In biochemistry, this 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.
There are 20 amino acids which are directly expressed in the genetic code. If a protein contains a different amino acid it must have been modified after translation. These modifications are often essential for the function of the protein. Over 500 amino acids 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).