Amino acids are the building blocks of proteins. There are 20 naturally occurring amino acids. Amino acids exist in proteins as L-optical isomers, however they can extist as D-isomers in isolated examples, e.g. some bacterial cell walls contain D-isomers.
Amino acids can also be characterized as polar or non-polar, these dictate the amino acid function. There are 10 non-polar amino acids, these are found in protein core. There are also 10 polar amino acids, these have enzymatic roles, they can be used to bind DNA, metals and other naturally occuring ligands. There are essential amino acids and non-essential amino acids. Essential amino acids are the ones that the body cannot synthesise on its own. The essential amino acids in humans are histidine, leucine, isoleucine, lysine, methionine, valine, phenylalanine, tyrosine and tryptophan. These amino acids have to be supplied to the body via digested proteins that are then absorbed in the intestine and transported in the blood to where they are needed. The digestion of cellular proteins is also an important source for amino acids. Non-essential amino acids can be synthesised from compounds already existing in the body.
Amino acids have been abbreviated into a 3 letter code as well as a 1 letter code. For example, glycine has the 3 letter code 'Gly' and is assigned the letter 'G' (see single letter amino acid codes).
List of the 20 Amino acids, single letter code, three letter code, their charges, and side chain polarity:
|Amino acid||single letter code||three letter code||charge||polarity|
Amino acid structure
All amino acids have a carboxyl terminus, and an amino terminus, but they differ in their residual groups. Amino acids are bonded together by a covalent linkage called a peptide bond . Amino acids contain both a carboxyl group (COOH) and an amino group (NH2). The core amino acid structure is:
where R is the side chain tha differ in all amino acids.
Large amino acids form the rigid region of the polypeptide backbone while the small amino acids form the flexible regions of the polypeptide allowing the protein to fold into it's three dimensional shape. The core of the polypeptide is made up of the hydrophobic amino acids like phenyalanine, tyrosine, and tryptophan .
Amino acids are referred to as chiral due to the alpha carbon being connected to four different groups. They can exist as one of two mirror images referred to as the L isomer and the D isomer with only the L form of the amino acid isomer present within proteins. 
Amino acids in Translation
Berg.J, Tymoczko J, Stryer L,:Biochemistry Sixth Edition(2007), WH Freeman, New York p.27
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