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Varki A, Cummings RD, Esko JD, et al., editors. Essentials of Glycobiology
Chapter 24Viridiplantae and Algae
Malcolm A. O"Neill, Alan G. Darvill, Marilynn E. Etzler, Debra Mohnen, and Serge Perez.
Published online: 2017.
Recent research on plant glycan framework and function has frequently emphasized model plants such as Arabidopsis and also plants of commercial importance. However, over there is boosting interest in examining the glycans produced by plants from all the significant orders that the Viridiplantae. Together studies, along with the ease of access of transcriptomic data for many green algae and also land plants, have begun to disclose a affluent diversity in glycan structures and also insight into just how some of these frameworks have adjusted during the development of the Viridiplantae. In this chapter, we administer an synopsis of the current knowledge of green plant glycan frameworks with an emphasis on the functions that are unique to land plants.
Viridiplantae (green plants) room a clade of photosynthesis organisms the contain chlorophylls a and also b, produce and also store their photosynthetic products inside a double-membrane-bounded chloroplast, and have cell wall surfaces that typically contain cellulose. The Viridiplantae are comprised of 2 clades—the Chlorophyta and also the Streptophyta. The Chlorophyta contain most of the organisms generally referred to as “green algae.” The term “algae” is likewise used for several other groups of photosynthesis eukaryotes, consisting of diatoms and also the red, brown, golden, and yellow-green algae. The Streptophyta comprise several various other lineages the are also referred to together “green algae” and the soil plants. Floor plants include the liverworts, mosses, hornworts, lycopods, ferns, gymnosperms, and also flowering plants.
PLANT GLYCAN DIVERSITY
Green tree synthesize diverse glycans that vary in their structural complexity and also molecular size. Raffinose oligosaccharides (raffinose, stachyose, and also verbascose) are virtually ubiquitous in the plant kingdom and also rank second only to sucrose in abundance together soluble carbohydrates. This oligosaccharides space derivatives the sucrose that contain one or an ext α-Gal residues. Plants also synthesize numerous low molecular load glycoconjugates that contain either fragrant (e.g., phenolic glycosides) or aliphatic (glycolipids) aglycones.
Plant polysaccharides are straight or branched polymers written of the same or different monosaccharides. For example, cellulose is created of 1-4-linked β-D-Glc residues (Figure 24.1A), whereas the structurally complicated plant cell wall pectic polysaccharide, described as rhamnogalacturonan II (RG-II), has 12 different monosaccharides connected together by up to 21 different glycosidic linkages (Figure 24.2). Tree proteoglycans are structurally diverse glycans in which carbohydrate account for as much as 90% of the molecule and also is O-linked come the protein via hydroxyamino acids (Figure 24.3). Plant glycoproteins generally contain 15% or less of carbohydrate in the kind of N-linked oligomannose, complex, hybrid, and also paucimannose oligosaccharides (see figure 24.4). Land tree also kind O-GlcNAc-modified nuclear and cytosolic proteins (Chapter 19).
Glycosyl sequences of cellulose and selected hemicelluloses existing in plant cell walls.
NUCLEOTIDE SUGARS—THE building BLOCKS
Nucleotide sugars are the donors supplied for synthesis of glycoconjugates and glycosylated secondary metabolites (Chapter 5). In plants, the majority of these activated monosaccharides exist together their nucleotide-diphosphates (e.g., UDP-Glc), although at least one monosaccharide, Kdo, exists together its cytidine monophosphate derivative (CMP-Kdo). Nucleotide street are created from the carbohydrate generated by photosynthesis, indigenous the monosaccharides exit by hydrolysis that sucrose and storage carbohydrates, and also by recycling monosaccharides indigenous glycans and the cell wall. Nucleotide sugars are additionally formed by interconverting preexisting caused monosaccharides. To date, 30 different nucleotide sugars and also at least 100 gene encoding proteins affiliated in your formation and interconversion have been identified in plants.
PLANT GLYCOSYLTRANSFERASES and GLYCAN-MODIFYING ENZYMES
Plant genomes contain countless genes encoding proteins associated in the synthesis and modification of glycans. This proteins space spread across many enzyme class in the Carbohydrate-Active enzyme (CAZy) database (Table 24.1). Countless of this proteins may be associated in the formation and modification of the polysaccharide-rich cell wall. Indeed, the unicellular alga Ostreococcus tauri, which is just one of the couple of plants the does not kind a cell wall, has a much smaller number of genes predicted come be connected in glycan metabolism.
