Chapter 7 CARBOHYDRATES AND GLYCOBIOLOGY
Carbohydrates are the most abundant biomolecules on Earth. Each year, photosynthesis converts more than 100 billion metric tons of and into cellulose and other plant products. The carbohydrates in these plant products are a dietary staple in most parts of the world, and the oxidation of carbohydrates is the central energy-yielding pathway in most nonphotosynthetic cells. Carbohydrate polymers called glycans serve as structural and protective elements in the cell walls of bacteria, fungi, and plants, and in the connective tissues of animals. Other carbohydrate polymers lubricate skeletal joints and participate in cell-cell recognition and adhesion. In addition, some complex carbohydrate polymers covalently attached to proteins or lipids act as signals that determine the intracellular destination or metabolic fate of these hybrid molecules, called glycoconjugates.
Carbohydrates are aldehydes or ketones with at least two hydroxyl groups, or substances that yield such compounds on hydrolysis. Many, but not all, carbohydrates have the empirical formula ; some also contain nitrogen, phosphorus, or sulfur. There are three major size classes of carbohydrates: monosaccharides, oligosaccharides, and polysaccharides (the word “saccharide” is derived from the Greek sakcharon, meaning “sugar”). Monosaccharides, or simple sugars, consist of a single polyhydroxy aldehyde or ketone unit. The most abundant monosaccharide in nature is the six-carbon sugar d-glucose, sometimes referred to as dextrose.
Oligosaccharides consist of short chains of monosaccharide units, or residues, joined by characteristic linkages called glycosidic bonds. The most abundant are the disaccharides, with two monosaccharide units. Sucrose (table sugar), for example, consists of the six-carbon sugars d-glucose and d-fructose. All common monosaccharides and disaccharides have names ending with the suffix “-ose.” In cells, most oligosaccharides consisting of three or more units do not occur as free entities but are joined to nonsugar molecules (lipids or proteins) in glycoconjugates.
The polysaccharides are sugar polymers containing more than 10 monosaccharide units; some have hundreds or thousands of units. Some polysaccharides, such as cellulose, are linear chains; others, such as glycogen, are branched. Both cellulose and glycogen consist of recurring units of d-glucose, but they differ in the type of glycosidic linkage and consequently have strikingly different properties and biological roles.
This chapter introduces the major classes of carbohydrates and glycoconjugates and provides examples of their many structural and functional roles. Learning the structures and chemical properties of biomolecules is essential, because they are the vocabulary and the grammar of biochemistry. As you read about carbohydrates, note how specific cases illustrate these general principles that underlie all of biochemistry.
Carbohydrates can have multiple chiral carbons; the configuration of groups around each carbon atom determines how the compound interacts with other biomolecules. As we saw for l-amino acids in proteins, with rare exceptions, biological evolution selected one stereochemical series (d-series) for sugars.
Monomeric subunits, monosaccharides, serve as the building blocks of large carbohydrate polymers. The specific sugar, the way the units are linked, and whether the polymer is branched determine its properties and thus its function.
Storage of low molecular weight metabolites in polymeric form avoids the very high osmolarity that would result from storing them as individual monomers. If the glucose in liver glycogen were monomeric, the glucose concentration in liver would be so high that cells would swell and lyse from the entry of water by osmosis.
The sequences of complex polysaccharides are determined by the intrinsic properties of the biosynthetic enzymes that add each monomeric unit to the growing polymer. This is in contrast with DNA, RNA, and proteins, which are synthesized on templates that direct their sequence.
Polysaccharides assume three-dimensional structures with the lowest-energy conformations, determined by covalent bonds, hydrogen bonds, charge interactions, and steric factors. Starch folds into a helical structure stabilized by internal hydrogen bonds; cellulose assumes an extended structure in which intermolecular hydrogen bonds are more important.
Molecular complementarity is central to function. The recognition of oligosaccharides by sugar-binding proteins (lectins) results from a perfect fit between lectin and ligand.
An almost infinite variety of discrete structures can be built from a small number of monomeric subunits. Even short polymers, when arranged in different sequences, joined through different linkages, and branched to specific degrees, present unique faces recognized by their molecular partners.
Carbohydrates can have multiple chiral carbons; the configuration of groups around each carbon atom determines how the compound interacts with other biomolecules. As we saw for 
Monomeric subunits, monosaccharides, serve as the building blocks of large carbohydrate polymers. The specific sugar, the way the units are linked, and whether the polymer is branched determine its properties and thus its function.
Storage of low molecular weight metabolites in polymeric form avoids the very high osmolarity that would result from storing them as individual monomers. If the glucose in liver glycogen were monomeric, the glucose concentration in liver would be so high that cells would swell and lyse from the entry of water by osmosis.
The sequences of complex polysaccharides are determined by the intrinsic properties of the biosynthetic enzymes that add each monomeric unit to the growing polymer. This is in contrast with DNA, RNA, and proteins, which are synthesized on templates that direct their sequence.
Polysaccharides assume three-dimensional structures with the lowest-energy conformations, determined by covalent bonds, hydrogen bonds, charge interactions, and steric factors. Starch folds into a helical structure stabilized by internal hydrogen bonds; cellulose assumes an extended structure in which intermolecular hydrogen bonds are more important.
Molecular complementarity is central to function. The recognition of oligosaccharides by sugar-binding proteins (lectins) results from a perfect fit between lectin and ligand.
An almost infinite variety of discrete structures can be built from a small number of monomeric subunits. Even short polymers, when arranged in different sequences, joined through different linkages, and branched to specific degrees, present unique faces recognized by their molecular partners.