Mechanisms in Cellulose Biosynthesis

Cellulose is a major industrial biopolymer in the forest products, textile, and chemical industries. It also forms a large portion of the biomass useful in the generation of energy. Moreover, cellulose-based biomass is a renewable energy source that can be used for the generation of ethanol as a fuel. Cellulose is synthesized by a variety of living organisms, including plants, algae, bacteria, and animals. It is the major component of plant cell walls with secondary cell walls having a much higher content of cellulose. The biosynthesis of cellulose essentially proceeds by the polymerization of glucose residues using an activated substrate UDP-glucose. In plants, cellulose is synthesized on the plasma membrane by the enzyme cellulose synthase that is present in the membrane. In the bacterium Acetobacter xylinum, the enzyme cellulose synthase is present on the cytoplasmic membrane, and the cellulose product is obtained extracellularly. However, in other organisms, cellulose is found to be synthesized in other regions of the cell. In the alga Pleurochrysis, cellulose scales are formed in the Golgi apparatus and then deposited on the cell surface. Cellulose is an aggregate of glucan chains that are arranged in a specific manner to give rise to a crystalline state. Although cellulose produced by different organisms has the same chemical composition (polymer of β-1,4linked glucose residues), there are remarkable differences in the physical properties of the cellulose product, mainly in the length of the glucan chains (as represented by degree of polymerization) and the crystallinity and crystalline form of the cellulose product. Depending upon the specific organism, this crystalline state is different, and it defines the physical properties of the product such as its strength, solubility in various solvents, and accessibility to various modifying reagents. Cellulose biosynthesis proceeds in at least two stages – polymerization and crystallization. The first stage is catalyzed by the enzyme cellulose synthase, and the second stage is dependent on the organization of the cellulose synthases possibly with other proteins such that the glucan chains are assembled in a crystalline form.