The biological degradation of cellulose

Abstract

Cellulolytic microorganisms play an important role in the biosphere by recycling cellulose, the most abundant carbohydrate produced by plants. Cellulose is a simple polymer, but it forms insoluble, crystalline microfibrils, which are highly resistant to enzymatic hydrolysis. All organisms known to degrade cellulose efficiently produce a battery of enzymes with different specificities, which act together in synergism. The study of cellulolytic enzymes at the molecular level has revealed some of the features that contribute to their activity. In spite of a considerable diversity, sequence comparisons show that the catalytic cores of cellulases belong to a restricted number of families. Within each family, available data suggest that the various enzymes share a common folding pattern, the same catalytic residues, and the same reaction mechanism, i.e. either single substitution with inversion of configuration or double substitution resulting in retention of the beta-configuration at the anomeric carbon. An increasing number of three-dimensional structures is becoming available for cellulases and xylanases belonging to different families, which will provide paradigms for molecular modeling of related enzymes. In addition to catalytic domains, many cellulolytic enzymes contain domains not involved in catalysis, but participating in substrate binding, multi-enzyme complex formation, or possibly attachment to the cell surface. Presumably, these domains assist in the degradation of crystalline cellulose by preventing the enzymes from being washed off from the surface of the substrate, by focusing hydrolysis on restricted areas in which the substrate is synergistically destabilized by multiple cutting events, and by facilitating recovery of the soluble degradation products by the cellulolytic organism. In most cellulolytic organisms, cellulase synthesis is repressed in the presence of easily metabolized, soluble carbon sources and induced in the presence of cellulose. Induction of cellulases appears to be effected by soluble products generated from cellulose by cellulolytic enzymes synthesized constitutively at a low level. These products are presumably converted into true inducers by transglycosylation reactions. Several applications of cellulases or hemicellulases are being developed for textile, food, and paper pulp processing. These applications are based on the modification of cellulose and hemicellulose by partial hydrolysis. Total hydrolysis of cellulose into glucose, which could be fermented into ethanol, isopropanol or butanol, is not yet economically feasible. However, the need to reduce emissions of greenhouse gases provides an added incentive for the development of processes generating fuels from cellulose, a major renewable carbon source.

Biosolutions to the energy problem

“…In addition to its function of binding other members of the cellulose complex to itself, it also binds to cellulose [10]. Its nucleotide sequence revealed a deduced protein size of 196,800 Da, a cellulose binding domain (CBD) [8] and nine enzyme receptor domains of about 150-166 amino acid residues each. The nine repeated sequences, called 'cohesins' by Bayer et al [5], are quite similar to each other, i.e., exhibiting between 60 and 100% identity, with six of the nine domains being at least 90% identical.…”

“…Integration of cellulase production, sacchariWcation and fermentation into the microbial cell involves the use of thermophilic, anaerobic, ethanologenic bacteria such as Clostridium thermocellum [8,19,74]. C. thermocellum can convert both waste cellulose (i.e.…”

Evidence that linker sequences and cellulose-binding domains enhance the activity of hemicellulases against complex substrates

“…Analysis of the molecular architecture of cellulases and xylanases has shown that many of these enzymes have a modular structure in which the functional domains are spatially separated by linker sequences [1][2][3]. These interdomain regions vary in size from six to over 300 amino acids, and generally consist of single amino acids [4] or of tandem repeats of dipeptides [5].…”

Optimization of Cultural Condition and Synergistic Effect of Lactose with Carboxymethyl Cellulose on Cellulase Production by <i>Bacillus</i> sp. Isolated from Fecal Matter of Elephant (<i>Elephas maximus</i>)

“…Microbial cellulases have become the important biocatalyst due to their complex nature and wide spread applications. The potential applications of cellulose are in food, animal feed, textile, fuel and chemical industries [1,2]. The major industrial applications of cellulases are in textile industry for biopolishing of fabrics and producing stonewashed look of denims, as well as household laundry detergents for improving fabric softness and brightness [3].…”