This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Degradation/Utilization/Assimilation → Carbohydrates Degradation → Sugars Degradation|
Some taxa known to possess this pathway include : Escherichia coli K-12 substr. MG1655
It is well known that Escherichia coli can use glucose as a sole source of carbon and energy. The D isomer of glucose is widely found in nature and the β-D-glucopyranose anomer is predominant in aqueous solution [Franks87]. Exogenous β-D-glucopyranose enters the cell through outer membrane porins and is then actively transported into the cell via the inner membrane phosphotransferase system (PTS) which transforms it into β-D-glucose 6-phosphate as it crosses the cell membrane. β-D-glucose 6-phosphate is also produced biosynthetically during gluconeogenesis. β-D-glucose 6-phosphate is one of the basic precursor metabolites for biosynthetic pathways. It is also a substrate for the central degradative pathways glycolysis and the pentose phosphate cycle.
Escherichia coli can also grow on exogenously supplied glucose-1-phosphate (minimal medium containing glucose 1-phosphate) as sole carbon source [Pradel91]. Endogenous α-D-glucopyranose 1-phosphate is an intermediate in the metabolism of glycogen and D-galactose. It is a building block for the sugar nucleotide UDP-α-D-glucose, which is used in some biosynthetic pathways. Reviewed by Mayer, C. and W. Boos in [ECOSAL] (see below).
About This Pathway
The dashed line connecting D-glucose with β-D-glucopyranose is meant to show that the pathway is possible, but incompletely defined. The anomeric form (α or β) of the D-glucose product of EC 18.104.22.168 is not specified by the EC and it was not found in the literature for the indicated phosphatases. However, if α-D-glucopyranose is produced, it may either spontaneously convert to β-D-glucopyranose, or Escherichia coli aldose-1-epimerase (mutarotase, EC 22.214.171.124) could convert it to β-D-glucopyranose [Bouffard94] and in [Mulhern73].
Substrates β-D-glucopyranose and α-D-glucopyranose 1-phosphate may be derived from exogenous sources, or endogenously produced, as indicated by the input pathway links. In general, the ability to utilize sugars and their modes of utilization are strain-dependent in Escherichia coli.
Exogenous β-D-glucopyranose uptake via the PTS curbs the utilization of other exogenous sugars, which is known as the glucose effect. This effect is lost if β-D-glucopyranose becomes limiting. Under these conditions β-D-glucopyranose can also enter the cell without phosphorylation, via outer membrane porins and the Mgl ABC transporter (not shown).
Endogenous β-D-glucopyranose can be produced by pathways for the degradation of glucose-containing disaccharides such as maltose (see pathway glycogen degradation I) α,α-trehalose, α-lactose and melibiose, as shown in the pathway links. In contrast to exogenous β-D-glucopyranose which is phosphorylated by the PTS, endogenous β-D-glucopyranose is phosphorylated by glucokinase before entering central metabolism, as shown in the pathway links (in [Meyer97]). More recently, a role for glucokinase and glucose in a complex regulatory mechanism for maltose utilization involving Glk, MalT, Mlc and PtsG has been proposed [Lengsfeld09].
It is possible that high levels of β-D-glucopyranose could accumulate inside the cell under certain conditions. It has been shown that the maltose acetyltransferase product of gene maa efficiently acetylates both maltose and β-D-glucopyranose (not shown). Evidence suggests that acetylation could be a detoxification mechanism in which acetylated β-D-glucopyranose diffuses from the cell [Boos81, Brand91].
There is evidence that β-D-glucopyranose can be oxidized to D-glucono-1,5-lactone (glucono-δ-lactone) by inner membrane glucose dehydrogenase. However, the fate of the D-glucono-1,5-lactone remains unclear. It has been reported that membrane vesicles from glucose-grown Escherichia coli oxidized glucose to gluconate in the presence of pyrroloquinoline quinone, a cofactor for glucose dehydrogenase [vanSchie85]. A gluconolactonase (EC 126.96.36.199) has been partially characterized in Escherichia coli, but its D-gluconate product was not specifically identified [Hucho72] and no gene encoding this enzyme has been identified. D-gluconate can be degraded by a glucose utilization pathway that was described early [Cohen51], as shown in the pathway link. In addition, more recent work suggested possible excretion of D-gluconate although this compound was not specifically identified [Sashidhar10].
α-D-glucopyranose 1-phosphate is reversibly converted by phosphoglucomutase to α-D-glucose 6-phosphate. α-D-glucopyranose 1-phosphate is used in glycogen biosynthesis (see glycogen biosynthesis I (from ADP-D-Glucose)) and is produced during glycogen degradation (see glycogen degradation I). α-D-glucose 6-phosphate may spontaneously convert to β-D-glucose 6-phosphate in the physiological pH range [Salas65]. In addition, a glucose-6-phosphate 1-epimerase had been identified in Escherichia coli ATCC 9637 that could catalyze this production of β-D-glucose 6-phosphate.
Several phosphatases may catalyze the production of D-glucose (anomeric form unspecified) from α-D-glucopyranose 1-phosphate. The product of gene agp is a periplasmic enzyme that scavenges glucose and allows Escherichia coli to grow with glucose-1-phosphate as sole carbon source [Pradel91] and in [Lee03d].
Reviewed in Mayer, C. and W. Boos (2005) "Hexose/Pentose and Hexitol/Pentitol Metabolism." EcoSal module 3.4.1 [ECOSAL].
Unification Links: EcoCyc:GLUCOSE1PMETAB-PWY
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