The metabolism of gluconate in Escherichia coli: A study in continuous culture

Abstract

The gluconate metabolism in Escherichia coli involves duplicate activities of transport and phosphorylation for gluconate. In both cases, these activities can be differentiated in vifro by their different affinities for the substrate. In addition, the two gluconokinases can be differentiated by their heat sensitivities. The technique of continuous culture was used to investigate the influence of the growth rate on this metabolism in an E. coli HfrG6 strain during gluconate-limited growth under conditions of high and low oxygen concentrations. The transport and phosphorylation for gluconate, induced when the cells are cultivated in media with gluconate were differently influenced by the culture dilution rate. These activities were induced under the two conditions investigated; however, the low affinity transport system for gluconate and the thermosensitive gluconokinase were not detected under conditions of high and low oxygen concentrations, respectively. The induction of the dehydratase was favoured under conditions of low oxygen concentration. The experimental data suggest that induction and repression work together to regulate the levels of these activities during gluconate-limited growth conditions. Furthermore, that an effector molecule distinct from gluconate might be involved in the induction of the dehydratase.The initial metabolism of gluconate in E. coli has proved to be complex. Two sets of enzymes catalysing the gluconate transport and phosphorylation have been described. These two sets of enzymes are specified by two distinctly regulated groups of genes, which are locatated in distinct regions of the bacterial chromosome. The region bioH-asd (75 min) contains the gntT, gntU and gntK genes. These code for high and low affinity transport systems for gluconate and a thermoresistant gluconokinase (its activity is stable for 3 h at 30 "C), respectively. Together these activitities constitute the GntI system. The fdp-vafS region (96 min) , includes the gntS and gntV genes. These code for another high affinity transport system for gluconate and a thermosensitive gluconokinase (it loses 75% or more of its activity if incubated 3 h at 30 "C), respectively, These activities constitute the GntII system. E. coli gntT mutants are severely affected in the utilization of gluconate; moreover, some E. coli bioH-asd deleted mutants are unable to grown on this substrate (NAGEL DE ZWAIG et al. 1973, FAIK and KORNBERG 1973ISTURIZ et al. 1979. These and other observations suggested the GntI and GntII systems involved as main and subsidiary respectively, in the capture and phosphorylation ofgluconate (BACHI and KORNBERG 1975 a).The GntI and GntII activities convert external gluconate to intracellular 6-phosphogluconate, which can either be decarboxylated to pentose-phosphate by the action of the enzyme 6-phosphogluconate dehydrogenase (EC 1.1.1.44., gene gnd) or metabolized to pyruvate and glyceraldehyde 3-phosphate via the ENTNER-DOUDOROFF pathway (EISEN-BERG and DOBROGOSZ 1967). The first enzyme involved in the latter pa...

The subsidiary GntII system for gluconate metabolism in Escherichia coli: Alternative induction of the gntV gene

“…Moreover, in gluconate-limited mineral medium continuous culture, the total gluconate kinase activity, consisting of GntV at very low dilution rates (D), is gradually repressed as the induction of GntK increases as a consequence of the progressive increase of D and corresponding increment in the concentration of limiting substrate. These findings indicated that gntV induction, contrary to that of gntK, occurs mainly at low gluconate concentrations (Coello and Istúriz, 1992).…”

Involvement ofgntS in the control of GntI, the main system for gluconate metabolism inEscherichia coli

“…Since GntV is induced together with GntI activities when cells grow in presence of gluconate (ISTÚRIZ et al 1986, COELLO andISTÚRIZ 1992), or in presence of an idonate supplemented medium (BAUSCH et al 1998), there should be some coordination in the expression of these activities. Since GntV is induced together with GntI activities when cells grow in presence of gluconate (ISTÚRIZ et al 1986, COELLO andISTÚRIZ 1992), or in presence of an idonate supplemented medium (BAUSCH et al 1998), there should be some coordination in the expression of these activities.…”

The gntP gene of Escherichia coli involved in gluconate uptake

“…The GntII system, a subsidiary system for gluconate transport and phosphorylation, located in the 96-min region, includes the gntS gene, encoding another high-affinity transport system, and the gntV gene, encoding a thermosensitive glucokinase (15). Various observations have suggested that the genes of the GntI and the GntII systems are differentially regulated (1,5,19). The GntI system is specifically induced by gluconate and is regulated by the gntR gene (also located in the 75-min region) product, a repressor of the genes of the GntI system and the Entner-Doudoroff pathway but not of the genes of the GntII system (9, 19, 32).…”