High-Level Expression of a Thermally Stable Alginate Lyase Using Pichia pastoris, Characterization and Application in Producing Brown Alginate Oligosaccharide

An alginate lyase encoding gene sagl from Flavobacterium sp. H63 was codon optimized and recombinantly expressed at high level in P.pastoris through high cell-density fermentation. The highest yield of recombinant enzyme of sagl (rSAGL) in yeast culture supernatant reached 226.4 g/mL (915.5 U/mL). This was the highest yield record of recombinant expression of alginate lyase so far. The rSAGL was confirmed as a partially glycosylated protein through EndoH digestion. The optimal reaction temperature and pH of this enzyme were 45 degrees C and 7.5; 80 mM K+ ions could improve the catalytic activity of the enzyme by 244% at most. rSAGL was a thermal stable enzyme with T50(15) of 57-58 degrees C and T50(30) of 53-54 degrees C. Its thermal stability was better than any known alginate lyase. In 100 mM phosphate buffer of pH 6.0, rSAGL could retain 98.8% of the initial activity after incubation at 50 degrees C for 2 h. Furthermore, it could retain 61.6% of the initial activity after 48 h. The specific activity of the purified rSAGL produced by P. pastoris attained 4044 U/mg protein, which was the second highest record of alginate lyase so far. When the crude enzyme of the rSAGL was directly used in transformation of sodium alginate with 40 g/L, 97.2% of the substrate was transformed to di, tri, tetra brown alginate oligosaccharide after 32 h of incubation at 50 degrees C, and the final concentration of reducing sugar in mixture reached 9.51 g/L. This is the first report of high-level expression of thermally stable alginate lyase using P. pastoris system.