Screening and characterization of a cellulase with endocellulase and exocellulase activity from yak rumen metagenome

Cellulose saccharification is an important process in conversion from lignocellulosic biomass to biofuels and other chemicals, and requires concerted action of endocellulase, exocellulase and β-glucosidase. Thus, it is very interesting to discover and develop multifunctional cellulase in order to convert cellulose to glucose more efficiently. Here we report an endo/exocellulase Rucel5B with 336 amino acids cloned from yak rumen uncultured microorganism, and its recombinant expression in Escherichia coli. This cellulase possesses endo-β-1,4-glucanase activity of 220 U mg−1 against carboxymethylcellulose and exo-β-1,4-glucanase activity of 52.9 U mg−1 against 4-nitrophenyl-β-d-cellobioside, and is able to hydrolyze not only amorphous cellulose (carboxymethylcellulose, barley glucan, lichenan, phosphate acid swollen cellulose, etc.), but also crystalline cellulose (filter paper, avicel, etc.). The exo-type action mode of Rucel5B was confirmed by its release of cellobiose from cellooligosaccharides and crystalline cellulose, and its endo-type action mode was confirmed by a time-dependent decrease in the polymerization degree of hydrolysates when Rucel5B was incubated with soluble amorphous cellulose. Therefore, the enzymatic activities, the endo/exo-mode of action and the ability in saccharification of both amorphous and crystalline cellulose make Recul5B a very interesting candidate for efficient saccharification of cellulose.