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You are watching: Two glucose molecules joined together form

Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. Brand-new York: W H Freeman; 2002.


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Because street contain countless hydroxyl groups, glycosidic bonds can join one monosaccharide come another. Oligosaccharides are developed by the affiliation of two or more monosaccharides by O-glycosidic bonds (Figure 11.10). In maltose, for example, two d-glucose residues space joined by a glycosidic linkage in between the α-anomeric form of C-1 top top one sugar and also the hydroxyl oxygen atom on C-4 that the surrounding sugar. Together a linkage is called an α-1,4-glycosidic bond. The fact that monosaccharides have multiple hydroxyl groups means that various glycosidic linkages room possible. Indeed, the vast array of these linkages in concert v the wide selection of monosaccharides and their plenty of isomeric creates makes complex carbohydrates information-rich molecules.


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Figure 11.10

Maltose, a Disaccharide. Two molecules the glucose are connected by one α-1,4-glycosidic shortcut to form the disaccharide maltose.


11.2.1. Sucrose, Lactose, and also Maltose room the common Disaccharides

A disaccharide is composed of two sugars joined by one O-glycosidic bond. Three abundant disaccharides are sucrose, lactose, and also maltose (Figure 11.11). Sucrose (common table sugar) is obtained commercially indigenous cane or beet. The anomeric carbon atoms of a glucose unit and also a furustos unit space joined in this disaccharide; the construction of this glycosidic linkage is α for glucose and also β because that fructose. Sucrose deserve to be cleaved right into its component monosaccharides by the enzyme sucrase.


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Lactose, the disaccharide that milk, consists of galactose joined to glucose by a β-1,4-glycosidic linkage. Lactose is hydrolyzed to this monosaccharides by lactase in human beings (Section 16.1.12) and also by β-galactosidase in bacteria. In maltose, two glucose units room joined by one α-1,4 glycosidic linkage, as proclaimed earlier. Maltose originates from the hydrolysis the starch and is consequently hydrolyzed come glucose by maltase. Sucrase, lactase, and also maltase are situated on the outer surfaces the epithelial cells lining the tiny intestine (Figure 11.12).


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Figure 11.12

Electron Micrograph the a Microvillus. Lactase and other enzymes the hydrolyze carbohydrate are current on microvilli that job from the outer face of the plasma membrane of minister epithelial cells.

11.2.2. Glycogen and also Starch room Mobilizable stores of Glucose

Large polymeric oligosaccharides, created by the linkage of many monosaccharides, are dubbed polysaccharides. Polysaccharides play an important roles in energy storage and also in maintaining the structure integrity of an organism. If every one of the monosaccharides space the same, this polymers are dubbed homopolymers. The most common homopolymer in animal cells is glycogen, the storage form of glucose. As will be taken into consideration in information in thing 21, glycogen is a an extremely large, branched polymer that glucose residues. Many of the glucose devices in glycogen are attached by α-1,4-glycosidic bonds. The branches are developed by α-1,6-glycosidic bonds, present about once in 10 systems (Figure 11.13).


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Figure 11.13

Branch point in Glycogen. Two chains of glucose molecules join by α-1,4-glycosidic bond are connected by an α-1,6-glycosidic link to create a branch point. Such an α-1,6-glycosidic bond develops at about every 10 glucose units, (more...)


The nutritional reservoir in tree is starch, of which there space two forms. Amylose, the unbranched kind of starch, consists of glucose residues in α-1,4 linkage. Amylopectin, the branched form, has around 1 α-1,6 link per 30 α-1,4 linkages, in comparable fashion to glycogen other than for that lower level of branching. More than fifty percent the carbohydrate ingested by human beings is starch. Both amylopectin and amylose are swiftly hydrolyzed through α-amylase, one enzyme secreted by the salivary glands and also the pancreas.


11.2.3. Cellulose, the major Structural Polymer the Plants, consists of direct Chains the Glucose Units

Cellulose, the other significant polysaccharide that glucose discovered in plants, serves a structural quite than a nutritional role. Cellulose is one of the most abundant essential compounds in the biosphere. Some 1015 kg that cellulose is synthesized and degraded on earth each year. It is one unbranched polymer that glucose residues joined by β-1,4 linkages. The β configuration allows cellulose to type very long, directly chains. Fibrils are formed by parallel chains that communicate with one another through hydrogen bonds. The α-1,4 linkages in glycogen and also starch create a an extremely different molecular style from that of cellulose. A hollow helix is created instead that a right chain (Figure 11.14). This differing results of the α and also β linkages are biologically important. The directly chain created by β linkages is optimal because that the construction of fibers having actually a high tensile strength. In contrast, the open helix developed by α linkages is well suited to forming an obtainable store that sugar. Mammals lack cellulases and therefore cannot digest wood and also vegetable fibers.


Figure 11.14

Glycosidic Bonds recognize Polysaccharide Structure. The β-1,4 linkages favor directly chains, which are optimal for structural purposes. The α-1,4 linkages favor bent structures, i beg your pardon are an ext suitable because that storage.


