Glucan synthase

In enzymology, a cellulose synthase (EC 2.4.1.12, UDP-glucose:(1→4)-β-D-glucan 4-β-D-glucosyltransferase) is an enzyme that catalyzes the chemical reaction In enzymology, a cellulose synthase (EC 2.4.1.12, UDP-glucose:(1→4)-β-D-glucan 4-β-D-glucosyltransferase) is an enzyme that catalyzes the chemical reaction Thus, the two substrates of this enzyme are UDP-glucose and n, whereas its two products are UDP and n+1. This enzyme is involved in the synthesis of cellulose. A similar enzyme utilizes GDP-glucose, cellulose synthase (GDP-forming) (EC 2.4.1.29). Cellulose is an aggregation of unbranched polymer chains made of β-(1→4)-linked glucose residues that makes up a large portion of primary and secondary cell walls. Although important for plants, it is also synthesized by most algae, some bacteria, and some animals. Worldwide, 2 × 1011 tons of cellulose microfibrils are produced, which serves as a critical source of renewable biofuels and other biological-based products, such as lumber, fuel, fodder, paper and cotton. These microfibrils are made on the surface of cell membranes to reinforce cells walls, which has been researched extensively by plant biochemists and cell biologist because 1) they regulate cellular morphogenesis and 2) they serve alongside many other constituents (i.e. lignin, hemicellulose, pectin) in the cell wall as a strong structural support and cell shape. Without these support structures, cell growth would cause a cell to swell and spread in all directions, thus losing its shape viability Plant cellulose synthases belong to the family of glycosyltransferases, which are proteins involved in the biosynthesis and hydrolysis of the bulk of earth's biomass. Cellulose is synthesized by large cellulose synthase complexes (CSCs), which consist of synthase protein isoforms (CesA) that are arranged into a unique hexagonal structure known as a “particle rosette” 50 nm wide and 30–35 nm tall. There are more than 20 of these full-length integral membrane proteins, each of which is around 1000 amino acids long. These rosettes, formerly known as granules, were first discovered in 1972 by electron microscopy in green algae species Cladophora and Chaetomorpha (Robinson et al. 1972). Solution x-ray scattering have shown that CesAs are at the surface of a plant cell and are elongated monomers with a two catalytic domains that fuse together into dimers. The center of the dimers is the main point of catalytic activity. Since cellulose is made in all cell walls, CesA proteins are present in all tissues and cell types of plants. Nonetheless, there are different types of CesA, some tissue types may have varying concentrations of one over another. For example, the AtCesA1 (RSW1) protein is involved in the biosynthesis of primary cell walls throughout the whole plant while the AtCesA7 (IRX3) protein is only expressed in the stem for secondary cell wall production. Cellulose biosynthesis is the process during which separate homogeneous β-(1→4)-glucan chains, ranging from 2,000 to 25,000 glucose residues in length, are synthesized and then immediately hydrogen bond with one another to form rigid crystalline arrays, or microfibrils. Microfibrils in the primary cell wall are approximately 36 chains long while those of the secondary cell wall are much larger, containing up to 1200 β-(1→4)-glucan chains. Uridine diphosphate-glucose (UDP), which is produced by the enzyme sucrose synthase (SuSy) that produces and transports UDP-glucose to the plasma membrane is the substrate used by cellulose synthase to produce the glucan chains. The rate at which glucose residues are synthesized per one glucan chain ranges from 300 to 1000 glucose residues per minute, the higher rate being more prevalent in secondary wall particles, such as in the xylem. Microfibril synthesis is guided by cortical microtubules, which lie beneath the plasma membrane of elongating cells, in that they form a platform on which the CSCs can convert glucose molecules into the crystalline chains. The microtubule–microfibril alignment hypothesis proposes that cortical microtubules, which lie beneath the plasma membrane of elongating cells, provide tracks for CSCs that convert glucose molecules into crystalline cellulose microfibrils. The direct hypothesis postulates some types of direct linkage between CESA complexes and microtubules. Additionally, the KORRIGAN (KOR1) protein is thought to be a critical component of cellulose synthesis in that it acts as a cellulase at the plasma membrane-cell wall interface. KOR1 interacts with a two specific CesA proteins, possibly by proof-reading and relieving stress created by glucan chain synthesis, by hydrolyzing disordered amorphous cellulose. Cellulose synthesis activity is affected by many environmental stimuli, such as hormones, light, mechanical stimuli, nutrition, and interactions with the cytoskeleton. Interactions with these factors may influence cellulose deposition in that it affects the amount of substrate produced and the concentration and/or activity of CSCs in the plasma membrane.