Crystal structure of a novel homodimeric L-ribulose 3-epimerase from Methylomonus sp.

D-allulose has potential as a low calorie sweetener which can suppress fat accumulation. Several enzymes capable of D-allulose production have been isolated, including D-tagatose 3-epimerases. Here, we report the isolation of a novel protein from Methylomonas sp. expected to be a putative enzyme based on sequence similarity to ketose 3-epimerase. The synthesized gene encoding the deduced ketose 3-epimerase was expressed as a recombinant enzyme in E. coli, and it exhibited the highest enzymatic activity toward L-ribulose, followed by D-ribulose and D-allulose. The X-ray structure analysis of L-ribulose 3-epimerase from Methylomonas sp. (MetLRE) revealed a homodimeric enzyme, the first reported structure of dimeric L-ribulose 3-epimerase. The monomeric structure of MetLRE is similar to that of homotetrameric L-ribulose 3-epimerases, but the short C-terminal α-helix of MetLRE is unique and different from those of known L-ribulose 3 epimerases. The length of the C-terminal α-helix was thought to be involved in tetramerization and increasing stability; however, the addition of residues to MetLRE at the C-terminus did not lead to tetramer formation. MetLRE is the first dimeric L-ribulose 3-epimerase identified to exhibit high relative activity toward D-allulose.