Have you ever thought about
what makes up your body? Only 20 amino acids! Take a look at the graphic below,
to discover the structure of each of these, plus information on the notation
used to represent them.
Source: Compound Interest. Click to enlarge.
Amino acids are organic
compounds containing amine (-NH2) and carboxyl (-COOH) functional groups, along
with a side chain (R group) specific to each amino acid. The key elements of an
amino acid are carbon, hydrogen, oxygen, and nitrogen, although other elements
are found in the side chains of certain amino acids. About 500 amino acids are
known and can be classified in many ways. They can be classified according to
the core structural functional groups’ locations as alpha- (α-), beta- (β-),
gamma- (γ-) or delta- (δ-) amino acids; other categories relate to polarity, pH
level, and side chain group type (aliphatic, acyclic, aromatic, containing
hydroxyl or sulfur, etc.). In the form of proteins, amino acid residues form
the second-largest component (water is the largest) of human muscles and other
tissues. Beyond their role as residues in proteins, amino acids participate in
a number of processes such as neurotransmitter transport and biosynthesis.
In biochemistry, amino acids
having both the amine and the carboxylic acid groups attached to the first
(alpha-) carbon atom have particular importance. They are known as 2-, alpha-,
or α-amino acids (generic formula H2NCHRCOOH in most cases, where R is an
organic substituent known as a “side chain”); often the term
“amino acid” is used to refer specifically to these. They include the
22 proteinogenic (“protein-building”) amino acids, which combine into
peptide chains (“polypeptides”) to form the building-blocks of a vast
array of proteins. These are all L-stereoisomers (“left-handed”
isomers), although a few D-amino acids (“right-handed”) occur in
bacterial envelopes, as a neuromodulator (D-serine), and in some antibiotics. 

Twenty of the proteinogenic amino acids are encoded directly by triplet codons
in the genetic code and are known as “standard” amino acids. The
other two (“non-standard” or “non-canonical”) are
selenocysteine (present in many noneukaryotes as well as most eukaryotes, but
not coded directly by DNA), and pyrrolysine (found only in some archea and one
bacterium). Pyrrolysine and selenocysteine are encoded via variant codons; for
example, selenocysteine is encoded by stop codon and SECIS element.
N-formylmethionine (which is often the initial amino acid of proteins in
bacteria, mitochondria, and chloroplasts) is generally considered as a form of
methionine rather than as a separate proteinogenic amino acid. Codon–tRNA
combinations not found in nature can also be used to “expand” the
genetic code and create novel proteins known as alloproteins incorporating
non-proteinogenic amino acids.
Many important proteinogenic
and non-proteinogenic amino acids have biological functions. For example, in
the human brain, glutamate (standard glutamic acid) and gamma-amino-butyric
acid (“GABA”, non-standard gamma-amino acid) are, respectively, the
main excitatory and inhibitory neurotransmitters. Hydroxyproline, a major
component of the connective tissue collagen, is synthesised from proline.
Glycine is a biosynthetic precursor to porphyrins used in red blood cells.
Carnitine is used in lipid transport.
Nine proteinogenic amino acids
are called “essential” for humans because they cannot be created from
other compounds by the human body and so must be taken in as food. Others may
be conditionally essential for certain ages or medical conditions. Essential
amino acids may also differ between species.

Because of their biological
significance, amino acids are important in nutrition and are commonly used in
nutritional supplements, fertilizers, and food technology. Industrial uses
include the production of drugs, biodegradable plastics, and chiral catalysts.
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