Carbohydrates and Their Polymers 141 Monosaccharides

Like amino acids there are a large number of significant carbohydrates found in nature. Carbohydrates are known either as aldoses or as alde-hydic polyols because of the aldehydic carbonyl at carbon atom 1 (C1) and extensive hydroxyl (OH) substitution. Carbohydrates also exist as ketoses having an internal carbonyl or keto group (=O) generally at C2, and again, many alcohol (OH) substituents on the carbon chain. Figure 1.9 illustrates these structures for a 6-carbon aldohexose and a 5-carbon aldoribose. Although there are many different kinds of carbohydrate monomers, three members of the family, glucose, ribose, and 2-deoxyribose dominate in biological importance. Glucose is important because it is the key product of photosynthesis, and thus ultimately the main dietary source of energy for nonphotosynthesizing forms of life. Glucose is most familiarly known as the dominant monomer of the very large molecular weight, polymeric compounds glycogen, starch, and cellulose. Ribose and 2-deoxyribose (lacking the OH on C2) dominate because of their role in the polymeric backbone structure of the genetic materials ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), respectively.

A single molecule of glucose or any other similar sugar is known as a monosaccharide. Figure 1.9 shows the open-chain chemical structures of the D enantiomer of glucose, one member of a large family of 6-carbon monosaccharides (or hexoses), and that of the D enantiomer of the 5-carbon monosaccharide ribose (generically a pentose), along with their biologically most active cyclized D and L enantiomers. Chemically, glucose and ribose are aldehydes as indicated by the aldehydic oxygen on C1 of both open-chain structures, and thus these compounds and other isomers of their respective families are often known as aldohexoses and aldopentoses, respectively. In both cases, the open-chain structures

HO-C3-H

D-Glucose

HO CH

HO CH

a-D-Glucopyranose

a-D-Glucopyranose

P-D-Glucopyranose

P-D-Glucopyranose

H-C2"OH

H-C2"OH

H-C3"OH OH OH

H |C|-OH a-D-Ribofuranose

H-CHOH

I5 H

D-Ribose

HO-CH2 (OKI

)H OH P-D-Ribofuranose

Figure 1.9 Structure of common sugars. Glucose is the most common pyranose (6 atom ring) monomer in many polymeric carbohydrates and the most important carbohydrate source of energy for living organisms. Ribose and 2-deoxyribose are the most prominent furanoses (5 atom rings) because of their prominence in DNA and RNA formation. 2-deoxyribose (not shown) lacks the OH group on the 2 position. The shaded atoms on the open chain structure indicate the atoms involved in intramolecular ring closure and the shaded OH groups on the pyranose and furanose rings indicate the alternative location of the OH residues that define the a- and the b-stereochemical forms of the molecules.

readily undergo intramolecular cyclization to produce the closed-ring structures shown, with the new intramolecular bond being chemically known as an acetal.

The closed-ring acetal forms of the 5-carbon, 6-carbon, and 7-carbon monosaccharides are the forms involved in biological reactions such as synthesis of polymeric glucose structures including starch, glycogen, and plant cell walls. Likewise, the cyclic structures shown are the forms of the compound involved in energy and nucleic acid metabolism and which are ultimately broken down to CO2 and H2O (mainly glucose) during the course of metabolism producing usable energy to maintain life.

Given the complexity of substitutions on the carbon chain, it should be apparent that the monosaccharides like glucose and ribose can exist as a number of structurally different isomers. The main isomer of glucose that is found in nature is the compound shown in Fig. 1.9, which is known as the D-enantiomer. The D-enantiomer form of monosaccha-rides is determined by the configuration of substituents around the asymmetric carbon atom farthest from the main functional group of the molecule. In glucose the main functional group is the C1 aldehyde. Consequently, in glucose C5 is the asymmetric carbon farthest from the aldehyde and with the hydroxyl written to the right as shown, we have the D form. The L form (not shown) has the OH group on C5 written to the left with all other substituents on the molecule remaining as shown for D glucose. During intramolecular cyclization or acetal bond formation, ring closure takes place between C1 and C5 and results in C1 becoming a new asymmetry center, with the newly asymmetric C1 atom being known as the anomeric carbon. Since ring closure can take place with the new OH group on either side of the plane of the ring, two isomers are possible. These isomeric forms have been designated a and b Figure 1.9 illustrates the structure of the a and b isomers, or anomers, around C1 for the pyranose forms of glucose and the furanose forms of ribose. In the a forms, the OH groups on C1 and C2 are on the same side of the ring, generally depicted downward, while in the b forms the C1 and C2 OH groups are on opposite sides of the ring with the C1 OH generally being depicted upward. Although we will not discuss them here, there are a very large number of other structural isomers of hexoses and pentoses as well as isomeric forms of trioses (3 carbon), tetroses (4 carbon), and heptoses (7 carbon), many of which have significant albeit specialized roles in biology that will be encountered throughout this book.

