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The UDP-forming form of cellulose synthase () is the main enzyme that produces
cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wa ...
. Systematically, it is known as ''UDP-glucose:(1→4)-β-D-glucan 4-β-D-glucosyltransferase'' in
enzymology Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. ...
. It catalyzes the
chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking ...
: :
UDP-glucose Uridine diphosphate glucose (uracil-diphosphate glucose, UDP-glucose) is a nucleotide sugar. It is involved in glycosyltransferase reactions in metabolism. Functions UDP-glucose is used in nucleotide sugar metabolism as an activated form of gl ...
+ 1→4)-β-D-glucosylsub>n = UDP + 1→4)-β-D-glucosylsub>n+1 A similar enzyme utilizes GDP-glucose,
cellulose synthase (GDP-forming) In enzymology, a cellulose synthase (GDP-forming) () is an enzyme that catalyzes the chemical reaction :GDP-glucose + (1,4-beta-D-glucosyl)n \rightleftharpoons GDP + (1,4-beta-D-glucosyl)n+1 Thus, the two substrates of this enzyme are GDP-gluco ...
( EC 2.4.1.29). This family of enzymes is found in
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...
and
plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae excl ...
s alike. Plant members are usually known as ''CesA'' (cellulose synthase) or the tentative ''CslA'' (cellulose synthase-like), while bacterial members may additionally be known as ''BcsA'' (bacterial cellulose synthase) or ''CelA'' (simply "cellulose"). Plants acquired ''CesA'' from the endosymbiosis event that produced the
chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it i ...
. This family belongs to
glucosyltransferase Glucosyltransferases are a type of glycosyltransferase that enable the transfer of glucose. Examples include: * glycogen synthase * glycogen phosphorylase Glycogen phosphorylase is one of the phosphorylase enzymes (). Glycogen phosphorylase cat ...
family 2 (GT2). Glycosyltransferases are involved in the biosynthesis and hydrolysis of the bulk of earth's biomass. There are known to be about seven subfamilies in the plant ''CesA'' superfamily, or ten in the combined plant-algal superfamily.
Urochordate A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time c ...
s are the only group of animals possessing this enzyme, having acquired them by
horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offspring (reproduction). H ...
more than 530 million years ago.


Cellulose

Cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wa ...
is an aggregation of unbranched
polymer A polymer (; Greek '' poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
chains made of β-(1→4)-linked
glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...
residues that makes up a large portion of primary and secondary
cell walls A cell wall is a structural layer surrounding some types of cells, just outside the cell membrane. It can be tough, flexible, and sometimes rigid. It provides the cell with both structural support and protection, and also acts as a filtering mech ...
. 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.


Purpose of cellulose

Cellulose
microfibrils A microfibril is a very fine fibril, or fiber-like strand, consisting of glycoproteins and cellulose. It is usually, but not always, used as a general term in describing the structure of protein fiber, e.g. hair and sperm tail. Its most frequently o ...
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 Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity a ...
,
hemicellulose A hemicellulose (also known as polyose) is one of a number of heteropolymers (matrix polysaccharides), such as arabinoxylans, present along with cellulose in almost all terrestrial plant cell walls.Scheller HV, Ulvskov Hemicelluloses.// Annu Rev ...
,
pectin Pectin ( grc, πηκτικός ': "congealed" and "curdled") is a heteropolysaccharide, a structural acid contained in the primary lamella, in the middle lamella, and in the cell walls of terrestrial plants. The principal, chemical component o ...
) 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


Structure

Several structures of the bacterial cellulose synthase BcsAB has been resolved. The bacterial enzyme consists of two different subunits, the catalytic BcsA on the cytoplasmic side, and the regulatory BcsB on the periplasmic side. They are coupled by a series of transmembrane helices, termed by the
CATH database The CATH Protein Structure Classification database is a free, publicly available online resource that provides information on the evolutionary relationships of protein domains. It was created in the mid-1990s by Professor Christine Orengo and coll ...
as 4p02A01 and 4p02B05. (Divisions for other models, such as , follow similarly.) The enzyme is stimulated by
cyclic di-GMP Cyclic di-GMP (also called cyclic diguanylate and c-di- GMP) is a second messenger used in signal transduction in a wide variety of bacteria. Cyclic di-GMP is not known to be used by archaea, and has only been observed in eukaryotes in ''Dictyo ...
. ''In vivo'' but not ''in vitro'', a third subunit called BcsC made up of a 18-strand beta barrel is required. Some bacteria contain extra non-essential periplasmic subunits. BcsA follows a layout of cytoplasmic domains sandwiched between the N- and C-terminal transmembrane domain. It has a typical family 2 GT domain (4p02A02) with a GT-A fold structure. At the C-terminal end is a PilZ domain conserved in bacteria, which forms part of the cyclic di-GMP binding surface together with BcsB and the beta-barrel (4p02A03) domain. Besides the C-terminal TM domain, BcsB is made up of two repeats, each consisting of a carbohydrate-binding module 27 (CATH 2.60.120.260) and an alpha-beta sandwith (CATH 3.30.379.20). BcsA and BcsB together form a channel through with the synthesized cellulose exits the cell, and mutations to residues lining the channel are known to reduce the activity of this enzyme. A gating loop in BcsA closes over the channel; it opens when cyclic di-GMP is bound to the enzyme.


