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Cell Wall Polysaccharides
Ata Ullah, Lutufur Rahman, Muhammad Bilal Yazdani, Muhammad Irfan, Waheed S. Khan and Asma Rehman*
National Institute for Biotechnology and Genetic Engineering, Faisalabad, Pakistan
Abstract
Polysaccharides are considered the most abundant biomolecules in nature and are probably found in all organisms on the earth. It has diverse applications in living organisms ranging from their structure to biological signaling. Commercially, the polysaccharides have gold-standard value in pharmaceutical, paper, cosmetic and food industries. There are different sources of polysaccharides, but the cell walls of plants, algae, fungi and somehow bacteria are considered the best reservoirs of polysaccharides. The cell wall polysaccharide includes monosaccharides, disaccharides, oligosaccharides and other lipoglycans and proteoglycans. The current chapter will be focused on the cell wall polysaccharides of plants, green algae, fungi, and bacteria.
Keywords: Polysaccharides, biomolecules, cell wall, plants, microbes, structure, protection industry
2.1 Introduction to Cell Wall
Cell wall is a specialized extracellular matrix (ECM) which surrounds every cell of plant, algae, fungi and bacteria. It gives some distinguishable characteristics to the plant cells comparatively to the animal cells. Specifically, the cell wall mainly serves as the mechanical and structural support to the cells. In addition, the cell wall has multitudinous functions such as, it gives mechanical protection to living cells in adverse environment, provide porous medium for the circulation of water mineral and nutrients, provide stable physical structure to cell and act as site of storage for regulatory molecules that sense the pathogenic microbes [1–3].
The cell walls of various organisms have different compositional structures. Such as, the algal cell wall is similar to that of plant cell wall and both contain specific polysaccharides which are helpful in their taxonomy. Similarly, the fungal cell wall completely lacks polysaccharides and contain chitin. On the other hand, bacterial cell wall is distinguished by the presence of peptidoglycan while archaea is lacking this chemical composition. Difference in the compositional structure of cell wall helps in the taxonomy and classification of these organisms. So, the current chapter is exclusively focusing on the cell wall polysaccharides of plant, algae, fungi, bacteria and archaea [4, 5].
2.2 Plant Cell Wall Polysaccharides
The cell wall of the plant cell is a highly organized network that can vary in the cellular lifespan. Primary cell wall of the plant cell is newly synthesized through cellular division and their size dramatically increases as the cell grows. The outermost layer of the plant cell wall is known as middle lamella, which is considered as interface between the primary cell wall with other cells. In many cells, the secondary cell wall also developed with complex structure that impregnated with lignin. The plant cell wall may be imparting in structural integrity to support plasma membrane and sense external stimuli and mediate cellular signaling [6, 7].
The main components of plant cell wall including heterogenous mixture of polysaccharides, proteins, aromatic substances and water with trace numbers of ions. Specifically, the tensile strength of the cell wall of plant cells is due to cellulose, hemicellulose, pectic substances and their mutual interactions. Among the polysaccharide’s cellulose, hemicellulose and pectin are the most prominent composition of the cell wall with 30, 30 and 35%, abundance respectively. Cellulose and hemicellulose are responsible for rigidity in cell wall while pectin induces fluidity through gelatinous polysaccharides network (Figure 2.1) [2, 8]. The cellulose and hemicellulose embedded in the pectic are stabilized through proteins and phenolic compounds. Moreover, the hemicellulose attaches to the surface of cellulose channel to prevent the direct contact among the cellulose microfibrils while pectin is attached to hemicellulose to form gel phase [9–11]. Herein, different plant cell wall polysaccharides will be discussed.
2.2.1 Cellulose
It is considered the most frequently found polysaccharides in the nature which comprising 200–400 g/kg of the plant tissues. It is water insoluble, linear, unbranched structural polysaccharides with β-(1,4)-D-Glucose residues which are insoluble in water. Each D-glucose unit is rotated at 180° to its neighbor in a helical chain with length of 2,000–8,000 glucose units. Similarly, ~150 helical cellulose chain combined with each other through hydrogen binding for highly crystalline structure known as microfibrils. These microfibrils are approximately 3.6 nm wide and 100 nm apart without any gap, thus provide high mechanical strength and resistance to chemical and enzymatic digestion. The microfibrils have also some amorphous regions, that depend upon their source. Additionally, the algal cellulose has highly crystalline structure while the plants cellulose has least crystalline [5, 12].
Figure 2.1 The illustration depicts the cell walls of plant cell. The plant cell wall composed of cellulose microfibrils that have been cross linked with each other through glycans, pectin and other substances.
2.2.2 Hemicellulose
Hemicellulose is another heterogenous polysaccharide in plant cell wall with β (1–4) linked backbones of glucose, mannose, or xylose sugars. The term hemicellulose was coined at the archaic time when their structure and biosynthesis were not fully understood. By nature, it was not cellulose nor pectin, therefore the researchers have suggested the name of hemicellulose. Recently, the different features and structural properties of the hemicellulose has been explored but the term is still used by the researchers refereeing to the cell wall polysaccharides [13].
Hemicelluloses polysaccharides includes xylans, glucomannans, xyloglucans, mannans and β (1–3)–(1–4) glucans with the C1 and C4 equatorial configuration in their backbone. All these types of hemicelluloses are present in the cell wall terrestrial plants, expect of β (1–3)–(1–4) glucans, which only present in Poales (Order of flowering plants in the monocotyledons). The structure and abundance of the hemicellulose can vary among different special and cell wall types. The biological function of the hemicellulose is the strengthening of the cell wall of plant cells via deposition with lignin and cellulosic microfibrils [14].
2.2.2.1 Xyloglucan
Xyloglucan (XyG) is the most frequently occurring hemicellulose in the primary cell wall of the all land plants including mosses but not yet been found in the grasses and Charophytes. The hemicellulosic XyG have branched structure with alpha-D-xylose that is linked to C6 of glucose molecule. The XyG may be have β-D-galactose with less amount of L-fucose, (1–2) α-D-galactose and the galactose residues have been attached with acetyl moiety. In addition, the substituents of XyG are highly conserved and regulated during its biosynthesis. XyG is not found in the secondary cell wall although it plays a role in the interlacing of microfibrils in primary cell wall. Similarly, XyG plays an important role in the regulation of cell growth and expansion, particularly in the combination with enzyme such as xyloglucan endotransglycosylase. Most interestingly, the degraded XyG shows an anti-auxin effect, thus plays a pivotal role in cellular communication. It also helps in fruit maturing and fruit-ripening related softening [15–17].
2.2.2.2 Xylans
Xylans are also ranked as extensively found polysaccharide. It is abundantly found in the cereals, straw, sorghum, hardwood and grasses. Xylans are thought to be linked to the microfibrils of cellulose to contribute in their tensile strength properties, especially in