By exploiting nuclear magnetic resonance (NMR) techniques along with novel applications of saturation difference analysis, we deciphered the functions of the previously uncharacterized products of three bacterial genes, rbsD, fucU, and yiiL, which are part of the ribose, fucose, and rhamnose operons of Escherichia coli, respectively. We show that RbsD catalyzes the pyran to furan conversion of ribose, whereas FucU and YiiL are involved in the catalysis of the anomeric conversion of their respective sugars. It was observed that the anomeric exchange of only ribofuranose, not ribopyranose, occurs spontaneously in solution rationalizing its evolutionary incorporation into the nucleic acid. The RbsD and FucU proteins share sequence homology and belong to the same protein family that is found from eubacteria to human, whereas the YiiL homologues exist in archaebacteria and lower eukaryotes. These enzymes, including the galactose mutarotase, exhibit a certain degree of cross-specificity to structurally analogous sugars thereby encompassing all existing monosaccharides in terms of their reactivities. The ubiquitous presence of enzymes involved in the anomeric changes of monosaccharides highlights an importance of these activities in various cellular processes requiring efficient monosaccharide utilization.