A novel, cupin-type phosphoglucose isomerase in Escherichia coli

Escherichia coli cells contain a homolog of presumed 5-keto-4-deoxyuronate isomerase (KduI) from pectin-degrading soil bacteria, but the catalytic activity of the E. coli protein (o-KduI) was never demonstrated. The known three-dimensional structure of E. coli o-KduI was compared with the available structures of sugar-converting enzymes. Based on the results of this analysis, sugar isomerization activity of recombinant o-KduI was tested against a panel of D-sugars and their derivatives. The three-dimensional structure of o-KduI exhibits a close similarity with Pyrococcus furiosus cupin-type phosphoglucose isomerase. In accordance with this similarity, o-KduI was found to catalyze interconversion of glucose-6-phosphate and fructose-6-phosphate and, less efficiently, conversion of glucuronate to fructuronate. o-KduI was hexameric in crystals but represented a mixture of inactive hexamers and active dimers in solution and contained a tightly bound Zn2+ ion. Dilution, substrate binding and Zn2+ removal shifted the hexamer ⇆ dimer equilibrium to the dimers. Our findings identify o-KduI as a novel phosphosugar isomerase in E. coli, whose activity may be regulated by changes in oligomeric structure. More than 5700 protein sequences are annotated as KduI, but their enzymatic activity has not been directly demonstrated. E. coli o-KduI is the first characterized member of this group, and its enzymatic activity was found to be different from the predicted activity. •Escherichia coli contains a gene of presumed 5-keto-4-deoxyuronate isomerase (KduI).•KduI is structurally similar to bacterial cupin-type phosphoglucose isomerase.•KduI catalyzes interconversion of glucose-6-phosphate and fructose-6-phosphate.•KduI exists as a mixture of inactive hexamers and active dimers in solution.•KduI as a novel, alternative phosphosugar isomerase in Escherichia coli.