11.2.4. Glycosaminoglycans are Anionic Polysaccharide chains Made that Repeating Disaccharide Units

A different kind the repeating polysaccharide is current on the animal cell surface and also in the extracellular matrix. Many glycosaminoglycans are made the disaccharide repeating devices containing a derivative of one amino sugar, one of two people glucosamine or galactosamine (Figure 11.15). At least one the the sugars in the repeating unit has actually a negatively fee carboxylate or sulfate group. Chondroitin sulfate, keratan sulfate, heparin, heparan sulfate, dermatan sulfate, and also hyaluronate space the significant glycosaminoglycans.


Figure 11.15

Repeating systems in Glycosaminoglycans. Structural formulas for 5 repeating systems of necessary glycosaminoglycans show the range of modifications and also linkages that room possible. Amino groups are shown in blue and negatively charged groups in (more...)


Glycosaminoglycans are usually enclosed to protein to type proteoglycans. Heparin is synthesized in a nonsulfated form, i beg your pardon is climate deacet-ylated and also sulfated. Incomplete modification leads to a mixture of otherwise sulfated sequences. Few of them act as anticoagulants by binding specifically to antithrombin, which accelerates its sequestration of thrombin (Section 10.5.6). Heparan sulfate is favor heparin other than that it has actually fewer N- and O-sulfate teams and an ext acetyl groups.

Proteoglycans resemble polysaccharides more than proteins in as much as the carbohydrate makes up as much as 95% that the biomolecule by weight. Proteoglycans role as lubricants and also structural contents in connective tissue, mediate adhesion of cell to the extracellular matrix, and bind factors that stimulate cabinet proliferation.


11.2.5. Certain Enzymes are Responsible for Oligosaccharide Assembly

Oligosaccharides are synthesized v the activity of particular enzymes, glycosyltransferases, which catalyze the formation of glycosidic bonds. Each enzyme should be specific, to a better or lesser extent, come the sugars being linked. Provided the diversity of recognized glycosidic linkages, plenty of different enzymes are required. Keep in mind that this mode of assembly was standing in comparison with those provided for the other biological polymers heretofore discussed—that is, polypeptides and oligonucleotides. As these polymers are assembled, information about monomer succession is moved from a template, and also a solitary catalytic apparatus is responsible for all bond formation.

The general kind of the reaction catalyzed by a glycosyltransferase is shown in figure 11.16. The street to be added comes in the form of an triggered sugar nucleotide. Sugar nucleotides are necessary intermediates in many processes, and also we will encounter this intermediates again in Chapters 16 and also 21. Note that such reactions can proceed through either retention or turning back of construction at the glycosidic carbon atom in ~ which the brand-new bond is formed; a provided enzyme proceeds by one stereochemical path or the other.


Figure 11.16

General kind of a Glycosyltransferase Reaction. The street to be included comes from a sugar nucleotide—in this case, UDP-glucose.


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The human being ABO blood teams illustrate the results of glycosyl- transferases. Carbohydrates are attached come glycoproteins and glycolipids ~ above the surfaces of red blood cells. For one form of blood group, one of the three different structures, termed A, B, and also O, might be present (Figure 11.17). This structures have actually in common an oligosaccharide foundation called the O (or periodically H) antigen. The A and B antigens differ from the O antigen through the addition of one extra monosaccharide, one of two people N-acetylgalactosamine (for A) or galactose (for B) v an α-1,3 linkage to a galactose moiety of the O antigen.


Figure 11.17

Structures of A, B, and also O Oligosaccharide Antigens. Abbreviations: Fuc, fucose; Gal, galactose; GalNAc, N-acetylgalactosamine; GlcNAc, N-acetylglucosamine.


Specific glycosyltransferases add the extra monosaccharide come the O antigen. Each human inherits the gene because that one glycosyltransferase the this type from each parent. The kind A transferase especially adds N-acetylgalactosamine, conversely, the kind B transferase adds galactose. This enzymes are similar in all but 4 the 354 positions. The O phenotype is the result of a mutation the leads to premature discontinuation of translation and, hence, come the production of no energetic glycosyltransferase.

These structures have necessary implications for blood transfusions and other transplantation procedures. If one antigen no normally current in a human is introduced, the person"s immune system recognizes it as foreign. Adverse reactions deserve to ensue, initiated by the intravascular damage of the incompatible red blood cells.

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Why are various blood types present in the person population? intend that a pathogenic biology such as a parasite expresses ~ above its cell surface a carbohydrate antigen comparable to one of the blood-group antigens. This antigen may not be conveniently detected as foreign in a human with the blood type that matches the parasite antigen, and also the parasite will flourish. However, other world with various blood species will it is in protected. Hence, there will be selective push on human beings to differ blood type to stop parasitic mimicry and also a equivalent selective pressure on helminth to improve mimicry. The constant “arms race” in between pathogenic microorganisms and also human beings cd driver the development of diversity of surface antigens in ~ the person population.

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