1.4.2 Oligosaccharides and polysaccharides

Clearly, with the multitude of functional groups, monosaccharides have the ability to undergo a variety of chemical reactions to produce many new compounds. Among the more common of these are additions of car-boxyl, acetyl, and amino groups at various locations on the parent monosaccharide. However, by quantity, the most important kinds of reactions that monosaccharides undergo are their reaction with other monosac-charides to produce a family of familiar disaccharides and the very large carbohydrate polymers known as glycogen, starch, and cellulose.

The most familiar dietary disaccharides are maltose, lactose, and sucrose whose monosaccharide reactants and disaccharide product structures are shown in Fig. 1.10. As in earlier discussions of important biological covalent bonds, the covalent link between the anomeric carbon of one monosaccharide and the OH group of a second saccharide (or another molecule containing an OH) is known as a glycosidic bond, and the product molecules are generically referred to as glycosides. For example, lactose, maltose, and sucrose can be referred to as glycosidic molecules with the bond being designated as "1^4," "1^4," and "1^2" glycosidic bonds, respectively. The latter nomenclature is used to identify the carbon atoms involved in the glycosidic bond. Many other disaccha-rides, trisaccharides, and tetrasaccharides, collectively known as oligosac-charides, are found in nature with some having specialized biological ho—ch2

oh a-D-Glucose oh

ch2oh oh

Maltose h oh

Maltose ho—ch ho-ch2

Lactose h oh ho-ch2

Lactose ho.

Sucrose

Figure 1.10 Formation of glycosidic (sugar-sugar) bonds to form the familiar disaccharides maltose, lactose, and sucrose. Notice that sucrose is formed from glucose, an aldohexose, and fructose, an example of a ketohexose. Glycogen and starch can be viewed as maltose polymers.

ho o chnoh ho oh

h oh h functions while others are merely partial degradation products of larger molecules such as glycogen.

In humans and other animals, the most important polysaccharide is glycogen, also known as animal starch because of its similarity in molecular size, structure, and composition to plant starches which were discovered earlier than glycogen. In the human body, glycogen is found principally in liver and muscle where it acts as a reservior of glucose monomers, acting much like a glucose buffer in an animal's energy economy. Glycogen comprises about 1% of human muscle mass, and an adult human liver contains about 110 g of glycogen. This huge polymer has a molecular weight range of 106 to 107 Da, depending on the exact number of glucose residues in the molecule. Glycogen along with fat provide the main energy stores of the body. Glycogen and starch are constructed entirely of glucose monomers linked mainly by a 1^4 linkages like those that connect the glucose monomers in the disaccharide maltose (Fig. 1.10). Although shown more frequently in our glycogen model (Fig. 1.11), in nature 1 out of every 12 glucose monomers in a glycogen chain also is connected to a second glucose monomer in a 1^6 linkage, each such linkage producing a branch off the main glycogen chain as shown in Fig. 1.11. Multiple branches like those shown in the figure produce a highly compact spherical molecule.

al ^ 4 -Glycosidic bonds al ^ 4 -Glycosidic bonds

Figure 1.11 Section of a glycogen or starch molecule. The initiation point for synthesis is at the base of the chain and 1^6 branch points are added subsequent to main chain formation. The final glycogen molecule has a molecular weight of 1 X 106 to 1 X 107 Da, is spherical in three-dimensional structure, and highly hydrated.

Figure 1.11 Section of a glycogen or starch molecule. The initiation point for synthesis is at the base of the chain and 1^6 branch points are added subsequent to main chain formation. The final glycogen molecule has a molecular weight of 1 X 106 to 1 X 107 Da, is spherical in three-dimensional structure, and highly hydrated.

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