Question

Explain cellulose biosynthesis - substrate, catalytic process, resulting structure (primary, secondary, tertiary (supramolecular) cellulose structure)

Answer 1

Cellulose is an important structural component of the primary cell wall of green plants. Cellulose is a polysaccharide consisting of many D-glucose units linked to each other to form a linear chain. The glucose units are linked to each other via β(1→4) linkage (glycosidic bonds). Hydrogen bonds form between hydroxyl groups and oxygen atoms between the glucose units within the chain and/or between neighboring chains. The hydroxyl groups present on one glucose unit present in one chain bonds with oxygen atoms of the glucose unit located in the same chain or neighboring chain. The bonding results in holding of chains together to form microfibrils. Hydrogen bonding binds the glucan chains together and helps in parallel stacking of cellulose microfibrils to form a crystalline structure. This confers tensile strength in cell walls. Cellulose microfibrils aggregate into larger size cellulose microfibrils to form primary and secondary cell walls in higher plants. The length of cellulose microfibril depends upon the degree of polymerization and this varies among species.

In higher vascular plants, cellulose is synthesized at the plasma membrane by involving rosette terminal complexes (RTCs) which are hexameric protein structures containing enzyme cellulose synthase. This enzyme helps in the synthesis of cellulose units by catalyzing polymerization of glucan chains. The enzyme is encoded by CesA genes. Different type of CesA genes are involved in the synthesis of primary and secondary cell wall. The enzyme uses UDP-glucose to form β(1→4) linkages within glucose units to form cellulose. Cellulose synthase complex (CSC) is involved in biosynthesis of cellulose microfibrils containing 18 cellulose synthase (CesA) proteins. The process involves an initiation, elongation step. CesA glucosyl transferase initiates the polymerization of cellulose using UDP-glucose (uridine di phosphate glucose). UDP-glucose precursors are used to elongate the cellulose chain. The mature chain is cleaved by enzyme cellulase.Cellulose, the most abundant biopolymer synthesized on land, is made of linear chains of ß (1–4) linked D-glucose. As a major structural component of the cell wall, cellulose is important not only for industrial use but also for plant growth and development. Cellulose microfibrils are tethered by other cell wall polysaccharides such as hemicellulose, pectin, and lignin. In higher plants, cellulose is synthesized by plasma membrane-localized rosette cellulose synthase complexes. Despite the recent advances using a combination of molecular genetics, live cell imaging, and spectroscopic tools, many aspects of the cellulose synthesis remain a mystery. In this chapter, we highlight recent research progress towards understanding the mechanism of cellulose synthesis in Arabidopsis.

Cellulose biosynthesis is a regulated process. This means that the amount of cellulose synthesis,   degree of polymerization, the crystalline cellulose core size, and the orientation of cellulose microfibrils are the events regulated by genetic functions. In bacteria CseA genes control cellulose biosynthesis and microfibril assembly, though in plants no genes linked to CseA are reported.