The capacity of the metabolic network to compensate for alterations in the amount of an enzyme depends on the impact of the associated changes in metabolite concentrations on all the steps in the network. Enzymes that are sensitive to modulation by effectors, particularly metabolites from within the pathway, can compensate for decreased expression because small changes in the concentrations of substrates, products, inhibitors, and activators are likely to be sufficient to stimulate the activity of the residual enzyme. However, for enzymes that lack such regulatory properties, compensation can occur only through alterations in the concentrations of the immediate substrates and products of the enzyme. The extent to which this can occur is constrained in vivo by the effect that such changes can have on the operation of the other enzymes in the network. Thus, flux can be reduced because the changes in metabolite concentration that would be required to prevent the decrease have adverse effects on other sections of the pathway, rather than because the manipulated enzyme has insufficient catalytic capacity to support the flux. This explains why a moderate decrease in either plastidic aldolase (Haake et al., 1998, 1999) or transketolase (Henkes et al., 2001) inhibited the rate of CO2 fixation even though the maximum catalytic capacity of the residual enzyme was seemingly still in excess of that required to accommodate the normal rate of photosynthesis. The mechanisms that restrict flux through the pathway in these examples are considered in more detail below.
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