Amino Acid

In chemistry, an amino acid is any molecule that contains both amino and carboxylic acid functional groups.

In biochemistry, this shorter and more general term is frequently used to refer to alpha amino acids, that is, those amino acids in which the amino and carboxylate functionalities are attached to the same carbon. Some molecules like proline don't contain an amino group and chemically are not amino acids (technically proline is an imino acid), but are also classified as such because of functional similarity to real amino acids in living cells.

Amino acids are a biochemical building block. They form the building blocks of long chemical chains called proteins (see below). Amino acids are also the building blocks of shorter chains called peptides.

There are 20 amino acids that are encoded by the standard genetic code. Other amino acids contained in proteins are usually the result of modification after translation (protein synthesis). These modifications are often essential for the function of the protein. At least two amino acids other than the standard 20 are sometimes incorporated into proteins during translation:

• Selenocysteine is incorporated into some proteins at a UGA codon, which is normally a stop codon.
• Pyrrolysine is used by some methanogens in enzymes that they use to produce methane. It is coded for similarly to selenocysteine but with the codon UAG instead.

Over 500 amino acids have been found in nature.

As well as their role in protein synthesis, there are other biologically important amino acids such as the neurotransmitter GABA, carnitine (used in lipid transport within a cell), ornithine, and taurine.

Some of the 20 amino acids in the genetic code are essential amino acids, meaning that they cannot be synthesized by the body from other compounds through chemical reactions, but instead must be taken in with food. In humans, the essential amino acids are lysine, leucine, isoleucine, methionine, phenylalanine, threonine, tryptophan, valine, and (in children) histidine and arginine.

The general structure of alpha amino acids is:

   COOH
   |
 H-C-R
   |
   NH2