The inefficient aerobic energetics of Zymomonas mobilis: Identifying the bottleneck

To investigate the mechanisms of Zymomonas mobilis uncoupled aerobic metabolism, growth properties of the wild-type strain Zm6 were compared to those of its respiratory mutants cytB and cydB, and the effects of the ATPase inhibitor DCCD on growth and intracellular ATP concentration were studied. The effects of the ATPase inhibitor DCCD on growth and intracellular ATP concentration strongly indicated that the apparent lack of oxidative phosphorylation in aerobically growing Z. mobilis culture might be caused by the ATP hydrolyzing activity of the H⁺-dependent ATPase in all analyzed strains. Aerobic growth yields of the mutants, and their capacity of oxidative ATP synthesis with ethanol were closely similar, not supporting presence of one major, yet energetically inefficient electron transport branch causing the observed poor aerobic growth and lack of oxidative phosphorylation in Z. mobilis. We suggest that rapidly operating Entner-Doudoroff pathway generates too high phosphorylation potential for the weakly coupled respiratory system to shift the H⁺-dependent ATPase toward ATP synthesis.