PROTEINS SKIN, LEATHER, AND GLUE - Archaeological Chemistry - page 324

The term polypeptide is often used synonymously with protein . A
protein , however, may consist of one or more specific polypeptide chains.
The dividing line between the use of the terms polypeptides and proteins
is somewhat undefined, although polypeptides are less likely to constitute
the structural element of living bodies. The type of amino acids and the
order in which they are linked together within a polypeptide chain deter-
mine the properties of particular proteins. A change in just one amino acid,
out of a total of the several hundreds or even thousands that make up
many a protein, and even a change in the position of an amino acid within
the chain, results in proteins having different properties and therefore
being functionally different.
Some proteins may be very large, indeed. Titin , for example, a muscle
protein and the largest known protein, includes in its molecule over 26,000
combined amino acid units.
although they synthesize a very few from their food intake. Almost all of the
amino acids that animals require but cannot synthesize are taken up as food
from plants or from other animals, which, in turn, previously acquired them
from plants. Also, humans, like other animals, are unable to synthesize all
the amino acids they require for their regular living; nor can they assimilate
proteins in their food, because during digestion, proteins are broken down
into their component amino acids. They can, however, assimilate the amino
acids they ingest and use them to build all the proteins necessary for their
subsistence.
TEXTBOX 60
THE PROTEINS
The proteins are complex biological substances that make up the structural
elements of the body of animals and fulfill many body functions (see
Textbox 59). Each protein has different and unique functions. Their
uniqueness depends on the number and order of amino acids within their
polymeric chains. Proteins are required for the structure, function, and reg-
ulation of the cells, tissues, and organs of living organisms. Some proteins
make up the structural elements of the body, such as specific organs,
muscles, skin, blood, or part of the bones. Others perform specific body
functions; examples are some hormones , known as peptide hormones ,