THERMOPHILIC ENZYMES FOR BIOMASS CONVERSION

There is a growing interest in the conversion of lignocellulose biomasses into useful products since the exploitation of agro-industrial waste could provide alternative substrates for low-cost fuels and chemical productions. A key factor for the fruitful utilization of the biomasses is represented by an efficient hydrolysis (saccharification step) of cellulose and hemicellulose using enymes as alternative to the chemical hydrolysis. Since in lignocelluloses the chains of carbohydrates are embedded in a lignin matrix which hinders the enzyme action, a pre-treatment of the biomasses is required for a more effective saccharification. The drastic conditions required by many pre-treatment techniques, give rise to problems when using conventional enzymes in the saccharification step, for the lack of thermostability of most cellulases and hemicellulases. In this context, thermophilic bacteria have received considerable attention as source of cellulolytic and xylanolytic enzymes since they are thermostable, active at high temperatures (thermophylic) and resistant to solvents and detergents. These unusual properties make them attractive candidates for the development of enzymatic biomass conversion processes. Here we report on thermostable cellulolytic, hemicellulolytic and glycolytic activities, isolated from thermophilic bacteria and archaebacteria, that are of potential interest for biomass conversion. Some (hyper)thermophilic microorganisms have been tested for their ability to grow on complex carbohydrates as primary carbon and energy source. Several activities, able to degrade cellulose at acidic pH and high temperature, have been identified in the thermophilic bacterium Alicyclobacillus acidocaldarius. Moreover, a xylanolytic activity from the bacterium Thermus thermophilus has been detected when grown in medium added with xylan. The hyperthermophilic archaeon Sulfolobus solfataricus, originally isolated from a solfataric field in the area of Naples, is another interesting source of enzymes for biomass conversion. From this organism, a bi-functional xylanase/cellulase with optimal temperature and pH of 90/95°C and 4.0/3.5 respectively, is the last of a series of glycoside hydrolases, involved in cellulose and hemicellulose degradation, isolated and characterised from this source in our Institute. Enzymes involved in the hydrolysis of oligomers of starch, cellulose, xylan, mannan, and xyloglucan have also been identified and characterised. Remarkably, these enzymes are all active at temperatures higher than 90°C and acidic pH. Therefore, cocktails of endo- and exo-glycoside hydrolases from this source could convert, in “one pot”, at high temperatures and acidic pH a variety of complex polysaccharides into fermentable sugars. References - Cobucci-Ponzano B, Perugino G, Trincone A, Mazzone M, Di Lauro B, Giordano A, Rossi M and Moracci M (2003) Applications in biocatalysis of glycosyl hydrolases from the hyperthermophilic Archaeon Sulfolobus solfataricus. Biocatalysis and Biotransformation 21, 215-221. - Cannio R, Di Prizito N, Rossi M, Morana A (2004) A xylan-degrading strain of Sulfolobus solfataricus: isolation and characterization of the xylanase activity. Extremophiles 8, 117-124. - Morana A, Paris O, Maurelli L, Rossi M, Cannio R. (2007) Gene cloning and expression in Escherichia coli of a bi-functional b-D-xylosidase/a-L-arabinosidase from Sulfolobus solfataricus involved in xylan degradation. Extremophiles 11, 123-132. - Morana A, Esposito A, Maurelli L, Ruggiero G, Ionata E, Rossi M and La Cara F (2008) A novel thermoacidophilic cellulase from Alicyclobacillus acidocaldarius. Protein Pept. Letters 15, (available online)