Estimated number of genes encoding proteins associated in the synthetic and change of glycans in plants and humans
PLANT cell WALLS
A substantial section of the carbohydrate developed by photosynthesis is used to produce the polysaccharide-rich wall surfaces that surround tree cells. Major and an additional cell walls are distinguished by their composition, architecture, and also functions. A primary wall surface surrounds growing and also dividing plant cells and nongrowing cells in the soft organization of fruits and also leaves. These walls are qualified of controlled extension to permit the cell to grow and expand yet yet are sufficiently strong to resist the cells interior turgor pressure. A lot thicker and stronger an additional wall is often formed once a cell has ceased to grow. This wall surface is deposited between the plasma membrane and also the primary wall surface and is created of layers that differ in the orientation of your cellulose microfibrils. The secondary walls that vascular tissues connected in the activity of water and nutrients with the tree are further strengthened by the organization of lignin. The capability to form conducting tissues through lignified and also rigid second walls was an indispensable occasion in the evolution of vascular land plants, as it assisted in the transfer of water and also nutrients and allowed extensive upright growth. An additional cell walls account for many of the carbohydrate in tree biomass being considered as feedstock for the manufacturing of biofuels and value-added chemistry (Chapter 59).
PRIMARY CELL wall surface GLYCANS
Primary cabinet walls space composites the resemble fiber-reinforced porous gels. The complex structures and also functions of these walls an outcome from the assembly and interactions of a restricted number of structurally identified polysaccharides and proteoglycans. Wall structure and also organization may adjust during cell division and breakthrough and in solution to biotic and abiotic difficulties by the differential synthetic and alteration of the noncellulosic contents or by the enhancement of new components.
Primary wall surfaces of land plants contain cellulose, hemicellulose, and pectin, in different proportions. They also contain structural proteins/proteoglycans, enzymes, phenolics, and also minerals. Pectin and hemicellulose are existing in about equal amounts in the so-called kind I main walls the gymnosperms, dicots, and nongraminaceous monocots, conversely, hemicellulose is far more abundant 보다 pectin in the type II walls of the grasses. Much much less is known about the ingredient of the walls of avascular plants and green algae, although it is likely that the walls of these plants contain cellulose, pectin, and also hemicellulose.
Cellulose, the most abundant biopolymer in nature, is a direct polymer composed of 1-4-linked β-D-Glc residues (Figure 24.1A). Several of this chains space hydrogen bonded to one an additional to form paracrystalline microfibrils. Every microfibril is predicted to contain between 18 and also 24 glucan chains. The glucan chain is synthesized by a cellulose synthase (CESA) complicated that exists together a hexameric rosette structure on the cells plasma membrane. Three CESAs, encoded through three different genes, are thought to communicate to form a trimeric complex, which in turn assembles into a hexameric rosette. The catalytic website of each cellulose synthase is located in the cytosol and transfers glucose native UDP-Glc onto the elongating glucan chain. The mechanisms associated in the formation of a microfibril from separation, personal, instance glucan chains room not fine understood, return it may involve a self-assembly procedure that is assisted in by details proteins. The newly formed microfibrils are then deposited in the wall surface of a growing cell with an orientation the is transverse to the axis the elongation. This orientation may be guided in part by protein-mediated interactions between CESA proteins and also cortical microtubules.
Naturally occurring cellulose is a mixture of 2 crystalline forms, Iα and Iβ, together with surface chains and less crystalline material. The 1α and 1β polymorphs differ mostly in the packing setup of their hydrogen-bonded sheets. Countless properties of native cellulose depend on interaction that occur at the surface ar of the microfibrils. The surface chains are easily accessible and reactive vice versa, the hydroxyl teams of the inner chains in the crystal participate in considerable intra- and also intermolecular hydrogen bonding. Cellulose is insoluble in water and also somewhat resistant come hydrolysis by endo- and also exoglucanases thus highly packed arrangement of the glucan chains.
Several varieties of enzymes including endoglucanases cellobiohydrolases and β-glucosidases are required for cellulose depolymerization. Plenty of of this enzymes covers a catalytic domain linked to a cellulose-binding module. This module facilitates binding that the enzyme to the insoluble substrate. Part microorganisms also produce copper-dependent oxidases that render crystalline cellulose more susceptible come hydrolysis. Cellulases and other enzymes connected in cellulose hydrolysis often exist as macromolecular complexes described as cellulosomes. Boosting the performance of cellulosomes is an area of energetic research, to increase the switch of plant biomass come fermentable sugar (Chapter 59).
