At the most fundamental level, the kinetic properties of an enzyme and the displacement of its reaction from thermodynamic equilibrium in vivo do not provide a reliable indicator of the effect on pathway flux of a reduction in the amount of the enzyme. Thus, although Rubisco, plastidic fructose-1,6-bisphosphatase, and phosphoribulokinase have traditionally been considered to be important in the control of photosynthesis on the basis that they catalyze irreversible reactions and are subject to regulation by effectors and reversible posttranslational modification (Macdonald and Buchanan, 1997), a moderate decrease in the amount of any of these enzymes usually has little effect on the rate of CO2 fixation under normal growth conditions (Stitt and Sonnewald, 1995). This tendency for metabolic pathways to compensate for a decrease in the amount of an enzyme arises from the inevitable complementary changes that occur in the concentrations of metabolites throughout the reaction network. These changes may be sufficient to compensate for decreased expression of an enzyme by increasing the proportion of its catalytic capacity that is realized in vivo, as observed in tobacco lines with an 85–95% decrease in expression of phosphoribulokinase (Paul et al., 1995), or by altering the activation state of the targeted enzyme, thus increasing the catalytic capacity of the residual protein, as observed for Rubisco (Stitt and Schulze, 1994).
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