ABSTRACT
Cellulose is the major cell-wall component, which interacts strongly with hemicelluloses and lignin.
It is a linear polymer of β-1,4-D-glucan. In most crystal structures of cellulose, the molecule has a twofold helical conformation stabilized by intramolecular hydrogen bonds. Cellulose is a semi-crystalline polymer. It can crystallize into several different crystalline polymorphs, cellulose I and cellulose II being the most important types. The coordinated action of enzymatic polymerization associated with crystallization of native cellulose gives rise to nascent cellulose microfibrils which are assembled to form higher order structures such as layers, cell walls, and fibers.
Biosynthesis of cellulose is a multi-step process involving terminal complexes containing cellulose synthase enzymes (glycosyltransferases) organized in spinnerets at the cell membrane and using the uridine 5’-diphospho-glucose as substrate. Seven families of processive β-glycosyltransferase genes have been identified in plants.
Cellulose can be degraded in nature by enzymes (cellulases) as an essential part of the carbon cycle. A variety of cellulolytic bacteria and fungi works in association with related microorganisms to convert insoluble cellulose to soluble sugars, primarily cellobiose and glucose, which are then assimilated by the cell. Due to the structural complexity and rigidity of cellulosic substrates, their efficient degradation generally requires multienzyme systems. Two types of systems occur. One type, mainly produced by aerobic bacterial and fungal microorganisms, consists of a collection of independent extracellular cellulases (cellobiohydrolases, endoglucanases, exoglucanases, and/or β-glucosidases) that act synergistically. The second type, mainly produced by anaerobic bacteria, consists of an enzyme complex, called cellulosome.
