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.

There are 20 amino acide which are directly expressed in the genetic code. If a protein cotains a different amino acide it was modified after building the protein. These modifications are often esential for the function of the protein. Over 500 amino acides have been found in nature.

The generalized structure of alpha amino acids is:

    COOH
    |
  H-C-R
    |
    NH2

Where "R" represents a side chain specific to each particular amino acid. Amino acids are usually classified by properties of the side chain into four groups: acidic, basic, hydrophilic (polar), and hydrophobic (nonpolar).

Except for glycine, where R = H, amino acids occur in two possible optical isomers, called D and L; L amino acids represent the vast majority of amino acids found in proteins. D amino acids are found in some proteins produced by exotic sea-dwelling organisms, such as cone snails. Proteins are created by polymerization of amino acids by peptide bonds in a process called translation:

1. Amino acid; 2, zwitterion structure; 3, two amino acids forming a peptide bond.

Twenty amino acids are represented in the genetic code:

The chemical properties of the side chains are: