Amino acids are the chemical building
blocks of proteins, or polypeptides, and comprise a set of 20 different
molecules. Fully assembled proteins consist of a long chain of amino acids
held together by linkages known as peptide bonds. The general structure of an
amino acid is characterized by three chemical groups: an amino group, a
carboxylic acid group, and a side group that varies depending upon the type
of amino acid. At the pH found within most living cells, or physiological pH,
the amino group of the molecule has a plus (+) charge and the carboxylic acid
group has a minus (-) charge. The charge of the side group varies with the
type of amino acid. Amino acids may also adopt alternate tautomeric
forms, or structures, as shown here. This does not change the chemical
composition of the molecule, but simply rearranges its hydrogen atoms. The
alternate structure adopted by this amino acid allows it to form linkages
called peptide bonds with other amino acids. When a peptide bond is formed,
two adjacent amino acids together lose two hydrogen atoms and one oxygen
atom, resulting in production of a water molecule by a process called
condensation. As this occurs, the carbon atom linked to the remaining oxygen
of one amino acid forms a bond with the nitrogen atom of the other amino
acid. When this process is repeated for a large number of amino acids, a long
chain of molecules known as a polypeptide or protein is formed.
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