Relationship Between Enzyme Properties and Network Fluxes
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).
