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 Escherichia 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.