Enhanced Thermostability of an l-Rhamnose Isomerase for d-Allose Synthesis by Computation-Based Rational Redesign of Flexible Regions

d-Allose is a low-calorie rare sugar with great application potential in the food and pharmaceutical industries. The production of d-allose has been accomplished using l-rhamnose isomerase (L-RI), but concomitantly increasing the enzyme's stability and activity remains challenging. Here, we rationally engineered an L-RI from Clostridium stercorarium to enhance its stability by comprehensive computation-aided redesign of its flexible regions, which were successively identified using molecular dynamics simulations. The resulting combinatorial mutant M2-4 exhibited a 5.7-fold increased half-life at 75 °C while also exhibiting improved catalytic efficiency. Especially, by combining structure modeling and multiple sequence alignment, we identified an α0 region that was universal in the L-RI family and likely acted as a "helix-breaker". Truncating this region is crucial for improving the thermostability of related enzymes. Our work provides a significantly stable biocatalyst with potential for the industrial production of d-allose.

Improving Thermostability and Catalytic Behavior of l-Rhamnose Isomerase from Caldicellulosiruptor obsidiansis OB47 toward d-Allulose by Site-Directed Mutagenesis

d-Allose, a rare sugar, is an ideal table-sugar substitute and has many advantageous physiological functions. l-Rhamnose isomerase (l-RI) is an important d-allose-producing enzyme, but it exhibits comparatively low catalytic activity on d-allulose. In this study, an array of hydrophobic residues located within β1-α1-loop were solely or collectively replaced with polar amino acids by site-directed mutagenesis. A group of mutants was designed to weaken the hydrophobic environment and strengthen the catalytic behavior on d-allulose. Compared with that of the wild-type enzyme, the relative activities of the V48N/G59N/I63N and V48N/G59N/I63N/F335S mutants toward d-allulose were increased by 105.6 and 134.1%, respectively. Another group of mutants was designed to enhance thermostability. Finally, the t_1/2 values of mutant S81A were increased by 7.7 and 1.1 h at 70 and 80 °C, respectively. These results revealed that site-directed mutagenesis is efficient for improving thermostability and catalytic behavior toward d-allulose.

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 Co²⁺ . 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 Co²⁺ 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.