L-rhamnose isomerase: a crucial enzyme for rhamnose catabolism and conversion of rare sugars.

L-rhamnose isomerase (L-RhI) plays a key role in the microbial L-rhamnose metabolism by catalyzing the reversible isomerization of L-rhamnose to L-rhamnulose. Additionally, the enzyme exhibits activity on various other aldoses and ketoses, and its broad substrate specificity has attracted attention for its potential application in the production of rare sugars; however, improvement of the enzyme properties is desirable, such as thermal stability, enzymatic activity, and a pH optimum suitable for industrial usage. This review summarizes our current insights into L-RhIs with respect to their substrate recognition mechanism and their relationship with D-xylose isomerase (D-XI) based on structural and phylogenetic analyses. These two enzymes are inherently different, but recognize distinctly different substrates, and share common features that may be phylogenetically related. For example, they both have a flexible loop region that is involved in shaping active sites, and this region may also be responsible for various enzymatic properties of L-RhIs, such as substrate specificity and thermal stability. KEY POINTS: •L-RhIs share structural features with D-XI. •There are two types of L-RhIs: E. coli L-RhI-type and D-XI-type. •Flexible loop regions are involved in the specific enzyme properties.