Metabolic responses to substrate futile cycling in Escherichia coli

A cyclic pathway between phosphoenolpyruvate and oxaloacetate was created in Escherichia coli by simultaneous overexpression of phosphoenolpyruvate carboxykinase (encoded by pck) and phosphoenopyruvate carboxylase (encoded by ppc) from a multicopy plasmid under the control of the tac promoter. The simultaneous overexpression of these two enzymes stimulated oxygen and glucose consumption, reduced growth yields, and resulted in high level excretion of pyruvate and acetate. These responses were abolished when either pck or ppc was deleted from the plasmid or when both enzymes were inactivated by mutation. Therefore, the observed effects imply the existence of futile cycling. Incremental induction of futile cycling showed that stimulation of oxygen consumption was the first response, followed by the increased glucose consumption and the excretion of fermentation products. The specific growth rate of E. coli was insensitive to futile cycling per se, because the growth rate was also reduced by the overexpression of inactive enzymes at high levels, and the activity of the two enzymes did not inhibit growth further. Wild-type cells appear to be capable of compensating for the increased ATP drain due to futile cycling but cannot be as effective when a tricarboxylic acid cycle enzyme, alpha-ketoglutarate dehydrogenase, is defective.