Why is RuBisCO the Target

Balancing the rate of the conversion of photon energy into chemical energies such as NADPH and ATP, and those of the inflow of CO2 from the atmosphere and of the reduction of the fixed CO2, is the critical point for land plants. If the rate of the energy utilization is lowered by the decreased inflow of CO2 through the stomata, chloroplants are obliged to direct energies to photorespiration and reduction of oxygen molecules to superoxide radicals (O2-).' O2- is dismutated to hydrogen peroxide by superoxide dismutase. Hydrogen peroxide is a potent oxidant of some PCR enzymes, which lose their activity by oxidation of the functional vicinal sulfhydryl groups.

Plants have a machinery to decompose hydrogen peroxide in chloroplasts.1 It has been found recently, however, that the machinery itself is very labile under drought-stress conditions.2 To over come this liability of plants, we introduced a bacterial catalase into tobacco chloroplasts, where hydrogen

Fig. 16.1. Overall reactions of photosynthesis. PQ, plastoquinon; Cyt, cytochrome; Fd, ferredoxin; FNR, ferredoxin:NADP+ reductase; MDAR, monodehydroascorbate reductase; SOD, superoxide dismutase; APX, ascorbate peroxidase; RuBP, ribulose 1,5-bisphosphate; PGA, 3-phosphoglycer-ate; TP, triose phosphate; CF, coupling factor.

Fig. 16.1. Overall reactions of photosynthesis. PQ, plastoquinon; Cyt, cytochrome; Fd, ferredoxin; FNR, ferredoxin:NADP+ reductase; MDAR, monodehydroascorbate reductase; SOD, superoxide dismutase; APX, ascorbate peroxidase; RuBP, ribulose 1,5-bisphosphate; PGA, 3-phosphoglycer-ate; TP, triose phosphate; CF, coupling factor.

peroxide is formed in abundance. The introduced catalase decomposes the active oxygen to greatly protect chloroplasts from oxidative damage.

The above study clearly shows that it is possible to improve the endogenous active oxygen-scavenging system by introducing bacterial catalase into plant chloroplasts. However, one should not ignore the fact that the transformants can be alive for longer periods without any growth. This kind of approach to creating aridity-philic plants would not meet the desired goal by changing present plants into ones that can sequestrate atmospheric CO2 by growing on unused, deforested and arid lands. The plants we should seek will be ones that are still productive in photosynthesis under these growth conditions. A plausible target for this purpose is RuBisCO.3

The CO2 fixation step catalyzed by RuBisCO in photosynthesis is the important rate limiting step. The control coefficient of the enzyme in photosynthesis is over 0.5 in the presence of full sunlight. This fact tells us that improving the enzymatic efficiency is a meaningful direction to take for improvements in plant water use efficiency and crop productivity.

RuBisCO, even that of higher land plants, has several disadvantages as an enzyme.3 The reaction turnover rate is up to 3/sec/reaction site; 1/100 to 1/1000 that of most enzymes found in nature. The affinity of the enzyme for CO2 is 10 to 15 ^M; just a quarter of the enzyme in chloroplasts can participate in photosynthesis. Much worse is the occurrence of the unavoidable oxygenase reaction. Plant RuBisCO, well adapted to the present oxygenic atmosphere, still fixes O2 once for every 2 to 3 CO2 fixations in chloroplasts. A part of the reaction product is oxidized to CO2 in the subsequent glycolate pathway. In total, the oxygenase reaction reduces the productivity of plants up to 60%.

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