X-ray structures of Enterobacter cloacae allose-binding protein in complexes with monosaccharides demonstrate its unique recognition mechanism for high affinity to allose.

d -Allose is an aldohexose of the C3-epimer of d -glucose, existing in very small amounts in nature, called a rare sugar. The operon responsible for d -allose metabolism, the allose operon, was found in several bacteria, which consists of seven genes: alsR, alsB, alsA, alsC, alsE, alsK , and rpiB. To understand the biological implication of the allose operon utilizing a rare sugar of d -allose as a carbon source, it is important to clarify whether the allose operon functions specifically for d -allose or also functions for other ligands. It was proposed that the allose operon can function for d -ribose, which is essential as a component of nucleotides and abundant in nature. Allose-binding protein, AlsB, coded in the allose operon, is thought to capture a ligand outside the cell, and is expected to show high affinity for the specific ligand. X-ray structure determinations of Enterobacter cloacae AlsB (EtcAlsB) in ligand-free form, and in complexes with d -allose, d -ribose, and d -allulose, and measurements of the thermal parameters of the complex formation using an isothermal titration calorimeter were performed. The results demonstrated that EtcAlsB has a unique recognition mechanism for high affinity to d -allose by changing its conformation from an open to a closed form depending on d -allose-binding, and that the binding of d -ribose to EtcAlsB could not induce a completely closed form but an intermediate form, explaining the low affinity for d -ribose. [Display omitted] • The rare sugar allose is an aldohexose exitsing in very small amounts in nature. • The allose operon, responsible for allose metabolism, was found in several bacteria. • Allose-binding protein coded in the allose operon captures a ligand outside the cell. • Structures of the protein in complexes with various monosaccharides were determined. • The protein has an unique recognition mechanism for high affinity to allose. [ABSTRACT FROM AUTHOR]