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Introduction:
Proteins are probably the most important class of biochemical
molecules, although of course lipids and carbohydrates are also
essential for life. Proteins are the basis for the major structural
components of animal and human tissue.
Proteins are natural polymer molecules consisting of amino
acid units. The number of amino acids in proteins may range
from two to several thousand.
Amino Acids:
Although we are studying only about 20 amino acids, there
are about six more found in the body. Many others are also known
from a variety of sources. Amino acids are the building blocks
used to make proteins and peptides. The different amino acids
have interesting properties because they have a variety of structural
parts which result in different polarities and solubilities.
Each amino acid has at least one amine and one acid functional
group as the name implies. See graphic on the left. The different
properties result from variations in the structures of different
R groups. The R group is often referred to as the amino acid
"side chain". Amino acids have special common names,
however, a three letter abreviation for the name is used most
of the time. Consult the amino acid table on the next page for
structure, names, and abbreviations.
Zwitterion:
Amino acid physical properties indicate a "salt-like"
behavior. Amino acids are crystalline solids with relatively
high melting points, and most are quite soluble in water and
insoluble in non-polar solvents. In solution, the amino acid
molecule appears to have a charge which changes with pH.
An intramolecular neutralization reaction leads to a salt-like
ion called a zwitterion. The accepted practice is to show
the amino acids in the zwitterion form.
(1) The carboxyl group can lose a hydrogen ion to become negatively
charged.
(2) The amine group can accept a hydrogen ion to become positively
charged.
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