From Biomass to Biofuels: Breaking the Cellulose Barrier

Cellulases are of considerable current interest for converting the cellulosic content of biomass to fermentable sugars for biofuels production. To facilitate the enzymatic conversion of biomass to sugars, pre-treatment processes at high temperatures, low pH, and even in ionic liquids are candidates to reduce the recalcitrance of the lignocellulosic substrate. Cellulases that function at high temperatures and are stable to pre-treatment conditions offer the potential advantages of lower cooling costs, higher solids loadings, reduced risk of microbial contamination, and higher reaction rates.

Hyperthermophilic archaea and bacteria are potential sources of extremely stable cellulosic enzymes. To this end, bioprospecting for thermophilic cellulose degraders and their enzymes has resulted in the discovery of a unique cellulase, designated EBI244, which is most active at 109°C (a record for cellulases) and is the most heat tolerant enzyme found in any cellulose-digesting microbe, including bacteria. These results demonstrate the discovery of a novel hyperthermophilic cellulase from a hyperthermophilic enrichment, and establish that hyperthermophilic archaea are capable of growth on crystalline cellulose.

Another approach to develop more stable cellulases is to generate new enzymes by protein engineering. We have thus employed several protein-engineering strategies, including rational design and directed evolution, to create cellulases with improved thermostability and activity profiles tailored to a desired temperature range. These results will be discussed in connection with cellulase structure-stability relationships and the complementarity of bioprospecting and protein engineering for the discovery and generation of new cellulases.

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