Compensatory regulation in metabolic pathways--responses to increases and decreases in citrate synthase levels.

The level of citrate synthase was varied in Escherichia coli by recombinant DNA methods to elucidate regulatory interactions between the individual steps of the citric acid cycle. The effects of overproduction and underproduction of citrate synthase were assessed by measuring metabolite levels, rates of carbon flow, the phosphorylation state of isocitrate dehydrogenase, and the growth rate of the culture. This analysis revealed that the levels of citrate synthase and isocitrate dehydrogenase activity are co-ordinated for efficient growth on acetate. When citrate synthase was overproduced the isocitrate dehydrogenase reaction became rate limiting and prevented large increases in the flux through the citric acid cycle. Furthermore, changes in the level of citrate synthase were found to modulate the phosphorylation state of isocitrate dehydrogenase which regulates the distribution of carbon flow between the citric acid cycle and the glycoxylate shunt. These adjustments allowed the organism to maintain a relatively constant metabolic state despite changes in the level of a central metabolic enzyme. The interplay between citrate synthase and isocitrate dehydrogenase illustrates how living systems can compensate for variations in their internal environment.