Amino Acids, important
class of organic compounds that contain both the amino (8NH2) and carboxyl
(8COOH) groups. Of
these acids, 20 serve as the building blocks of proteins (see
Protein). Known as the standard, or alpha, amino acids, they
comprise alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid,
glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine,
phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine. All
20 are constructed according to a general formula:
As the formula shows, the amino and carboxyl groups are
both attached to a single carbon atom, which is called the alpha carbon atom.
Attached to the carbon atom is a variable group (R); it is in their R groups
that the molecules of the 20 standard amino acids differ from one another. In
the simplest of the acids, glycine, the R consists of a single hydrogen atom.
Other amino acids have more complex R groups that contain carbon as well as
hydrogen and may include oxygen, nitrogen, or sulfur, as well.
When a living cell makes protein, the carboxyl group of
one amino acid is linked to the amino group of another to form a peptide bond.
The carboxyl group of the second amino acid is similarly linked to the amino
group of a third, and so on, until a long chain is produced. This chainlike
molecule, which may contain from 50 to several hundred amino acid subunits, is
called a polypeptide. A protein may be formed of a single polypeptide chain, or
it may consist of several such chains held together by weak molecular bonds.
Each protein is formed according to a precise set of instructions contained
within the nucleic acid (see Nucleic Acids), which is the
genetic material of the cell. These instructions determine which of the 20
standard amino acids are to be incorporated into the protein, and in what
sequence. The R groups of the amino acid subunits determine the final shape of
the protein and its chemical properties; an extraordinary variety of proteins
can be produced from the same 20 subunits.
The standard amino acids serve as raw materials for the
manufacture of many other cellular products, including hormones (see
Hormone) and pigments. In addition, several of these amino
acids are key intermediates in cellular metabolism (see
Metabolism).
Most plants and microorganisms are able to use inorganic
compounds to make all the amino acids they require for normal growth. Animals,
however, must obtain some of the standard amino acids from their diet in order
to survive; these particular amino acids are called essential. Essential amino
acids for humans include lysine, tryptophan, valine, histidine, leucine,
isoleucine, phenylalanine, threonine, methionine, and arginine. They are found
in adequate amounts in protein-rich foods from animal sources or in carefully
chosen combinations of plant proteins.
In addition to the amino acids that form proteins, more
than 150 other amino acids have been found in nature, including some that have
the carboxyl and amino groups attached to separate carbon atoms. These unusually
structured amino acids are most often found in fungi and higher plants.
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