Improved thermostability and robustness of L-arabinose isomerase by C-terminal elongation and its application in rare sugar production

Rare sugar was defined as a sugar that occurs in very small quantities in nature. Among them, L-ribose and D-tagatose were of high added value and useful as pharmaceutical intermediate for anti-HBV drugs or low calorie sweetener in food industry. Bio-production of the two rare sugar from biomass waste has not been investigated. Hence, development of a feasible and efficient co-production method was of practical usage. However, lack of suitable biocatalyst has become a bottleneck. By sequence alignment and analysis, a C-terminal α-helix from L-arabinose isomerase (L-AI) family was selected as a tool for protein engineering. This α-helix was ligated to C-terminal of Lactobacillus fermentum L-AI (LFAI) and significantly enhanced its thermostability and robustness for both L-arabinose and galactose catalysis. The mutant LFAI-C4 enzyme was immobilized by alginate and antimicrobial peptide poly-L-lysine, and was used to convert pretreated corncob acid hydrolysate (PCAH) into L-ribulose and D-tagatose in the presence of boric acid. In addition, we identified and immobilized a novel thermostable mannose-6-phosphate isomerase from Bacillus subtilis (BsMPI-2) which was efficient in catalyzing retaining L-ribulose into L-ribose and showing no activity on D-tagatose. The dual immobilized enzymes (LFAI-C4 and BsMPI-2) system co-produced 191.9 g/L L-ribose and 80.1 g/L D-tagatose, respectively. Showing a total yield of 46.6% from L-arabinose to L-ribose, which was the highest among reported. The dual immobilized enzymes system preserved 82% activity after 40 batches reaction, showing excellent potentials for industrial use. This study presents a promising alternative for rare sugar production from low-value raw material and showed satisfied conversion rate, product concentration, and operation stability.