Chloroplast ribulose bisphosphate carboxylase

Photorespiration results from the oxygenation reaction catalyzed by ribulose bisphosphate carboxylase/ oxygenase. 2-Phosphoglycolate generated by the reaction moves from the chloroplast to the cytosol, where other enzymes break it down to C02, H20, and Pj. The oxygenation reaction, like carboxylation, does not require light directly. It occurs mainly during illumination, however, because the formation of the substrate, ribulose-1,5-bisphosphate, requires ATP and NADPH (see fig. 15.25). 

The term "molecular chaperone" is not novel and was first introduced (7) to describe the transient association of nucleoplasmin with the histones during the formation of nucleosomes. Subsequently, a similar transient association was found in the chloroplast of higher plants between nascent Large subunits (LSU) of die enzyme Ribulose bisphosphate carboxylase (Rubisco) and a SdOkDa protein complex composed of 12-14 60kDa subunits (2). The nascent chloroplast-synthesized LSU was found to bind this complex in a non-covalent manner and in sub-stoichiometric amounts. 

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 procedure for the isolation of chloroplasts from cell wall mutants of Chlamydomonas yields relatively pure, active chloroplasts. These chloroplast were photosynthetically active (3) and synthesized proteins in organello (4). They also imported, processed and assembled in vitro synthesized precursors for the small subunit of ribulose bisphosphate carboxylase and for the light harvesting chlorophyll a/b proteins (5, 6, 7). 

In spinach leaves we found a NADPH/NADP ratio of 1.8 during illumination. We conclude that this high ratio mainly reflects the NADPH/NADP in the stroma, because Dietz and Heber (l) obtained the same NADPH/NADP ratio of 1.8 for chloroplasts from illuminated spinach leaves employing nonaqueous separation of chloroplasts from frozen spinach leaves. One may argue that the ratio of NADPH/NADP contents could differ from the ratio of the corresponding free concentrations. It has been shown that NADPH can be bound to ribulose-bisphosphate carboxylase (2), but this binding was found to be substantial only in the absence of RuBP, whereas in the presence of 4 mM RuBP there was practically no binding of NADPH observed (3). Since in illuminated leaves the RuBP concentration is about 8 mM, it is not to be expected that under these conditions the ratio of free NADPH/NADP will be dramatically different from the ratio of contents. It can be therefore concluded that in illuminated leaves the NADPH/NADP ratio in the stroma is in the order of 1. 

THE pH STAT OF CHLOROPLASTS IN RELATION TO RIBULOSE BISPHOSPHATE CARBOXYLASE ACTIVITY AND INORGANIC NITROGEN AND SULPHATE ASSIMILATION... 

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. 

The pH Stat of Chloroplasts in Relation to Ribulose Bisphosphate Carboxylase Activity and Inorganic Nitrogen and Sulphate Assimilation 23... 

Ribulosa b/ phosphate carboxylase, ribulose bisphosphate carboxylase/oxygeiuise, Rubisco, car-boxydismutase, Fraetion-l protein (EC 4.1.1.39) the enzyme responsible for catalysing photosynthetic CO2 fixation in all photosynthetic organisms except green sulfur bacteria (Chlorobiaceae). In plants it occurs in the chloroplast stroma where it constitutes about 50% of the total protein it should be noted. 

The fundamental basis of photosynthetic carbon metabolism is the incorporation of carbon dioxide by ribulose-bisphosphate carboxylase (rubisco). This leads to the synthesis of three-carbon sugars which are either exported from the chloroplast or metabolized to regenerate the acceptor ribulose bisphosphate. Rubisco is a bifunctional enzyme in that, in parallel to carboxylation, it catalyzes an oxygenation reaction that leads to phospho-glycolate. This is the starting point for photorespiratory metabolism, which will be discussed below (Section 1.6.2). In C4 plants, the conventional C3 pattern of the photosynthetic carbon reduction Calvin cycle is confined to the bundle sheath cells. The surrounding mesophyll cells act as an ancillary carbon dioxide pump, fixing carbon dioxide via phosphoenolpyruvate carboxylase into C4 acids. These are transported to the bundle sheath for decarboxylation.In this way, photorespiration is limited because of the elevated carbon dioxide levels. 

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... 

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). 

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