In biochemistry, an -amino acid is an organic monomer consisting of an amino group, a carboxyl group, a hydrogen atom, and a residual group (commonly denoted as simply "R") covalently bonded to a central (alpha) carbon (C). Amino acids can link via peptide bonds to create short chains (polymers) called polypeptides, often referred to simply as peptides. Longer, more complex chains (or combinations of multiple chains) are called proteins, which have a vast array of biological functions. In peptides and proteins, the R-groups are often called "side chains", because they are not involved in the polymer backbone. Amino acids have two stereo isomers (mirror images of each other), and only the L-isomer is used in protein synthesis.
The Amino Acids
There are twenty common amino acids used by nearly all living organisms as well as several rarer ones. The difference between any two amino acids is the residual (R) group, and they can be categorized based on the chemical properties of the R-groups. Each amino acid has a three-letter abbreviation and a one-letter symbol.
Amino Acids with Non-Polar R-Groups
Amino Acids with Aromatic R-Groups
Amino Acids with Positively Charged R-Groups
Amino Acids with Negatively Charged R-Groups
Most organisms are unable to synthesize all twenty amino acids. Essential amino acids must be taken in from the environment, whereas nonessential amino acids can be produced by the organism. For humans, there are seven essential amino acids that must be obtained in the diet in order to survive; there are therefore thirteen non-essential amino acids for humans. The number of and the identity of the essential amino acids varies among species.
For humans, the nonessential amino acids can be synthesized in cells through a variety of pathways. For some animals, the amino acid carnitine is essential, but most animals can produce it from lysine.