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.
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. Proline is the only cyclic proteinogenic amino acid. 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.
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, citrulline, homocysteine, hydroxyproline, hydroxylysine, and sarcosine.
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 proteinogenic alpha amino acids is:
COOH | H-C-R | NH2Where "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. They are also abundant components of the cell walls of bacteria. 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. (Also see bond.)
Twenty amino acids are represented in the genetic code:
The chemical properties of the side chains are:
|Abbrev.||Full Name||Side chain type||Mass||Isoelectric point||Remarks|
|C||Cys||Cysteine||hydrophilic||121.16||5.05||Two cysteines can form a disulfide bond. This enforces tertiary structure, and such proteins as insulin have disulfide bonds.|
|G||Gly||Glycine||hydrophilic||75.07||6.06||Because of the two hydrogen atoms at the α carbon, glycine is not optically active.|
|P||Pro||Proline||hydrophobic||115.13||6.30||Can disrupt protein folding structures like α helix or β sheet.|
|hydrophobic||positive||negative||polar||charged||small||tiny||aromatic||aliphatic||van der Waals volume|