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Encyclopedia of Physical Science and Technology EN002C-64 May 19, 2001 20:39

Biopolymers 217

Other proteins in mammalian systems, the antibodies, acid side chains on the antibody surface at the binding
serve to protect against invasion by bacteria or viruses. site.
Foreign material such as bacterial protein or polysaccha- Some proteins can serve yet another purpose in living
ride, on entering the bloodstream, provokes the synthesis organisms, where they can act as regulators of biochemi-
of specific antibodies. The antibodies then combine with cal pathways. Hormones are substances that act on target
the “foreign” molecules, known as antigens, and render cells and markedly alter the metabolism of those cells.
them harmless. Stimulation of antibody synthesis in man Some hormones are small proteins or polypeptides, and it
can be brought about by immunization, whereby a harm- is believed that they interact with specific receptors on the
less form of disease-producing bacteria or virus is intro- outer surface of the target cell membrane. The receptors
duced into the body and specific antibodies are formed. themselves are proteins, and binding of the hormone to
These are active against virulent forms of the same bac- the receptor protein causes changes to occur within the
terium and may remain in the bloodstream for years, af- cell. In some cases it is known that a messenger molecule
fordingprotectionagainstthedisease.Unfortunately,“for- increases in concentration in the affected cell, and that
eign” tissue in the body elicits the same response, and so, this messenger modifies enzyme activities within the cell.
for organ transplants to be successful and not be rejected, Thus glucagon, a polypeptide of 29 amino acid residues
the immune response must be minimized by the adminis- secreted by the pancreas, acts on liver cells in this way and
tration of powerful immunosuppressant drugs. stimulates breakdown of glycogen (see Section II.B.6),
Antibodies constitute a group of proteins known as the giving an increase in blood glucose levels.
immunoglobulins. In an adult human there are five classes In many cases the direct effect (at the molecular level)
of immunoglobulin, and the most prevalent, the IgG an- of the hormone on its target cell is brought about by a
tibodies of blood, have been studied in some detail. Each complex “cascade” of reactions. This is true for insulin,
IgG molecule contains two identical heavy chains of ap- the most widely studied polypeptide hormone. Insulin is
proximately440aminoacidsandtwoidenticallightchains secreted by the pancreas and consists of two short chains
of around 220 amino acids, all linked by disulfide bonds. of 21 and 30 amino acids, linked by disulfide bonds (see
It is believed that the chains are constructed of domains of Fig. 4). The hormone acts mainly on cells of liver, muscle,
approximately 110 amino acids. Each domain has similar and fatty tissues, decreases glucose concentration in the
folding of the polypeptide chains and consists essentially blood, and promotes glycogen and fat synthesis. Amino
of two antiparallel pleated sheets. The light chains, there- acid side chains on the surface of the insulin molecule are
fore, contain two such domains, while the heavy chains important for interaction with the insulin receptor at its tar-
have four each. get cells. It is known that binding of insulin to the receptor
Antibodies are specific for the antigens with which they stimulates the activity of an enzyme within the cell which
combine. Therefore, many different IgG molecules exist, is intrinsic to the receptor. This enzyme, in turn, activates
but the primary structures of the three C-terminal domains other proteins and enzymes by adding phosphate groups
of a heavy chain and the C-terminal domain of a light chain to specific amino acid side chains. The newly activated
are very similar from one IgG molecule to another. These proteins then affect other proteins to increase or reduce
arereferredtoastheconstantregionsoftheheavyandlight their activity and utlimately bring about the effects asso-
chains, respectively. The N-terminal domains of both the ciated with insulin action. Thus, for example, the insulin
heavy and light chains, however, show great diversity in receptor enzyme activates a second enzyme that stimu-
amino acid sequence and are known as the variable regions lates glycogen synthase, which is capable of converting a
of the chains. modified glucose to the storage polysaccharide glycogen
Different classes of antibody vary in the nature of the (see Section II.B.6). Hence insulin helps to control blood
constant regions of the heavy chains, but within one class glucose levels.
(e.g., the IgG) the variable regions of the light and heavy Other protein regulators, such as the repressors, act not
chainsconferspecificityonanantibody.Thebindingsiteis at cell surfaces but interact with the nucleic acids which
believed to be situated in a cleft (for a small antigen) or on control protein synthesis. These are described later (Sec-
an irregular surface (complementary to part of the surface tions II.C.2, and II.E).
of a large antigen) formed by parts of two variable do- In addition to the functions discussed above, proteins
mains in close proximity, one domain belonging to a light can have a structural role in living organisms. Unlike many
chain and one to a heavy chain. Since each IgG molecule of the water-soluble proteins already described, which
has two light and two heavy chains, then each has two are often spherical in shape, structural proteins can be
antigen-binding sites. Binding of antigen is believed to in- insoluble and fibrous. They are synthesized in a soluble
volvenoncovalentbondingbetweentheantigenandamino form, processed to give the insoluble material, and then