of sorghum phosphoeno/pyruvate carboxylase involved in C4 photosynthesis. Nucleic Acids Research 18, 658. 

Hudspeth, R.L. Grula, J.W. (1989). Structure and expression of the maize gene encoding the phosphoeno/pyruvate carboxylase isozyme involved in C4 photosynthesis. Plant Molecular Biology 12, 579-89. 

Singh, R. (1989). Carbon dioxide fixation by PEP carboxylase in pod-walls of chickpea. In Photosynthesis, Molecular Biology and Bioenergetics, ed. G.S. Singhal et al., pp. 315-29. New Delhi Narosa Publishing House. 

Yanagisawa, S., Izui, K., Yamaguchi, Y., Shigesada, K. Katsuki, H. (1988). Further analysis of cDNA clones for maize phosphoeno/pyruvate carboxylase involved in C4 photosynthesis. FEBS Letters 229, 107-10. 

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. 

PEP carboxylase has a lower Km for C02 than does Rubisco. Further, 02 is a very poor substrate for this enzyme. This means that, at relatively low concentrations of C02, the delivery of carbon into photosynthesis products is more efficient than in C3 plants and oxygenation doesn t occur. 

PEP carboxylase is concentrated in special mesophyll cells in the outer part of the leaf. This means that the cells most exposed to the atmosphere are the most efficient at converting C02 into organic products. Photosynthesis involving Rubisco is more prominent in the bundle sheath cells located in the inner part of the leaf around the veins that carry compounds between different parts of the plant. 

Unlike C3 or C4 land plants, diffusion of CO2 limits photosynthesis in aquatic plants. Consequently, algae have adapted a mechanism that allows them to actively transport or pump either CO2 or HCO3 across their membrane, thereby accumulating DIC in the cell (Lucas and Berry, 1985). Although carbon fixation is through RuBP carboxylase,... 

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.]... 

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,... 

Although in each type of C4 pathway there is an initial carboxylation catalyzed by PEP carboxylase, the plant s ability to produce a net increase in fixed carbon depends on subsequent release of CO2 and refixation by the RPP cycle. In this sense, the RPP cycle is still the fundamental carboxylating mechanism of these plants. It should be noted that C3 plants also contain a cytosolic PEP carboxylase which is capable of fixing CO2. However, C3 plants lack the biochemical and structural specialization as well as the division of labor between cell types that make possible the classical C4 type of photosynthesis. 

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. 

---