EXCESS CAPACITY OF H+-ATPASE AND INVERSE RESPIRATORY CONTROL IN ESCHERICHIA-COLI

With succinate as free-energy source, Escherichia coli generating virtually all ATP by oxidative phosphorylation might be expected heavily to tax its ATP generating capacity. To examine this the H+-ATPase (ATP synthase) was modulated over a 30-fold range. Decreasing the amount of H+-ATPase reduced the growth rate much less than proportionally; the H+-ATPase controlled growth rate by <10%. This lack of control reflected excess capacity: the rate of ATP synthesis per H+-ATPase (the turnover number) increased by 60% when the number of enzymes was decreased by 40%. At 15% H+-ATPase, the enzyme became limiting and its turnover was increased even further, due to an increased driving force caused by a reduction in the total flux through the enzymes. At smaller reductions of [H+-ATPase] the total flux was not reduced, revealing a second cause for increased turnover number through increased membrane potential: respiration was increased, showing that in E. coli, respiration and ATP synthesis are, in part, inversely coupled. Indeed, growth yield per O2 decreased, suggesting significant leakage or slip at the high respiration rates and membrane potential found at low H+-ATPase concentrations, and explaining that growth yield may be increased by activating the H+-ATPase.