Characterization of a thermostable recombinant l ‐rhamnose isomerase from Caldicellulosiruptor obsidiansis OB47 and its application for the production of l ‐fructose and l ‐rhamnulose

BACKGROUND l ‐Hexoses are rare sugars that are important components and precursors in the synthesis of biological compounds and pharmaceutical drugs. l ‐Rhamnose isomerase (L‐RI, EC 5.3.1.14) is an aldose–ketose isomerase that plays a significant role in the production of l ‐sugars. In this study, a thermostable, l ‐sugar‐producing L‐RI from the hyperthermophile Caldicellulosiruptor obsidiansis OB47 was characterized. RESULTS The recombinant L‐RI displayed maximal activity at pH 8.0 and 85 °C and was significantly activated by Co 2+ . It exhibited a relatively high thermostability, with measured half‐lives of 24.75, 11.55, 4.15 and 3.30 h in the presence of Co 2+ at 70, 75, 80 and 85 °C, respectively. Specific activities of 277.6, 57.9, 13.7 and 9.6 U mg −1 were measured when l ‐rhamnose, l ‐mannose, d ‐allose and l ‐fructose were used as substrates, respectively. l ‐Rhamnulose was produced with conversion ratios of 44.0% and 38.6% from 25 and 50 g L −1 l ‐rhamnose, respectively. l ‐Fructose was also efficiently produced by the L‐RI, with conversion ratios of 67.0% and 58.4% from 25 and 50 g L −1 l ‐mannose, respectively. CONCLUSION The recombinant L‐RI could effectively catalyze the formation of l ‐rhamnulose and l ‐fructose, suggesting that it was a promising candidate for industrial production of l ‐rhamnulose and l ‐fructose. © 2017 Society of Chemical Industry