Page 15 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

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

Biopolymers 219

arrangement of fibers in a tissue depends on the nature of toxic molecule to target cells, which are then destroyed
the tissue. In tendon, which requires great strength, fibers by the radioactivity or toxicity. In this way, for example,
are arranged parallel to one another to give a structure cancerous tumors can be reduced. Hormones can be given
with the tensile strength of a light steel wire, while in skin to alleviate diseases caused by a deficiency of that hor-
where strength and flexibility are required, the fibers are mone. Thus, insulin is taken by those suffering from di-
randomly oriented and woven together as in felt. Where abetes mellitus; growth hormones can prevent dwarfism.
rigidity is needed, as in bone, calcium salts are deposited Yet other proteins that are involved in blood clotting can
to harden the tissue. The importance of the structural in- be used to prevent excessive bleeding in hemophiliacs.
tegrity of collagen is demonstrated by the human defi- Antibodies, because of their ability to bind specific
ciency disease scurvy, affecting gums, joints, and wound molecules, can be utilized in the preparation of purified
healing. This results from a lack of vitamin C, ascorbic antigens, where the antigen itself has useful properties.
acid, now known to be required for the action of enzymes The antibody can be immobilized on an inert solid sup-
that modify proline and lysine to give the hydroxy forms port; antigen solution is allowed to flow over the support,
during collagen synthesis. andtheantigensbecomeboundselectivelytotheantibody-
Other structural proteins of some economic importance support complex. Disruption of antigen-antibody binding
are the fibroins, the main constituents of silk, which ap- allows pure antigen to be released and collected. This tech-
pear to contain extensive antiparallel pleated sheets, and nique can be used with many other proteins substituting
the keratins which are the main structural proteins of fur for the antibodies, providing that these proteins show se-
and wool, as well as of hair, skin, horn, hooves, nails, lective binding of useful substances.
feathers, claws, and beaks. The keratin-containing struc- Fibrous proteins such as found in wool and silk have
tures are extremely complex, but in wool and hair, for been used for many centuries for clothing. The strength,
example, there are α-helix-containing fibrils embedded in flexibility, and water-holding abilities of the fibers have
an amorphous matrix. provided cloth which is both practical and comfortable.
Protein is also an essential foodstuff for man. We need
amino acids from which to synthesize the proteins of our
3. Utilization
own bodies; certain amino acids can be made by con-
The enzyme systems of microbes such as yeast have been version of other molecules in the body, but some—the
used for centuries in brewing and breadmaking. In the first essential amino acids (Trp, Phe, Lys, Met, Leu, Ile, and
case the yeast enzymes catalyze the conversion of glucose Val)—must be obtained in our diet from external protein.
to alcohol and in the second, carbon dioxide is produced Gelatin, the denatured water-soluble form of collagen,
and trapped in the dough to give a lighter texture to the is used widely in the food industry as a gelling agent.
bread. Microbial systems are also employed in the manu- It is commonly used in fruit-flavored jellies and canned
facture of cheeses and in the production of antibiotics. meats. The gel melts, however, at around 25 C, so these
◦
Nowadays purified enzymes are used on an industrial foodstuffs should be kept at lower temperatures.
scale, often in solution. Thus, purified starch-degrading
enzymes can be added in the early stages of brewing to
B. Polysaccharides
speed up production of glucose, and glucose syrups can
be made from starch by the action of enzymes. Enzymes Polysaccharides are composed of small sugars, the
can also be immobilized on an inert solid support over monosaccharides, and a great variety of structure is found.
which substrate is passed. Such immobilized enzymes are Although they occur in almost all living organisms, their
frequently employed to convert glucose to another sugar, functions seem to be more limited than those of pro-
fructose. teins. Quantitatively, the most important polysaccharides
Many proteins are used in modern medicine. Antibody are energy reserves or perform a structural role, but other
synthesiscanbedeliberatelystimulatedinthehumanbody polysaccharides can act as lubricants or have a protective
to provide protection against future exposure to disease- function.
producing viruses or bacteria. In some cases of severe Several polysaccharides have industrial importance;
infection, antibodies can be administered directly to a pa- these are usually polymers that can be obtained easily,
tient. Purified antibodies of very high specificity, called in good yield, from any one source. Commercially use-
monoclonal antibodies, can now be prepared and are used ful polysaccharides are often prepared from higher plants,
in diagnostic kits to confirm pregnancy or detect certain but marine algae and bacteria are also important sources.
diseases. In addition, radioactive or modified antibodies In some cases, however, polysaccharide mixtures may be
to specific cell surface antigens can be synthesized. These obtained as a by-product of an industrial process, e.g.,
antibodies can transport a radioactive isotope or a small hemicelluloses resulting from cellulose purification in