Carbohydrate literally means "hydrate of
carbon." However, carbohydrates do not contain individual
water molecules. Carbohydrates are so designated because the
empirical formulas of most of them can be expressed as Cx(H2O)y.
The molecular formula of glucose is C6H12O6
(A in Figure 1).
Figure 1. Schematic (A) and Cyclic (B) Structure of
Diagram from 725 (25.16) and 726
The extraordinary structural complexities
of carbohydrates are created by the nature of the process by
which monosaccharides are aligned to form disaccharides,
oligosaccharides, and polysaccharides. For example, two
identical monosaccharides can bond to produce 11 different
disaccharides. Beyond that, monosaccharides create branching
structures. Thus, a small number of
monosaccharides can unite to create a staggering
range of possible configurations to form various
carbohydrate compounds. That is in sharp contrast to the
case of two amino acids that can form only one
dipeptide, or two nucleotides
that can generate only one nucleic acid configuration. That
is so because amino acids and nucleic acids connect to each
other in only one way. To present two widely divergent
scenarios, four different nucleotides can generate only 24
distinct tetranucleotides, whereas four different
monosaccharides can be aligned in 35,560 unique
tetrasaccharides. The "letters of the
carbohydrate language" evidently are far more versatile than
those in protein or nucleic acid worlds.
Glycoproteins and Proteoglycans
Most mammalian proteins contain sugar
moieties and hence should be properly called
glycoconjugates. There are two main types:
proteoglycans. In general,
glycoproteins are highly branched and contain short
oligosaccharide (glycan) chains without repeating sequences.
By contrast, proteoglycans comprise long, linear, and
unbranched glycans and do not contain repeating
disaccharides. Glycoproteins include most of the molecules
that serve as biomembrane receptors, hormones, and mediators
of inflammatory and healing responses.
glycosphingolipids— are lipids
containing covalently bound sugars.
Sphingolipids are a group of amphipathic, polar
lipids. Glycolipids are divided into four groups:
cerebrosides, sulfatides, globosides, and gangliosides. All
contain a polar head group composed of sugars attached to
ceramide by a glycosidic bond. Glycolipids—and glycoproteins—are
not synthesized by template mechanisms as are nucleic acids
and proteins. Rather, these compounds are generated by
activities of enzymes—predominantly of glycotransferases—which
are expressed or activated differentially during
development, differentiation, and demise of cells.
Chemically, carbohydrates are polyhydroxy aldehydes and
polyhydroxy ketones. In solution, the aldehyde group of
glucose at carbon 1 reacts with hydroxyl group at carbon 5
to form two cyclic structures designated á-D-glucose and
â-D-glucose (B in Figure 1). The -OH group on the carbon 1
in the former is axial to the ring, whereas it is equatorial
to the ring in the latter. Based on structure, carbohydrates
are also divided into two broad classes: aromatic compounds
that contain a ring (benzene or related) structure; and
aliphatic compounds that do not contain such structures. In
general, aromatic carbohydrates are more stable and
resilient than their aliphatic counterparts. Thus, it does
not come as a surprise that all vitamins except ascorbic
acid have an aromatic configuration. This makes evolutionary
sense, since vitamins not only facilitate biochemical
reactions, but also serve as potent redox restorative,
oxystatic, and defense molecules, especially as far as their
roles as antioxidants. Most hormones are also aromatic. On
the negative side, many pesticides (such as DDT) and many
other synthetic chemicals (PCBs, PBBs, and related
compounds) are also aromatic and thus stable and resilient.
They have long half-lives and cause long-term toxicity.
What Is Insulin?
* What Is Glucose?