Hemicelluloses room branched polysaccharides through a backbone composed of 1-4-linked β-D-pyranosyl residues through an equatorial O-4 (Glc, Man, and Xyl). Xyloglucan, glucuronoxylan, arabinoxylan, and glucomannan (Figure 24.1B–E) are contained under this chemical meaning of hemicelluloses. Hemicelluloses and also cellulose have actually structural and also conformational similarity that permit them to kind strong, noncovalent associations with one one more in the cabinet wall, although the organic significance of this interactions is a topic of debate.
Xyloglucans are differentiated from one another by the variety of 1-4-linked β-D-Glcp backbone residual water that space branched. XXXG-type xyloglucans are composed of subunits in which 3 consecutive backbone residual water bear an α-Xyl substituent at O-6 and a fourth, unbranched backbone residue (Figure 24.1B). Each Xyl residue (X next chain) might itself be extended by the enhancement of one or much more monosaccharides consisting of β-D-Gal, α-L-Fuc, α-L-Ara, and also β-D-GalA. Eighteen structurally distinctive side chains have been figured out to date, although only a subset of these are synthesized by a single plant species. XXXG-type xyloglucan is current in the major walls the hornworts, lycopods, ferns, gymnosperms, a diverse range of dicots and all monocots v the exemption of the grasses. XXGG-type xyloglucans, which have only two consecutive backbone residual water bearing one α-D-Xyl substituent at O-6, are current in the main walls of the grasses, part Solanaceae, mosses, and liverworts. The Xyl may be expanded by the enhancement of Gal, Ara, or GalA, yet rarely, if ever, through Fuc.
Early models the dicot main walls predicted that xyloglucan acted together tethers between cellulose microfibrils and that controlled enzymatic cleavage the the xyloglucan facilitated wall expansion and thus plant cabinet growth. However, the identification of an Arabidopsis mutant that is can not to synthesize xyloglucan yet shows close to normal development and development challenged this notion and also led come the ide that pectin may have a more important role in controlling wall expansion than previously believed.
Glucuronoarabinoxylan (GAX, number 24.1C) is the predominant noncellulosic polysaccharide in the kind II wall surfaces of the grasses. The backbone is composed of 1-4-linked β-D-Xyl residues, countless of which space substituted at O-3 through α-L-Araf residues. This Araf residues might be further substituted in ~ O-2 through an α-L-Araf or a β-D-Xylp residue. A small variety of the backbone residues are substituted at O-2 with α-D-GlcpA and also its 4-O-methylated equivalent (MeGlcpA).
The presence of 1-3, 1-4-linked β-glucans (also referred to as mixed-linkage glucans) in the walls of grasses to be once considered to it is in a distinct feature of these plants. However, structurally connected mixed-linkage β-glucans have additionally been identified in the walls of Selaginella (lycopod) and also Equisetum (horsetails), although the evolution relationship between these β-glucans is not known.
Pectins space structurally complicated polysaccharides the contain 1-4-linked α-GalA. Three structurally unique pectins—homogalacturonan, substituted galacturonan, and rhamnogalacturonan—have been established in plant cell walls (Figure 24.2). Homogalacturonan, which might account for as much as 65% of the pectin in a main wall, is written of 1-4-linked α-GalA. The carboxyl team may it is in methyl-esterified and also the glycose itself might be acetylated at O-2 or O-3. The extent of methyl-esterification is regulated by pectin methyl-esterases existing in the wall surface and affects the capability of homogalacturonan-containing glycans to form ionic calcium cross-links through themselves and with various other pectic polymers. Together interactions change the mechanical properties that the wall and may influence plant growth and development.
Rhamnogalacturonans are polysaccharides with a backbone composed of GalA and rhamnose (Rha) residual water in the repeating disaccharide 4-α-D-GalpA-1-2-α-L-Rhap-1. Plenty of of the GalAs are acetylated in ~ O-2 and/or O-3. Relying on the plant, between 20% and also 80% that the Rha residues may be substituted at O-4 with straight or branched next chains composed mainly of Ara and Gal, together with smaller quantities of Fuc and GlcA (Figure 24.2). Little is known about the attributes of these next chains and also their donation to the nature of the major wall.
