Carbohydrates are the most abundant organic component of plants. Structurally,
carbohydrates are usually polyhydroxy aldehydes or polyhydroxy ketones (or compounds
that hydrolyze to yield polyhydroxy aldehydes and ketones). Since carbohydrates contain
carbonyl groups and hydroxyl groups, they exist primarily as acetals or hemiacetals.
8.1.5.1 Carbohydrates: Molecular Structure
Analogous to peptides, carbohydrates frequently exist as polymers of simpler building
blocks. The simplest building block, called a monosaccharide, is a simple carbohydrate
that cannot be hydrolyzed to simpler carbohydrates. A carbohydrate that can be
hydrolyzed to two monosaccharides is called a disaccharide. A trisaccharidecontains
three fundamental building blocks. Generalizing,oligosaccharides contain 3–10 mono-
saccharides, while polysaccharides(also known as glycans) contain more than 10
monosaccharide polymeric units. A polysaccharide that is a polymer of a single mono-
saccharide is called a homopolysaccharide; a polysaccharide composed from different
monosaccharides is a heteropolysaccharide. A homopolysaccharide composed only of
glucose monomeric units is called a glucan.In carbohydrate nomenclature, all mono-
saccharides and most polysaccharides have names ending in the suffix -ose.
Monosaccharides are the simplest, fundamental building block of carbohydrates.
Structurally, monosaccharides are classified in accord with two designations: (i) the
number of carbon atoms in the molecule, and (ii) the presence of either an aldehyde or
a ketone functional group. The number of carbon atoms in a monosaccharide (usually
3–7) is designated as tri-, tetr-, and so on.For example, a three-carbon monosaccharide
is a triose, while a six-carbon monosaccharide is a hexose. A monosaccharide contain-
ing an aldehyde group is called an aldose; one containing a keto group is termed a
ketose. These various designation systems are frequently used in conjunction; thus, a
five-carbon monosaccharide with a ketone is called a ketopentose, while a three-carbon
monosaccharide with an aldehyde is called an aldotriose.
8.1.5.2 Bioactive Carbohydrates and Stereochemistry
The nomenclature and classification of monosaccharides is further complicated by the
presence of chiral carbons within monosaccharides. In the late nineteenth century it was
ascertained that the configuration of the last chiral carbon in each of the naturally occur-
ring monosaccharides is the same as that for (+)-glyceraldehyde. This configuration
was designated as “D” and it was determined that all naturally occurring monosaccha-
rides were in the D configuration.
Since stereochemistry is crucial to modern drug design, it is imperative that the mul-
tiple conventions for representing stereochemistry should not be confused. There are
four conventions for representing stereochemical properties: (+)/(−);d/l; D/L; R/S. A
stereoisomer that rotates plane-polarized light to the right is said to be dextrorotatory;
its mirror image compound, which rotates plane-polarized light to the left, is levorota-
tory. This experimental direction of light rotation is specified in the name by (+) for
dextrorotatory and (−) for levorotatory; alternatively, the symbol dcan be used for
dextrorotatory and lfor levorotatory. Therefore, the (+)/(−) and d/lsystems are equivalent.
They do not tell us anything about the structure of the molecule; they merely tell us that
ENDOGENOUS MACROMOLECULES 475