Gene Cloning and Characterization of<scp>L</scp>-Ribulose 3-epimerase fromMesorhizobium lotiand Its Application to Rare Sugar Production

A gene encoding L-ribulose 3-epimerase (L-RE) from Mesorhizobium loti, an important enzyme for rare sugar production by the Izumoring strategy, was cloned and overexpressed. The enzyme showed highest activity toward L-ribulose (230 U/mg) among keto-pentoses and keto-hexoses. This is the first report on a ketose 3-epimerase showing highest activity toward keto-pentose. The optimum enzyme reaction conditions for L-RE were determined to be sodium phosphate buffer (pH 8.0) at 60 &#176;C. The enzyme showed of higher maximum reaction a rate (416 U/mg) and catalytic efficiency (43 M(-1) min(-1)) for L-ribulose than other known ketose 3-epimerases. It was able to produce L-xylulose efficiently from ribitol in two-step reactions. In the end, 7.2 g of L-xylulose was obtained from 20 g of ribitol via L-ribulose at a yield of 36%.