Applying metagenomics for the identification of bacterial cellulases that are stable in ionic liquids

Abstract

Ionic liquids (ILs) are novel and chemically inert solvents for a wide range of reactions in organic synthesis and biocatalysis, and at least one of them is known to dissolve cellulose. ILs would provide novel options for cellulose degradation in homogenous catalysis if cellulases were sufficiently stable and active. By screening metagenomic libraries 24 novel cellulase clones were identified and tested for their performance in the presence of ILs. Most enzyme clones showed only very poor or no activities. Three enzyme clones (i.e. pCosJP10, pCosJP20 and pCosJP24) were moderately active and stable in the presence of 1-butyl-1-methyl-pyrrolidinium trifluoromethanesulfonate. The corresponding genes of these environment-derived cosmids were similar to known cellulases from Cellvibrio japonicus and a salt-tolerant cellulase from an uncultured microorganism, S. Voget, H. L. Steele and W. R. Streit, J. Biotechnol., 2006, 126, 26-36.¹ The most active protein (CelA₁₀) belonged to GH5 family cellulases and was active at IL concentrations of up to 30% (v/v). Recombinant CelA₁₀ was extremely tolerant to 4 M NaCl and KCl. Furthermore improved cellulase variants of CelA₁₀ were isolated in a directed evolution experiment employing SeSaM-technology. Analysis of these variants revealed that the N-terminal cellulose binding domain plays a pivotal role for IL resistance.