The Escherichia coli high-affinity ribose-transport system consists of six proteins encoded by the rbs operon (rbsD, rbsA, rbsC, rbsB, rbsK and rbsR). RbsD was originally thought to be a high affinity ribose transport protein, but further analysis [1] shows that it is a D-ribose pyranase . It catalyzes the interconversion of beta-pyran and beta-furan forms of D-ribose. It also catalyzes the conversion between beta-allofuranose and beta-allopyranose. This family also includes FucU a component of the fucose operon and is a L-fucose mutarotase, involved in the anomeric conversion of L-fucose. It also exhibits a pyranase activity for D-ribose [1]. Both have been classified in the RbsD/FucU family of proteins. Members of this family are ubiquitous having been found in organisms from eubacteria to mammals.

1: Stepwise disassembly and apparent nonstepwise reassembly for the oligomeric RbsD protein. Feng Y, Jiao W, Fu X, Chang Z; Protein Sci. 2006;15:1441-1448. PMID:16731978