Functional characterisation of a thermophilic cellulase from a Malawian metagenomic library

Abstract

Biofuels are currently recognised as the most viable source of energy to replace depleting fossil fuel reserves, with bioethanol the most popular alternative alcohol fuel. Producing bioethanol from agricultural waste residues is a feasible socio-economic industrial process. Lignocellulose, from which plant material is composed, is highly recalcitrant to enzymatic degradation and therefore requires a suite of enzymes for complete hydrolysis of the biomass. Metagenomes, particularly from extreme environments, represent an unlimited resource for the discovery of novel biocatalysts for inclusion in industrial processes. Here we report on the cloning and functional characterisation of a novel thermophilic cellulase identified by the functional screening of a Malawian, hotspring sediment metagenomic library. The gene encoding the cellulase, celMHS, composed of 2,705 nucleotides and encoded a polypeptide of 905 amino acids with a predicted molecular mass of about 98 kDa. The in silico translated protein, CelMHS, contained a putative transmembrane domain, a family 4 carbohydrate binding motive (CBM 4), a truncated glycoside hydrolase family 42 (GH42) domain and a N-terminal region that does not have sequence similarity to any previously described domains. Functional characterisation of the recombinant CelMHS demonstrated that the protein displayed an optimal pH of 6.0 and temperature of 100°C. CelMHS had high specific activity toward substrates comprising of β-1,4 linked glucose subunits such as carboxymethyl cellulose, β-D-glucan from barley and lichenan, however, some activity was also observed against avicel, a crystalline cellulose substrate. HPLC analysis of the hydrolysis products produced by CelMHS indicates that this particular enzyme prefers longer chain oligosaccharides. This is, to the best of our knowledge, the first investigation describing the cloning and characterization of a carbohydrate hydrolysing enzyme comprised of the unique sequence architecture: a partial GH42 catalytic domain, a CBM 4 and a unique N-domain sequence. Key words: cellulose, cellulases, lignocellulosic biomass, bioethanol, saccharification, hydrolysis, metagenomic library, thermophilic