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

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. 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
A	Ala	Alanine	hydrophobic	89.09	6.11
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.
D	Asp	Aspartic acid	acidic	133.10	2.85
E	Glu	Glutamic acid	acidic	147.13	3.15
F	Phe	Phenylalanine	hydrophobic	165.19	5.49
G	Gly	Glycine	hydrophilic	75.07	6.06	Because of the two hydrogen atoms at the α carbon, glycine is not optically active.
H	His	Histidine	basic	155.16	7.60
I	Ile	Isoleucine	hydrophobic	131.17	6.05
K	Lys	Lysine	basic	146.19	9.60
L	Leu	Leucine	hydrophobic	131.17	6.01
M	Met	Methionine	hydrophobic	149.21	5.74
N	Asn	Asparagine	hydrophilic	132.12	5.41
P	Pro	Proline	hydrophobic	115.13	6.30	Can disrupt protein folding structures like α helix or β sheet.
Q	Gln	Glutamine	hydrophilic	146.15	5.65
R	Arg	Arginine	basic	174.20	10.76
S	Ser	Serine	hydrophilic	105.09	5.68
T	Thr	Threonine	hydrophilic	119.12	5.60
V	Val	Valine	hydrophobic	117.15	6.00
W	Trp	Tryptophan	hydrophobic	204.23	5.89
Y	Tyr	Tyrosine	hydrophilic	181.19	5.64

Amino Acid	hydrophobic	positive	negative	polar	charged	small	tiny	aromatic	aliphatic	van der Waals volume
Ala	X	-	-	-	-	X	X	-	-	67
Cys	X	-	-	-	-	X	-	-	-	86
Asp	-	-	X	X	X	X	-	-	-	91
Glu	-	-	X	X	X	-	-	-	-	109
Phe	X	-	-	-	-	-	-	X	-	135
Gly	X	-	-	-	-	X	X	-	-	48
His	X	X	-	X	X	-	-	X	-	118
Lys	X	X	-	X	X	-	-	-	-	135
Ile	X	-	-	-	-	-	-	-	X	124
Leu	X	-	-	-	-	-	-	-	X	124
Met	X	-	-	-	-	-	-	-	-	124
Asn	-	-	-	X	-	X	-	-	-	96
Pro	-	-	-	-	-	X	-	-	-	90
Gln	-	-	-	X	-	-	-	-	-	114
Arg	-	X	-	X	X	-	-	-	-	148
Ser	-	-	-	X	-	X	X	-	-	73
Thr	X	-	-	X	-	X	-	-	-	93
Val	X	-	-	-	-	X	-	-	X	105
Trp	X	-	-	X	-	-	-	X	-	163
Tyr	X	-	-	X	-	-	-	X	-	141