Substituted galacturonans have a backbone composed of 1-4-linked α-D-GalA acid residues that room substituted to varying levels with mono-, di-, or oligosaccharides. Xylogalacturonans contain solitary β-Xyl residues attached to O-3 of some of the backbone residues (Figure 24.2) whereas apiogalacturonans have β-D-apiose (Api) and apiobiose connected to O-2 of some of the backbone residues. Apiogalacturonans have only been determined in the walls of duckweeds and seagrasses.
Rhamnogalacturonan-II, which account for between 2% and 5% that the primary cell wall, is the most structurally complicated polysaccharide yet established in nature. The is written of 12 different monosaccharides linked together by approximately 21 various glycosidic linkages (Figure 24.2). 4 structurally different side chains room attached come the galacturonan backbone. 2 structurally conserved disaccharides (side chains C and also D) are linked to O-2 of the backbone. The A and also B next chains, i m sorry contain between 7 and also 9 monosaccharides, are attached to O-3 of the backbone. Numerous of the monosaccharides in RG-II are O-methylated and/or O-acetylated.
Virtually every one of the RG-II exists in the primary wall as a dimer cross-linked by a borate ester. The ester is formed between the Api residue in next chain A of each RG-II monomer (Figure 24.2). The dimer creates rapidly in vitro as soon as the RG-II monomer is reacted v boric acid and a divalent cation. However, the mechanism and site that dimer formation in planta has actually not been determined. Borate cross-linking the RG-II is likely to have an extensive effects top top the nature of pectin and the primary wall surface as RG-II is itself linked to homogalacturonan (Figure 24.2). Indeed, mutations that influence RG-II structure and also cross-linking result in plants through abnormal walls and also severe growth defects. Swollen major walls and also abnormal growth in addition to reduced RG-II cross-linking are additionally a properties of boron deficient plants.
Pectin is believed to exist in the cell wall as a macromolecular complex comprised of structure domains—homogalacturonan, rhamnogalacturonan, and substituted galacturonan—that room covalently linked to one another. However, there is just a restricted understanding of how these structural domain names are organized (Figure 24.2). Molecular modeling that a pectin (∼50 kDa) containing homogalacturonan (degree the polymerization ∼100) and also rhamnogalacturonan with arabinogalactan side chains, along with modeling of RG-II construction have started to provide insights into the conformations and also relative size of each pectin structural domain.
The conformation of the homogalacturonan chain is mostly unaffected through the conformation in ~ the glycosidic linkage, by transforms in its level of methyl-esterification or the existence of respond to ions. The homogalacturonan region has a persistence size of roughly 20 GalA residues, i m sorry is likely to be enough to stabilize junction zones developed with Ca++. In vitro studies indicate that the maximum stability of such junction area is obtained with oligomers containing around 15 nonesterified GalA residues. Thus managing the circulation of methyl-ester groups along the homogalacturonan backbone offers a system to manage the physical properties that pectin, consisting of its capability to type gels. Because that a gelatin to type and no to be brittle, other features including sequences the interrupt interchain associations in the pectin macromolecule may be important. For example the structural diversity and the conformational flexibility of the oligosaccharide next chains the the rhamnogalacturonan domain will certainly limit or protect against interchain pairing. The existence of 1-2-linked Rha residual water does not introduce kinks right into the backbone geometry that rhamnogalacturonan and also thereby border interchain associations. Rather, it is the side chains linked to this residues that are responsible for staying clear of or limiting interchain associations.
The conformations the the 4 side chain attached to the homogalacturonan backbone might lead RG-II to take on a “disk-like” shape. Well-defined tertiary structures room predicted for the monomer and the dimer. In the dimer, borate-ester cross-linking and also Ca++ interchain pairing further stabilizes the two disks. The apparent resistance the RG-II come wall-modifying enzymes in addition to the development of a cation-stabilized RG-II dimer likely results in a framework that resists temporal changes. In contrast, homogalacturonan is continually amendment by the action of wall surface enzymes and also its donation to wall architecture is because of this time dependent.
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Increased understanding of the physical properties the primary wall polysaccharides and also proteoglycans is forced to understand how modulating the amounts and structural functions of a few common polysaccharides and glycan domain names lead to main walls with varied properties and functionalities. Additional research is likewise needed to identify if wall structure and role results native the noncovalent interactions of polysaccharides and also proteoglycans or native the development of glycan-containing architectural devices with details structural and functional roles. The later scenario is analogous come the organization of proteoglycans and also O-linked mucins in the extracellular procession of pet cells (Chapters 10, 16, and also 17).