We have been trying to improve the enzyme based on the molecular and biochemical mechanisms of the evolution and adaptation of the enzyme to the present atmosphere after the appearance of the enzyme in nature. Information on the structure-function relationship of RuBisCO evolution will be highly likely to give us various approaches useful for the improvement of the enzyme. Particularly, removing the oxygenase reaction will render plants resistant to drought, and the plant with such a RuBisCO will show significant net CO2 fixation in photosynthesis under drought conditions.3
The biphasic reaction course, fallover, of carboxylation catalyzed by RuBisCO has been known as a characteristic of the enzyme from higher land plants.5 Fallover consists of hysteresis in the reaction, seen during the initial several minutes, and a subsequent, very slow suicide inhibition by inhibitors formed from the substrate ribulose-1,5-bisphosphate (RuBP).6 We have examined the relationship between occurrence of fallover, the putative hysteresis-inducible sites (Lys-21 and Lys-305 of the large subunit in spinach RuBisCO), and the relative specificity in the carboxylase and oxygenase reactions among RuBisCOs from a wide variety of photosynthetic organisms. Figure 16.2 shows the evolutionary relationships among them.6
The phylogenetic tree for the evolution of the gene for the large subunits of RuBisCO, rbcL, has been well accepted.6 Occurrence of fallover and the hysteresis-inducible sites followed well the sequence of adaptation of photosynthetic organisms to a terrestrial habitat, or the increase in the relative specificity of RuBisCO.
From this line of study, we expected that introduction of the hysteresis-inducible sites into the photosynthetic bacterial (y) enzyme would give rise to an increase in the relative specificity of the bacterial enzyme. However, this was not the case. The mutant Chroma-tium vinosum RuBisCO, having lysine residues at 21 and 305, showed fallover, but its relative specificity was very similar to that of the wild enzyme.
Another interesting point in Figure 16.2 is the occurrence of hysteresis-inducible sites in the RuBisCOs of p-purple bacteria and non-green algae. The relative specificity between carboxylase and oxygenase reactions (Sr) of the non-green algal enzyme was much higher than that of higher C3 plants. Interestingly, red algae are divided into two groups in the phylogenetic tree of rbcL. The group including Porphiridium and Porphyra live at moderate temperatures in the presence of salts. The other group contains Cyanidium and Galdieria, which grow at higher temperatures. The relative specificity of RuBisCOs from the latter group were the extremes of RuBisCOs examined so far.7 The higher specificity for CO2 fixation in these RuBisCOs was due partly to their higher affinities for CO2 (6.6 ^M) and partly to a higher activation energy in the oxygenation reaction (28.6 kcal mol-1).7
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