In view of rising prices of crude oil due to increasing fuel demands, the need for alternative sources of bioenergy is expected to increase sharply in the coming years. Among potential alternative bioenergy resources, lignocellulosics have been identified as the prime source of biofuels and other value-added products. Lignocelluloses as agricultural, industrial and forest residuals account for the majority of the total biomass present in the world. To initiate the production of industrially important products from cellulosic biomass, bioconversion of the cellulosic components into fermentable sugars is necessary. A variety of microorganisms including bacteria and fungi may have the ability to degrade the cellulosic biomass to glucose monomers. Bacterial cellulases exist as discrete multi-enzyme complexes, called cellulosomes that consist of multiple subunits. Cellulolytic enzyme systems from the filamentous fungi, especially Trichoderma reesei, contain two exoglucanases or cellobiohydrolases (CBH1 and CBH2), at least four endoglucanases (EG1, EG2, EG3, EG5), and one β-glucosidase. These enzymes act synergistically to catalyse the hydrolysis of cellulose. Different physical parameters such as pH, temperature, adsorption, chemical factors like nitrogen, phosphorus, presence of phenolic compounds and other inhibitors can critically influence the bioconversion of lignocellulose. The production of cellulases by microbial cells is governed by genetic and biochemical controls including induction, catabolite repression, or end product inhibition. Several efforts have been made to increase the production of cellulases through strain improvement by mutagenesis. Various physical and chemical methods have been used to develop bacterial and fungal strains producing higher amounts of cellulase, all with limited success. Cellulosic bioconversion is a complex process and requires the synergistic action of the three enzymatic components consisting of endoglucanases, exoglucanases and β-glucosidases. The co-cultivation of microbes in fermentation can increase the quantity of the desirable components of the cellulase complex. An understanding of the molecular mechanism leading to biodegradation of lignocelluloses and the development of the bioprocessing potential of cellulolytic microorganisms might effectively be accomplished with recombinant DNA technology. For instance, cloning and sequencing of the various cellulolytic genes could economize the cellulase production process. Apart from that, metabolic engineering and genomics approaches have great potential for enhancing our understanding of the molecular mechanism of bioconversion of lignocelluloses to value added economically significant products in the future.

Bioconversion of lignocellulosic biomass: biochemical and molecular perspectives

Hemicellulolytic microorganisms play a significant role in nature by recycling hemicellulose, one of the main components of plant polysaccharides. Xylanases (EC 3.2.1.8) catalyze the hydrolysis of xylan, the major constituent of hemicellulose. The use of these enzymes could greatly improve the overall economics of processing lignocellulosic materials for the generation of liquid fuels and chemicals. Recently cellulase-free xylanases have received great attention in the development of environmentally friendly technologies in the paper and pulp industry. In microorganisms that produce xylanases low molecular mass fragments of xylan and their positional isomers play a key role in regulating its biosynthesis. Xylanase and cellulase production appear to be regulated separately, although the pleiotropy of mutations, which causes the elimination of both genes, suggests some linkage in the synthesis of the two enzymes. Xylanases are found in a cornucopia of organisms and the genes encoding them have been cloned in homologous and heterologous hosts with the objectives of overproducing the enzyme and altering its properties to suit commercial applications. Sequence analyses of xylanases have revealed distinct catalytic and cellulose binding domains, with a separate non-catalytic domain that has been reported to confer enhanced thermostability in some xylanases. Analyses of three-dimensional structures and the properties of mutants have revealed the involvement of specific tyrosine and tryptophan residues in the substrate binding site and of glutamate and aspartate residues in the catalytic mechanism. Many lines of evidence suggest that xylanases operate via a double displacement mechanism in which the anomeric configuration is retained, although some of the enzymes catalyze single displacement reactions with inversion of configuration. Based on a dendrogram obtained from amino acid sequence similarities the evolutionary relationship between xylanases is assessed. In addition the properties of xylanases from extremophilic organisms have been evaluated in terms of biotechnological applications.

Molecular and biotechnological aspects of xylanases

Xylanases are hydrolases depolymerizing the plant cell wall component xylan, the second most abundant polysaccharide. The molecular structure and hydrolytic pattern of xylanases have been reported extensively and the mechanism of hydrolysis has also been proposed. There are several models for the gene regulation of which this article could add to the wealth of knowledge. Future work on the application of these enzymes in the paper and pulp, food industry, in environmental science, that is, bio-fueling, effluent treatment, and agro-waste treatment, etc. require a complete understanding of the functional and genetic significance of the xylanases. However, the thrust area has been identified as the paper and pulp industry. The major problem in the field of paper bleaching is the removal of lignin and its derivatives, which are linked to cellulose and xylan. Xylanases are more suitable in the paper and pulp industry than lignin-degrading systems.

Biotechnology of microbial xylanases: enzymology, molecular biology, and application.

The present review article aims to summarize the microbiological and technological background of the dark fermentation processes for hydrogen generation, emphasising on the exploitation of biomass and wastes as potential feedstocks. The basic principles, the microbiology and the current technology of the processes are outlined. Subsequently, the use of different types of biomass and wastes that have so far been tested as feedstocks is analysed focusing on the advantages, possible limitations and future prospects of their exploitation. Moreover, different types of so far suggested pretreatment methods for better utilisation of the feedstocks are presented, pointing out the advantages and disadvantages of each method. Finally, methods for possible further utilisation of the generated by-products are laid out as well as the present status of the real scale applications.

Biohydrogen Production from Biomass and Wastes via Dark Fermentation: A Review

Biosynthesis and biodegradation of wood components

Bacillus thermoalkalophilus isolated from termite-infested mound soils of the semi-arid zones of India had the ability to produce good amounts of xylanase(s) from cheap agricultural wastes. Of the two hemicellulosic substrates tested, bagasse was found to be the better inducer for xylanase production. Alkali treatment of bagasse and rice husk had varied effects on enzyme production. The enzyme preparation had activity optima at 60° C and 70° C and a half-life of 60 min at 65° C. The enzyme was stable for 24 h over a pH range of 4.0–6.0, while maximum activity was observed at pH 6.0–7.0. Enzyme production and activity were inhibited by the end-product of xylan hydrolysis, xylose.

Production and characterization of xylanase from Bacillus thermoalkalophilus grown on agricultural wastes

Hepatotoxicity is a common cause of severe metabolic disorders and even death. Paracetamol (acetaminophen) is widely used as an antipyretic and analgesic, and it produces acute liver damage if administrated in excess. Since the conventional treatment of liver diseases is associated with a wide range of adverse effects, botanical agents are commonly used. In this study we examined the therapeutic effect methanolic extract of bark of Albizia Odoratissima on paracetamol induced hepatotoxicity in rats.A total of 30 animals were randomly divided into five groups, and each group consisted of six animals and received the below mentioned treatment for the period of 21 days. Group I (Normal group): Received vehicle only. Group II (Negative group): Received the Paracetamol (PCM) at dose of 3g/kg body weight orally. Group III (Test group -I): Received the Paracetamol (PCM) at dose of 3g/kg body weight orally and Methanolic extract of bark of Albizia Odoratissima at dose of 250 mg/kg orally. Group IV (Test group – II): Received the Paracetamol (PCM) at dose of 3g/kg body weight orally and Methanolic extract of bark of Albizia Odoratissima at dose of 500 mg/kg orally: PCM + MEAO (250mg/kg).

Sridevi Rajaram

Papers

Production and characterization of xylanase from Bacillus thermoalkalophilus grown on agricultural wastes

Attenuation of paracetamol induced hepatotoxicity by albizia odoratissima in rats