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Structures of Bdellovibrio bacteriovorus phosphoglucose isomerase reveal novel rigidity in the active site of a selected subset of enzymes upon substrate binding

Glycolysis and gluconeogenesis are central pathways of metabolism across all domains of life. A prominent enzyme in these pathways is phosphoglucose isomerase (PGI) which mediates the interconversion of glucose-6-phosphate and fructose-6-phosphate (F6P). The predatory bacterium Bdellovibrio bacteriovorus leads a complex lifecycle, switching between intraperiplasmic replicative and extracellular “hunter” attack-phase stages. Passage through this complex lifecycle involves different metabolic states. Here we present the unliganded and substrate bound structures of the Bdellovibrio bacteriovorus PGI, solved to 1.74Å and 1.67 Å, respectively. These structures reveal that an induced-fit conformational change within the active site is not a pre-requisite for the binding of substrates in some PGIs. Crucially, we suggest a phenylalanine residue, conserved across most PGI enzymes but substituted for a glycine in Bdellovibrio and other select organisms, is central to the induced-fit mode of substrate recognition for PGIs. This enzyme also represents the smallest conventional PGI characterised to date and likely represents the minimal requirements for a functional PGI.