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 Ca2+) were obtained to maximize the potential of OsRpiB in preparing L-rhamnulose. The catalytic properties of OsRpiB, including Km, kcat, and catalytic efficiency (kcat/Km), 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.