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6.4Carbohydrates

There are many types of carbohydrates, which is a generic name for organic compounds represented chemically by the molecular formula (CH2O)n. A two-carbon carbohydrate is called a diose, a three-carbon carbohydrate a triose, and so on, with pentose and hexose being representative carbohydrates. While both D and L forms are optical isomers, carbohydrates within organisms are in the D form, with rare exceptions, although the reason for this is a mystery.

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6.4.1

Pentose and Hexose

Pentose is a component of nucleic acids (see Chapter 5). Hexoses include glucose, galactose, fructose, and mannose (Fig. 6-7A) and have a high affinity for water (highly soluble in water) with several hydroxyl groups. In addition to being an important energy source, glucose is the raw material that can be converted to a range of compounds within the body. Blood sugar is glucose found in the blood. Hexoses are represented by the molecular formula C6H12O6, with at least 24 isomers including those mentioned above. There are D and L forms with further division into six (pyranose form)- and five (furanose form)-membered rings. They can be further classified by grouping in terms of attached hydroxyl group orientation; however, this is not within our current scope of discussion.

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Linear and Ring-shaped Carbohydrate Molecules

Pentoses and hexoses can exist as linear and ring-shaped (Fig. 6-7A) molecules. An equilibrium exists between these forms in an aqueous solution. The equilibrium lies overwhelmingly on the ring-shaped side (above 99%), although it fluctuates. If pure β-D-glucose is dissolved in water, it will become a linear molecule for a short time, and the orientation of the hydroxyl group (1-position) will then reverse to produce α-D-glucose.

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6.4.2

Glycosidic Linkages

Glycosidic linkages are formed between the hydroxyl groups of neighboring carbohydrates with loss of one water molecule. The carbohydrate units (e.g., glucose) are termed monosaccharides, with two carbohydrates linked by glycosidic linkages forming a disaccharide, three forming a trisaccharide, 10 to several forming an oligosaccharide, and a structure with many glycosidic linkages forming a polysaccharide. Representative disaccharides include maltose (malt sugar), sucrose (cane sugar), and lactose (milk sugar) (Fig. 6-7B). In terms of linkages between carbon number 1 and 4, the downward direction of the 1st hydroxyl group is called α and the upward direction is called β, with maltose containing an α (1→4) bond and lactose containing a β (1→4) bond.

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6.4.3

Polysaccharides

There are many types of polysaccharides consisting of many linked carbohydrates, with representative members containing many linked glucoses, including starch (amylose), glycogen, and cellulose (Fig. 6-7C). Amylose is a linear polysaccharide with glucoses joined through α (1→4) bonds, while amylopectin is a branched polysaccharide with (1→6) bonds every 10–20 glucose residues; both are present in plants as starch granules. Glycogen is a similarly branched polysaccharide present in animals as a storage carbohydrate. Cellulose, which maintains the structure of plants, is very different with β (1→4) bonds. Other polysaccharides include mannan contained in konjac, agarose contained in agar, and chitin, which forms the exoskeleton of insects, shrimp, and crabs.

Glycosaminoglycans are another major group of polysaccharides and are also called mucopolysaccharides. They include hyaluronic acid, chondroitin sulfate, heparan sulfate, and keratan sulfate that are present in large amounts in connective tissue, mucous membranes, and mucous. These macromolecules are composed of residual carbohydrate chains of one hundred units long. They are characterized by their bonding to many surrounding water molecules as they possess amino and sulfate groups in addition to hydroxyl groups. These polysaccharides often covalently or non-covalently bind to a protein core to form an extremely large complex. This is termed a proteoglycan. Proteoglycans in cartilage form complexes consisting of more than 100 polysaccharide molecules, equal in size to a bacterium.

Fig. 6-7 Sugar and Sugar Chain Structure

(A) Representative hexose (B) Representative disaccharide (C) Representative polysaccharide of polymeric glucose

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Glycobiology

There are many types of monosaccharides in addition to those mentioned here, including those containing amino, carboxyl, and sulfate groups. These monosaccharides contain many hydroxyl groups and thus have many potential modification sites. There exists a range of branched carbohydrate chains in addition to those linked to linear chains. There are endless possibilities for carbohydrate chains attached to carbohydrates.

Oligosaccharide and polysaccharide chains are bound to the side of the proteins in the cell membrane facing the external environment and to the surface of proteins secreted extracellularly. In addition, oligosaccharides of glycolipids are bound to the side of lipids comprising the cell membrane facing the external environment.

Although analysis of these carbohydrate chains is difficult, their structure and synthesis have gradually been clarified by recent advances. The biologically important functions of carbohydrate chains have also been revealed. In addition to the fact that the properties of red blood cells and carbohydrate chains determine the ABO blood types discovered a fairly long time ago, we have learned that carbohydrate chains function in a range of fields including the fact that these chains, found on the surface of proteins, are responsible for the characteristics that were believed to be the specific function of proteins. They are also found to play a role in cell–cell recognition through cell–cell contact and subsequent cell behaviors. Glycobiology, the biology of carbohydrate chains, is an emerging field.

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6.4.4

Glycoproteins

More than half the proteins in eukaryotic organisms are glycoproteins, covalently bound to oligosaccharide or polysaccharide chains. Oligosaccharide chains are bound to the part of the cell membrane proteins facing the external environment and to the surface of proteins secreted extracellularly. Carbohydrate chains bound to the side chain amino group of an asparagine residue of a protein are termed N-linked glycans, while those bound to the hydroxyl group of a serine or threonine residue of a protein are termed O-linked glycans. There are several oligosaccharides per protein, depending on the type of protein. Although the previously mentioned proteoglycan is also a complex between a polysaccharide and a protein, it is limited to complexes originating from covalent or non-covalent bonds with specific types of glycosaminoglycans.

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Blood Type

Cell surface protein- or lipid-attached carbohydrate chains determine the ABO blood type. There are three types of simple structure oligosaccharide chains (Column Fig. 6-2). These determine the blood group of an individual as A, B, AB, or O. Blood type is determined using erythrocytes because of the ease of sample collection and because there are numerous erythrocytes throughout the body.

These differences are because of slight differences among individuals in the amino acid sequence of the enzyme glycosyltransferase, which adds terminal carbohydrate chains, and subsequent differences that arise in substrate specificity. There are seven base substitutions between types A and B among the 1062 base pairs of enzyme genes on chromosome 9, resulting in four differences in amino acids (Column Fig. 6-3). These differences alter enzyme specificity, transposing different carbohydrates in types A and B. The deletion of a single base in type O results in an immediately subsequent stop codon, producing only short proteins without enzyme activity. Since somatic cells are diploid, people have two of these genes, with people of genotypes AA and AO having A-type carbohydrate chains.

Individuals with type O blood possess both anti-A and anti-B antibodies. Individuals with type A blood have anti-B but not anti-A antibodies. Conversely, those with type B blood have anti-A but not anti-B antibodies. Individuals with type AB blood have neither anti-A nor anti-B antibodies. Accordingly, if a person with type A blood is transfused with type B blood, type B erythrocytes will coagulate because of the anti-B antibodies in the plasma. The same can be observed if a person with type O blood is transfused with type B blood.

However, when a person with type A blood is transfused with type O blood, type A erythrocytes do not coagulate although anti-A antibodies have been introduced. This is because the anti-A antibodies in the transfused plasma are rapidly diluted, which prevents coagulation of erythrocytes. Thus, rather than there being absolutely no effect, a few anti-A antibodies can bind to some A antigens throughout the body, including erythrocytes.

Column Fig. 6-2 Blood type saccharides

Column Fig. 6-3 Enzyme genes that determine blood type

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