POLYSACCHARIDES
Polysaccharides, which also are referred to as glycans, contain monosaccharides linked together by glycosidic bonds. they're classified as homopolysaccharides or heteropolysaccharides if they contain one type or quite one sort of monosaccharide residue. Homopolysaccharides could also be further classified consistent with the identity of their monomeric unit. For instance, glucans are polymers of glucose, whereas galactans are polymers of galactose. Although monosaccharide sequences of heteropolysaccharides can, in theory, be as varied as those of proteins, they're usually composed of only a couple of sorts of monosaccharides that alternate during a repetitive sequence.
Polysaccharides, in contrast to proteins and nucleic acids, form branched also as linear polymers. This is often because glycosidic linkages can be made to any of the hydroxyls of a monosaccharide. Fortunately for structural biochemists, many polysaccharides are linear and people that branch tend to try to do so in just a couple of well-defined ways.
A. Carbohydrate Analysis
The purification of carbohydrates can, by and enormous, be affected by chromatographic and electrophoretic procedures almost like those utilized in protein purification, although thin layer chromatography is additionally widely used. Affinity chromatography, using immobilized proteins referred to as lectins (Latin: Legere, to select or choose), maybe a particularly powerful technique in this regard. Lectins are sugar-binding proteins that were discovered in plants but are now known to occur altogether organisms, where they participate during a big variety of signaling, cell-cell recognition, and adhesion processes, also as in targeting newly synthesized proteins to specific cellular locations. Lectins recognize one or more specific monosaccharides with particular linkages to other sugars in oligosaccharides, usually with exquisite specificity. Their protein-carbohydrate interactions typically include multiple hydrogen bonds, which frequently include bridging water molecules, and therefore the packing of hydrophobic sugar faces against aromatic side chains. Among the simplest characterized lectins is wonder bean concanavalin A, which specifically binds -D-glucose and -D-mannose residues, and nutriment agglutinin (so named because it causes cells to agglutinate or clump together), which specifically binds -N-acetylmuramic acid and -N-acetylneuraminic acid. Characterization of an oligosaccharide requires that the identities, anomers, linkages, and order of its component monosaccharides be elucidated. The linkages of the monosaccharides could also be determined by methylation analysis (also called permethylation analysis), a way pioneered by Norman Haworth in the 1930s.
Although all aldoses and ketoses with an equivalent number of C atoms are isomers and hence have identical molecular masses, they need characteristic fragmentation patterns. The sequence and anomeric configurations of the monosaccharides in an oligosaccharide are often determined through the utilization of specific exoglycosidases. These enzymes specifically hydrolyze their corresponding monosaccharides from the nonreducing ends of oligosaccharides (the ends lacking a free anomeric carbon atom) during a manner analogous to the actions of exopeptidases on proteins. For instance, -galactosidase excises the terminal anomers of galactose, whereas -mannosidase does so with the anomers of mannose. A number of these exoglycosidases also exhibit specificity for the aglycone, the sugar chains to which the monosaccharide to be excised (the glycone) are linked. Through the utilization of mass spectrometry, the sequence of a polysaccharide could also be deducted from the mass decrements generated by exoglycosidases. the utilization of endoglycosidases (hydrolases that cleave glycosidic bonds between nonterminal sugar residues) of varying specificities also can supply useful sequence information. The proton and 13C NMR spectra of oligosaccharides can provide the entire sequence of an oligosaccharide if sufficient material is out there. Moreover, two-dimensional NMR techniques can reveal oligosaccharide structure.
B. Disaccharides
Sucrose, the foremost abundant disaccharide, occurs throughout the Plantae and is familiar to us as common table sugar. Its structure was established by methylation analysis as described above and was later confirmed by its X-ray structure. To call a polysaccharide systematically, one must specify its component monosaccharides, their ring types, their anomeric forms, and the way they're linked together. Sucrose is, therefore, O- -D-glucopyranosyl-(1 S 2)- -D-fructofuranoside, where the symbol (1 S 2) indicates that the glycosidic bond links C1 of the glucose residue to C2 of the fructose residue. Note that since these two positions are the anomeric carbon atoms of their respective monosaccharides, sucrose isn't a reducing sugar (as the suffix -ide implies). The hydrolysis of sucrose to D-glucose and D-fructose is amid a change in optical rotation from Dextro to levo. Consequently, hydrolyzed sucrose is usually called carbohydrate, and therefore the enzyme that catalyzes this process, -D-fructofuranosidase, is archaically named invertase.
