Klebsiella pneumoniae is presently unique among bacterial species in its ability to metabolize not only sucrose but also its five linkage-isomeric alpha-d-glucosyl-d-fructoses: trehalulose, turanose, maltulose, leucrose, and palatinose. Growth on the isomeric compounds induced a protein of molecular mass approximately 50 kDa that was not present in sucrose-grown cells and which we have identified as an NAD(+) and metal ion-dependent 6-phospho-alpha-glucosidase (AglB). The aglB gene has been cloned and sequenced, and AglB (M(r) = 49,256) has been purified from a high expression system using the chromogenic p-nitrophenyl alpha-glucopyranoside 6-phosphate as substrate. Phospho-alpha-glucosidase catalyzed the hydrolysis of a wide variety of 6-phospho-alpha-glucosides including maltose-6'-phosphate, maltitol-6-phosphate, isomaltose-6'-phosphate, and all five 6'-phosphorylated isomers of sucrose (K(m) approximately 1-5 mm) yet did not hydrolyze sucrose-6-phosphate. By contrast, purified sucrose-6-phosphate hydrolase (M(r) approximately 53,000) hydrolyzed only sucrose-6-phosphate (K(m) approximately 80 microm). Differences in molecular shape and lipophilicity potential between sucrose and its isomers may be important determinants for substrate discrimination by the two phosphoglucosyl hydrolases. Phospho-alpha-glucosidase and sucrose-6-phosphate hydrolase exhibit no significant homology, and by sequence-based alignment, the two enzymes are assigned to Families 4 and 32, respectively, of the glycosyl hydrolase superfamily. The phospho-alpha-glucosidase gene (aglB) lies adjacent to a second gene (aglA), which encodes an EII(CB) component of the phosphoenolpyruvate-dependent sugar:phosphotransferase system. We suggest that the products of the two genes facilitate the phosphorylative translocation and subsequent hydrolysis of the five alpha-d-glucosyl-d-fructoses by K. pneumoniae.