Hemicelluloses enzymatic degradation

The plant cell wall is a polymeric mesh consisting of cellulose, hemicellulose, pectin and protein. Cellulose and hemicellulose are integral components of the cell wall, but pectic substances are located mainly in the outer wall regions within the middle lamella (McNeil et ai, 1984). Pectic substances are more susceptible to enzymatic degradation, because they are more exposed than other cell wall components. Therefore, pectin-degrading enzymes may play a central role in the penetration of plant tissue by bacteria. 

Cellulose is found in nature in combination with various other substances, the nature and composition of which depend on the source and previous history of the sample. In most plants, there are three major components cellulose, hemicelluloses, and lignin. Efficient utilization of all three components would greatly help the economics of any scheme to obtain fuel from biomass. Hemicelluloses, lignocellulose and lignin remaining after enzymatic degradation of the cellulose in wood would require chemical or thermal treatment - as distinct from biochemical - to produce a liquid fuel. 

Acacia is a natural gum obtained from the acacia trees. It is a complex, loose aggregate of sugars and hemicelluloses. It is commercially available in a powdered form, a granular form, or as a spray-dried product. As a tablet binder, it is used in an aqueous solution or added in dried form prior to moistening with water. Acacia forms very hard tablets, which disintegrate slowly. Aqueous solutions are susceptible to bacterial and enzymatic degradation. It is incompatible with amidopyrine, cresol, phenol, ethanol, ferric salts, and a number of other substances. Acacia, which was widely used in the past as a tablet binder, is rarely used today, in favor of one of the many synthetic polymers. 

Due to the complex structures of hemicelluloses, several different enzymes are involved in their enzymatic degradation and modification (Table II) (7-70). Enzymes are nature s own catalysts, which act in mild conditions. Consequently, the use of enzymes offers an excellent alternative to engineer the properties of hemicelluloses in a controlled way. This paper discusses properties of hemicellulases and the possibilities for enzyme-aided modifications of hemicelluloses. 

Additionally, lignin presents a great barrier to cellulosic biomass utilisation (Kamm et al. 2011 Wackett 2008). The protective layer that lignin forms is an obstacle to use of cellulose and hemicellulose. Since lignin is a heterogeneous polymer, its enzymatic degradation is relatively difficult and slow, and its separation from the raw biomass presents an energy-intensive process. 

The foregoing observations confirm the conclusions derived from former experiments with beechwood holocellulose (10) (1) A partial degradation of the hemicelluloses is imperative before the cellulose fibrils can be attacked. (2) The hemicelluloses seem to be deposited between the cellulose fibrils or even to be encrusting them. (3) The enzymatic hydrolysis of the cellulose is governed by the porosity of the tissue (enzyme diffusion), the impediment of the hemicelluloses, and the properties of the cellulose (e.g., crystallinity). 

Carbohydrates would be the predominant raw materials for future biorefineries. The major polysaccharides found in nature are cellulose, hemicellulose and starch (see Chapter 1). These molecules would be mainly utilised after they are broken down to their respective monomers via enzymatic hydrolysis, thermochemical degradation or a combination of these two. Cellulose and hemicellulose, together with lignin, constitute the main structural components of biomass. Starch is the major constituent of cereal crops. This section would focus on the potential utilisation of carbohydrates and lignocellulosic biomass for chemical production. 

---