Characterization of Ribose-5-Phosphate Isomerase B from Newly Isolated Strain Ochrobactrum sp. CSL1 Producing L-Rhamnulose from L-Rhamnose

In this study, we attempted to find new and efficient microbial enzymes for producing rare sugars. A ribose-5-phosphate isomerase B (OsRpiB) was cloned, overexpressed, and preliminarily purified successfully from a newly screened Ochrobactrum sp. CSL1, which could catalyze the isomerization reaction of rare sugars. A study of its substrate specificity showed that the cloned isomerase (OsRpiB) could effectively catalyze the conversion of L -rhamnose to L -rhamnulose, which was unconventional for RpiB. The optimal reaction conditions (50°C, pH 8.0, and 1 mM Ca 2+ ) were obtained to maximize the potential of OsRpiB in preparing L -rhamnulose. The catalytic properties of OsRpiB, including K m , k ca , and catalytic efficiency ( k ca / K m ), were determined as 43.47 mM, 129.4 sec -1 , and 2.98 mM/sec. The highest conversion rate of L -rhamnose under the optimized conditions by OsRpiB could reach 26% after 4.5 h. To the best of our knowledge, this is the first successful attempt of the novel biotransformation of L -rhamnose to L -rhamnulose by OsRpiB biocatalysis.