Plants

In plants, cellulose is synthesized by large cellulose synthase complexes (CSCs), which consist of synthase protein
isoform A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene or gene family and are the result of genetic differences. While many perform the same or similar biological roles, some is ...
s (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 An integral, or intrinsic, membrane protein (IMP) is a type of membrane protein that is permanently attached to the biological membrane. All ''transmembrane proteins'' are IMPs, but not all IMPs are transmembrane proteins. IMPs comprise a signif ...
, each of which is around 1000
amino acids Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
long. These rosettes, formerly known as granules, were first discovered in 1972 by electron microscopy in green algae species ''
Cladophora ''Cladophora'' is a genus of reticulated filamentous Ulvophyceae (green algae). The genus ''Cladophora'' contains many species that are very hard to tell apart and classify, mainly because of the great variation in their appearances, which is a ...
'' and ''
Chaetomorpha ''Chaetomorpha'' is a genus of green algae in the family Cladophoraceae.Guiry, M. D. (2004)''Chaetomorpha''.''In:'' Guiry, M. D. & Guiry, G. M. (2017). AlgaeBase. Accessed through: World Register of Marine Species on 9 August 2017. Members of thi ...
'' (Robinson et al. 1972). Solution
x-ray scattering X-ray scattering techniques are a family of non-destructive analytical techniques which reveal information about the crystal structure, chemical composition, and physical properties of materials and thin films. These techniques are based on observ ...
have shown that CesAs are at the surface of a plant cell and are elongated monomers with a two catalytic
domain Domain may refer to: Mathematics *Domain of a function, the set of input values for which the (total) function is defined ** Domain of definition of a partial function ** Natural domain of a partial function **Domain of holomorphy of a function * ...
s that fuse together into
dimer Dimer may refer to: * Dimer (chemistry), a chemical structure formed from two similar sub-units ** Protein dimer, a protein quaternary structure ** d-dimer * Dimer model, an item in statistical mechanics, based on ''domino tiling'' * Julius Dimer ( ...
s. The center of the dimers is the main point of catalytic activity, and the lobes are presumed to contain the plant specific PC-R and CS-R. 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. Compared to the bacterial enzyme, plant versions of the synthase are much harder to crystallize, and as of August 2019 no experimental atomic structures of the plant cellulose synthase catalytic domain is known. However, at least two high-confidence structures have been predicted for these enzymes. The broader of the two structures (Sethaphong 2013), which includes the entire middle cytoplasmic domain (again sandwiched between TM helices), gives a useful view of the enzyme: two plant-specific insertions called the PC-R (plant-conserved region, similar in all plants) on the N-terminal end and CS-R (class-specific region, determines the subclass number after CesA) on the C-terminal end punctuate the usual GT catalytic core, probably providing the unique rosette-forming function of plant CesA. (Some CesA proteins possess an additional insertion.) The structure seems to explain the effects of many known mutations. The positioning of the two insertions, however, do not match the scattering result from Olek 2014. An 2016 experimental model of the PC-R () domain helps to fill in this gap, as it greatly improves the fit against Olek's previous result. It also matches the Sethaphong 2015 prediction of an antiparallel coiled-coil well. The two groups continue to further their understandings of the ''CesA'' structure, with Olek et al focusing on experimental structures and Sethaphong et al focusing on plant studies and building better computer models. Other differences from the bacterial ''BcsA'' includes a different TM helice count (''BcsA'' has 4 helices on each end; ''CesA'' has two on the N-terminal and 6 on the C-terminal), and the presence of a zinc finger () at the N-terminus.


Activity

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 The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (t ...
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.


Supporting structures

Microfibril synthesis is guided by cortical
microtubules Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27  nm and have an inner diameter between 11 a ...
, 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.


Environmental influences

Cellulose synthesis activity is affected by many environmental stimuli, such as hormones, light, mechanical stimuli, nutrition, and interactions with the
cytoskeleton The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is co ...
. 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.


References


Further reading

* {{Portal bar, Biology, border=no EC 2.4.1 Enzymes of known structure