Lactose [O- -D-galactopyranosyl-(1 S 4)-D-glucopyranose] or lactose occurs naturally only in milk, where its concentration ranges from 0 to 7% counting on the species. The free anomeric carbon of its glucose residue makes lactose a reducing sugar. Infants normally express the intestinal enzyme -D galactosidase or lactase that catalyzes the hydrolysis of lactose to its component monosaccharides for absorption into the bloodstream. Many adults, however, including most Africans and most Asians, have a coffee level of this enzyme (as do most adult mammals, since they normally don't encounter milk). Consequently, much of the lactose in any milk they drink moves through their alimentary canal to the colon, where its bacterial fermentation produces large quantities of CO2, H2, and aggravating organic acids. This leads to an embarrassing and sometimes painful digestive upset termed lactase deficiency. Perhaps this is often why Chinese cuisine, which is noted for the big variety of foodstuffs it employs, is barren of milk products. However, adult members of populations with a practice of herding cattle, mainly northern Europeans and certain African groups continue expressing the lactase gene and hence can drink milk without a drag. Modern food technology has come to the help of milk lovers who develop lactose intolerance: Milk products during which the lactose has been hydrolyzed enzymatically and lactase-containing pills are now widely available.
There are several common glucosyl–glucose disaccharides. These include maltose [O- -D-glucopyranosyl-(1 S 4)- D-glucopyranose], an enzymatic hydrolysis product of starch; isomaltose, its (1 S 6) isomer; and cellobiose, its (1 S 4) isomer, the repeating disaccharide of cellulose. Only a couple of tri- or higher oligosaccharides occur in significant natural abundance. Not surprisingly, all of them occur in plants.
C. Structural Polysaccharides: Cellulose and Chitin
Plants have rigid cell walls that, so as to take care of their shapes, must be ready to withstand pressure differences between the extracellular and intracellular spaces of up to twenty atm. In large plants, like trees, the cell walls even have a load-bearing function. Cellulose, the first structural component of plant cell walls , accounts for over half the carbon within the biosphere: 1015 kg of cellulose are estimated to be synthesized and degraded annually. Although cellulose is predominantly of vegetable origin, it also occurs within the stiff outer mantles of marine invertebrates referred to as tunicates (urochordates).
The primary structure of cellulose is decided through methylation analysis. Cellulose may be a linear polymer of up to 15000 D-glucose residues (a glucan) linked by (1 S 4) glycosidic bonds. As is usually true of huge polysaccharides, it's no defined size since, in contrast to proteins and nucleic acids, there's no genetically determined template that directs its synthesis.
X-ray studies of cellulose fibers led Anatole Sarko to tentatively propose the model. This highly cohesive, hydrogen-bonded structure gives cellulose fibers exceptional strength and makes them water-insoluble despite their hydrophilicity. In-plant cell walls, the cellulose fibers are embedded in and cross-linked by a matrix of several polysaccharides that are composed of glucose also as other monosaccharides. Inwood, this cementing matrix also contains an outsized proportion of lignin, a plastic-like phenolic polymer. One has only to observe a tall tree during a wind to understand the big strength of plant cell walls. In engineering terms, they are “composite materials,” as is concrete reinforced by steel rods. Composite materials can withstand large stresses because the matrix evenly distributes the stresses among the reinforcing elements.
Although vertebrates themselves don't possess an enzyme capable of hydrolyzing the (1 S 4) linkages of cellulose, the digestive tracts of herbivores contain symbiotic microorganisms that secrete a series of enzymes, collectively referred to as cellulose, that do so. An equivalent is true of termites. Nevertheless, the degradation of cellulose may be a slow process because its tightly packed and hydrogen-bonded glucan chains aren't easily accessible to cellulase and don't separate readily even after many of their glycosidic bonds are hydrolyzed. The digestion of fibrous plants like grass by herbivores is that therefore a more complex and time-consuming process than is the digestion of meat by carnivores (cows, e.g., have multi-chambered stomachs and must chew their cud). Similarly, the decay of dead plants by fungi, bacteria, and other organisms, and therefore the consumption of wooden houses by termites, often takes years.
Chitin is that the principal structural component of the exoskeletons of invertebrates like crustaceans, insects, and spiders and is additionally a serious cell membrane constituent of most fungi and lots of algae. it's estimated that 1014 kg of chitin are produced annually, most of it within the oceans, and thus that it's almost as abundant as is cellulose. Chitin may be a homopolymer of (1 S 4)-linked N-acetyl-D-glucosamine residue. It differs chemically from cellulose only therein each C2-OH group is replaced by an acetamido function. X-ray analysis indicates that chitin and cellulose have similar structures.
References :
1. Biochemsitry 4th edition by Donald Voet and Judith G. Voet .
2. The image is from freepik.com.
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