Plant cell wall constituents, importance

In addition to the 20 common amino acids, proteins may contain residues created by modification of common residues already incorporated into a polypeptide (Fig. 3-8a). Among these uncommon amino acids are 4-hydroxyproline, a derivative of proline, and 5-hydroxylysine, derived from lysine. The former is found in plant cell wall proteins, and both are found in collagen, a fibrous protein of connective tissues. 6-N-Methyllysine is a constituent of myosin, a contractile protein of muscle. Another important uncommon amino acid is y-carboxyglutamate, found in the bloodclotting protein prothrombin and in certain other proteins that bind Ca2+ as part of their biological function. More complex is desmosine, a derivative of four Lys residues, which is found in the fibrous protein elastin. 

Structural polysaccharides Important constituents of plant cell wall. 

Keto sugar derivatives probably occur as intermediates since the reaction is NAD+-dependent. The pentoses formed may be polymerized to yield arabanes and xylanes which are important cell wall constituents of plants (D 1.4.1). Together with polyuronides (see above) they form the group of hemicelluloses, in which cellulose fibrils (D 1.4.1) are embedded. 

Carbohydrates are essential components of cell walls in animals and in plants, form the scaffold for DNA and RNA, and are found in many biologically important compounds. Stereochemistiy is an essential component of carbohydrates in biological systems. This point is illustrated with two common polysaccharides cellulose (81), which is a principal constituent of plant cell walls, and amylose (82), which is a component of starch (see Chapter 28, Section 28.2) ... 

Many plant biotechnological strategies are directed towards introducing fundamental changes in Ae properties of (woody) plant cell walls and their biopolymers. Alternatively, other approaches attempt to alter levels of specific constituents such as those imparting defense functions. The purposes of such manipulations are manifold To optimize wood properties for lumber and paper production, to produce new biopolymers which are expected to be valuable as replacements for petroleum-derived polymers, to alter nutritional qualities of foodstuffs, to increase the production of pharmacologically important compounds, and to either introduce or enhance novel defense functions in plants (i.e., antioxidants, biocidal properties and the like). Such approaches, in fact, represent exploitation of ongoing evolutionary processes by calculated human intervention. 

Figure 9.12 Deoxy derivatives. These contain one less oxygen atom than the monosaccharide from which they are derived. 2-Deoxyribose is a most important deoxy pentose and is a major constituent of deoxyribonucleic acid (DNA). Deoxy hexoses are widely distributed among plants, animals and microorganisms especially as components of complex polysaccharides. Examples are rhamnose (6-deoxymannose), a component of bacterial cell walls, and fucose (6-deoxygalactose), which is often found in glycoproteins and is an important constituent of human blood group substances.

Carbohydrates are among the most abundant constituents of plants, animals, and microorganisms. Polymeric carbohydrates function as important food reserves, and as stmctural components in cell walls. Animals and most microorganisms are dependent upon the carbohydrates produced by plants for their very existence. Carbohydrates are the first products formed in photosynthesis, and are the products from which plants S5mthesize their own food reserves, as well as other chemical constituents. These materials then become the foodstuffs of other organisms. The main pathways of carbohydrate biosynthesis and degradation comprise an important component of... 

The nature and amounts of low molecular weight phenolic constituents in cell walls of graminaceous plants (grasses and cereals) are reviewed and relationships discussed between these constituents and wall biodegradability. The formation in cell walls of 4,4 -dihydroxy-truxillic acid and other cyclodimers of p-coumaric and ferulic acid is suggested as an important mechanism for limiting the biodegradability of wall polysaccharides. 

Pectin occurs in most plant materials but is particularly high in young and fruit tissues. It is an important constituent of the cell wall where it may be involved in interlinking of other structural polysaccharides and proteins (Ij). Pectin is also the major component of the middle lamella which has led to the classical view that it is an intercellular adhesive. 

Cellulose, (C iHjOjoln, the most important glucose polymer (1—4) -P-D-glucan), is found in grass and all higher plants where it is an essential constituent of cell walls. It is the most abundant organic compound in the world and is the chief constituent of wood ( 50%) and of cotton (>95%). 

The teichoic acids, discovered by Baddiley around 1950, are important constituents of the cell walls of plants. They also occur in (Gram-positive) bacteria and in cell walls generally, and all are highly... 

Phenolic compounds are important bioactive constituents derived from plants and are active ingredients in traditional medicine, pharmaceuticals, and nutraceuticals. Extraction process for phenolic acids requires standardization as within plant tissue they can stay either in the soluble fraction or as conjugates with carbohydrates or linked to cell wall components. HPLC is by far the most popular method of separation. Mass spectrometers, especially LC-MS and MALDI-TOF, provide a viable rapid and accurate method of identification and characterization of the simple phenolic acids. 

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