Carbohydrates (also called saccharides) are—on the basis of mass—the most abundant class of biological molecules on Earth. Although all organisms can synthesize carbohydrate, much of it is produced by photosynthetic organisms, including bacteria, algae, and plants. Carbohydrates can be described by the number of monomeric units they contain. Monosaccharides are the smallest units of carbohydrate structure. The name carbohydrate, “hydrate of carbon,” refers to their empirical formula (CH 2 O) n where n is 3 or greater (n is usually 5 or 6 but can be up to 9). Oligosaccharides are polymers of 2 to about 20 monosaccharide residues. The most common oligosaccharides are disaccharides, which consist of two linked monosaccharide residues. Polysaccharides are polymers that contain many (usually more than 20) monosaccharide residues. Oligosaccharides and polysaccharides do not have the empirical formula (CH 2 O) n because water is eliminated during polymer formation.

Most Monosaccharides Are Chiral Compounds Monosaccharides are water-soluble, white, crystalline solids that have a sweet taste. Examples include glucose and fructose. Chemically, monosaccharides are polyhydroxy aldehydes or aldoses, or polyhydroxy ketones or ketoses. They are classified by their type of carbonyl group and their number of carbon atoms. As a rule, the suffix -ose is used in naming carbohydrates, although there are a number of exceptions. All monosaccharides contain at least three carbon atoms. One of these is the carbonyl carbon, and each of the remaining carbon atoms bears a hydroxyl group. In aldoses, the most oxidized carbon atom is designated C-1 and is drawn at the top of a Fischer projection. In ketoses, the most oxidized carbon atom is usually C-2.

Proteoglycans are complexes of proteins and a class of polysaccharides called glycosaminoglycans. These glycoconjugates occur predominately in the extracellular matrix (connective tissue) of multicellular animals. Glycosaminoglycans are unbranched heteroglycans of repeating disaccharide units. As the name glycosaminoglycan indicates, one component of the disaccharide is an amino sugar, either D-galactosamine (GalN) or D-glucosamine (GlcN). The amino group of the amino–sugar component can be acetylated, forming Nacetylgalactosamine (GalNAc) or GlcNAc, respectively.

Figure 8.31 Structure of the peptidoglycan of Staphylococcus aureus. (a) Repeating disaccharide unit, tetrapeptide, and pentaglycine components. (b) Cross-linking of the peptidoglycan macromolecule.

Glycoproteins, like proteoglycans, are proteins that contain covalently bound oligosaccharides (i.e., proteins that are glycosylated: proteoglycans are a type of glycoprotein.)