Читать книгу An Introduction to Molecular Biotechnology - Группа авторов - Страница 14

Monosaccharides occur in cells either as aldoses or as ketoses (Figure 2.1a). The most important monosaccharides have a chain length of three, five, and six carbon atoms and are called trioses, pentoses, and hexoses. Under physiological conditions, pentoses and hexoses can form ring structures through hemiacetal and hemiketal formation (Figure 2.1b).

Figure 2.1 Composition and structure of sugar molecules. (a) Structures of the most important aldoses and ketoses. (b) Ring structures of pentoses and hexoses (hemiacetal and hemiketal formations), important isomers of glucose. (c) Important derivatives of glucose and galactose. (d) Formation of disaccharides and polysaccharides (starch [amylose], amylopectin, glycogen, cellulose).

Many important nitrogen‐containing derivatives of these monosaccharides (Figure 2.1c) use galactose and glucose as a base. Examples include glucosamine, N‐acetylglucosamine, and glucuronic acid. These derivatives can be present either as glycosides or as part of a polysaccharide.

Condensation reactions between sugar molecules result in the formation of glycosidic bonds with the elimination of a water molecule. As hydroxyl groups can be present in either the α or β position, the stereochemistry of sugar molecules is of great importance. The condensation of two sugar molecules results in the formation of a disaccharide (Figure 2.1d); that of three sugar molecules, correspondingly, is a trisaccharide. Oligosaccharides are built from a few sugar monomers, and polysaccharides (e.g. starch, glycogen, cellulose, chitin, etc.) are made up of many sugar monomers.

Sugar molecules can be easily activated through esterification with an acid, one important example being esterification with phosphoric acid. Sugar phosphates are important in glycolysis.

The most important polysaccharide in animal cells is glycogen, which is stored as an energy source in liver and muscle. Glycogen can be quickly converted into glucose‐1‐phosphate and then channeled into glycolysis. Glycogen is a branched polysaccharide formed from glucose molecules linked by α‐(1→4)‐glycosidic bonds or α‐(1→6)‐glycosidic bonds (Figure 2.1d). This results in many free ends on which the enzyme glycogen phosphorylase can begin degradation simultaneously.

Starch or amylose (Figure 2.1d) consists of glucose residues linked by α‐(1→4)‐glycosidic bonds. In amylopectin, additional glucose residues linked by α‐(1→6)‐glycosidic bonds are built in. Amylopectin, therefore, has a similar structure to glycogen but is less strongly branched. Starch is formed by photosynthesis in plant cells, where it is stored in amyloplasts. Starch can be broken down easily by animals and is therefore an important part of human nutrition.

Glucose is also used as a building block for cellulose (Figure 2.1d), which is necessary for formation of the plant cell wall. Cellulose is an unbranched polymer made from glucose molecules linked by β‐(1→4)‐glycosidic bonds. Cellulose cannot be broken down in the human digestive tract. Conversely, the rumen (first stomach) of ruminants (animals that chew the cud) contains microorganisms that produce cellulase – an enzyme that makes it possible for cows, for example, to use cellulose as a nutrient. Additional polymers present in the plant cell wall include polysaccharides, so‐called glycans made up of cellulose fibers linked together in a diagonal fashion, pectin (basic unit: galacturonic acid), and lignin (made from the coumaroyl, coniferoyl, and sinapoyl alcohols). Using cellulases, it is possible to digest the cell walls of plant cells. Cells without cell walls are called protoplasts. They are important in plant biotechnology because they are easily transformable by genetic engineering (see Chapter 30). In many plant species it is possible to regenerate intact plant cells from protoplasts. Cell walls of fungi and the exoskeletons of insects are composed of chitin, which has N‐acetylglucosamine as a building block in β‐(1→4)‐glycosidic bonds.

Further important polysaccharides are found in animals. Hyaluronic acid is made up of many disaccharide building blocks, which themselves consist of glucuronic acid and N‐acetylglucosamine. Hyaluronic acid has a very high viscosity and is therefore found in synovial fluid in the joints and in the vitreous humor in the eye. Furthermore, polysaccharides made from disaccharides consisting of sulfated glucuronic acid and N‐acetylglucosamine or N‐acetylgalactosamine units, respectively, are found in the connective tissues. Examples include chondroitin‐4‐sulfate, chondroitin‐6‐sulfate, dermatan sulfate, and keratin sulfate. Heparin, involved in the control of blood coagulation, also falls into this structural group. These polysaccharides are charged molecules under physiological conditions and can therefore interact with cellular macromolecules, such as proteins and nucleic acid by form hydrogen bridges and ion bonds.