Bacterial respiratory chain has two types of NADH-quinone reductase (NQR): one is energy-coupled (type-1) and the other has no energy-transducing capacity, that is, energy-uncoupled (type-2). Each of the NADH-reacting flavoprotein subunits of NQR-1 from Escherichia coli and the marine Vibrio alginolyticus reduced quinone to semiquinone radicals by the one-electron transfer pathway and was very sensitive to preincubation with NADH. On the other hand, the NQR-2 from these bacteria reduced quinone to quinol by the two-electron transfer pathway and was insensitive to preincubation with NADH. Since the NQR-1 from E. coli functions as a proton pump, whereas that from the marine V. alginolyticus functions as a sodium pump, the formation of semiquinone radicals as an intermediate is likely to be a common mechanism to functioning as either proton or sodium pump.
