Identification of glucoselysine-6-phosphate deglycase, an enzyme involved in the metabolism of the fructation product glucoselysine.

The metabolism of the glycation product fructose-epsilon-lysine in Escherichia coli involves its ATP-dependent phosphorylation by a specific kinase (FrlD), followed by the conversion of fructoselysine 6-phosphate into glucose 6-phosphate and lysine by fructoselysine-6-phosphate deglycase (FrlB), which is distantly related to the isomerase domain of glucosamine-6-phosphate synthase. As shown in the present work, several bacterial operons comprise: (1) a homologue of fructoselysine-6-phosphate deglycase; (2) a second homologue of the isomerase domain of glucosamine-6-phosphate synthase, more closely related to it; and (3) components of a novel phosphotransferase system, but no FrlD homologue. The FrlB homologue (GfrF) and the closer glucosamine-6-phosphate synthase homologue (GfrE) encoded by an Enterococcus faecium operon were expressed in E. coli and purified. Similar to FrlB, GfrF catalysed the reversible conversion of fructoselysine 6-phosphate into glucose 6-phosphate and lysine. When incubated with fructose 6-phosphate and elevated concentrations of lysine, GfrE catalysed the formation of a compound identified as 2-epsilon-lysino-2-deoxy-6-phospho-glucose (glucoselysine 6-phosphate) by NMR. GfrE also catalysed the reciprocal conversion, i.e. the formation of fructose 6-phosphate (but not glucose 6-phosphate) from glucoselysine 6-phosphate. The equilibrium constant of this reaction (0.8 M) suggests that the enzyme serves to degrade glucoselysine 6-phosphate. In conclusion, GfrF and GfrE serve to metabolize glycation products formed from lysine and glucose (fructoselysine) or fructose (glucoselysine), via their 6-phospho derivatives. The latter are presumably formed by the putative phosphotransferase system encoded by gfrA-gfrD. The designation gfr (glycation and fructation product degradation) is proposed for this operon. This is the first description of an enzyme participating in the metabolism of fructation products.