Glucose metabolism by K+-limitedKlebsiella aerogenes: Evidence for the involvement of a quinoprotein glucose dehydrogenase

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Like other members of the Enterobacteriaceae, Klebsiel la aerogenes transports glucose into the cell by means of a PEP-dependent phosphotransferase system (PTS; for review see [1]). This system has a high affinity for glucose and, not surprisingly, was found to be exceedingly active in cells of a slowly growing glucose-limited chemostat culture (apparent specific activity > 250 nmo l /min . (mg dry wt cells)-1 at D = 0 .1 /h [2]). Interestingly, the apparent activity of this system declined sharply as the growth rate was increased such that, at growth rates greater than 0.7/h, its activity was lower than that necessary to account for the actual rate of glucose uptake by the growing cells. More significantly, glucose-sufficient cultures (that were K +-, phosphateor ammonia-limited) consumed glucose at substantially higher rates than the corresponding glucose-limited culture, yet seemingly possessed uniformly low glucose-PTS activities. These discrepancies suggested that, at the higher growth rates a n d / o r with glucose-sufficient conditions, ancillary systems might contribute substantially to the overall rate of glucose flux, though such a supposition rested heavily on the assumption that the apparent glucose-PTS activity measured in vitro approximated closely to the actual activity of the undisturbed cells. However, the further observation that some glucose-sufficient cultures excreted gluconic and 2-ketoghiconic acid in substantial amounts [3] again pointed to the possibility of there being present in K. aerogenes processes of glucose catabolism that might not directly involve the glucose-PTS. Hence we undertook a search for this putative alternative glucosemetabolizing system which resulted in the finding of a quinoprotein glucose dehydrogenase (EC 1.1.99.17). To the best of our knowledge, this enzyme has not previously been reported to be present in K. aerogenes, or in related species.