Thermophilic Fungi and Their Enzymes for Biorefineries

Abstract

The biorefinery concept involves production of fuel and chemicals from lignocellulosic biomass and thus can be considered as an analog to chemical refineries, that produce fuels and products from petroleum. Bioconversion of lignocellulosic residues involves the use of microbial hydrolytic enzymes, namely cellulases and hemicellulases, which release fermentable sugars from the delignified (pretreated) substrate. These enzymes are secreted in higher amounts by filamentous fungi to obtain energy from the biomass in natural habitats. Therefore, cellulases and xylanases to be used in biorefineries are usually produced from fungi. However, an efficient and economical process for the development of commercial biorefineries is still not formulated. This is because most of the present-day biomass conversion processes use mesophilic hydrolytic enzymes, which get denatured at increased temperatures, resulting in consumption of large amounts of enzyme and thus, half of the process cost in biorefineries is attributed to the enzyme production process. Therefore, employment of thermostable enzymes for hydrolysis of lignocellulosic biomass at higher temperatures can help in increasing reaction rates tremendously, thereby requiring lesser enzyme, in turn reducing process cost. In addition, consolidated bioprocessing (CBP) of untreated substrate by thermophilic filamentous fungi, which co-produce ligninases as well as cellulases and hemicellulases can deconstruct lignin as well as cellulose and hemicellulose to fermentable sugars at elevated temperature, leading to the feasibility of future biorefineries. This review discusses lignocellulosic bioconversion by the application of thermostable lignocellulolytic enzyme from thermophilic fungi into biofuels and commodity chemicals.