Amino acids (//, //, or //) are biologically important organic compounds composed of amine (-NH2) and carboxylic acid (-COOH)functional groups, along with a side-chain specific to each amino acid. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen, though other elements are found in the side-chains of certain amino acids. About 500 amino acids are known and can be classified in many ways. Structurally they can be classified according to the functional groups' locations as alpha- (α-), beta- (β-), gamma- (γ-) or delta- (δ-) amino acids; other categories relate to polarity, pH level, and side chain group type (aliphatic, acyclic, aromatic, containing hydroxyl or sulfur, etc.) In the form of proteins, amino acids comprise the second largest component (after water) of human muscles, cells and other tissues. Outside proteins, amino acids perform critical roles in processes such as neurotransmitter transport and biosynthesis.
Amino acids having both the amine and carboxylic acid groups attached to the first (alpha-) carbon atom have particular importance in biochemistry. They are known as 2-, alpha-, or α-amino acids (generic formula H2NCHRCOOH in most cases where R is an organic substituent known as a "side-chain"); often the term "amino acid" is used to refer specifically to these. They include the 22 proteinogenic ("protein-building") amino acids which combine into peptide chains ("polypeptides") to form the building blocks of a vast array of proteins. These are all L-stereoisomers ("left-handed" isomers) although a few D-amino acids ("right-handed") occur in bacterial envelopes and some antibiotics. Twenty of the proteinogenic amino acids are encoded directly by triplet codonsin the genetic code and are known as "standard" amino acids. The other two ("non-standard" or "non-canonical") are pyrrolysine (found in methanogenic organisms and other eukaryotes) and selenocysteine (present in many noneukaryotes as well as most eukaryotes). For example, 25 human proteins include selenocysteine (Sec) in their primary structure, and the structurally characterized enzymes (selenoenzymes) employ Sec as the catalytic moiety in their active sites. Pyrrolysine and selenocysteine are encoded via variant codons; for example, selenocysteine is encoded by stop codon and SECIS element. Codon–tRNA combinations not found in nature can also be used to "expand" the genetic code and create novel proteins known as alloproteinsincorporating non-proteinogenic amino acids.
Many important proteinogenic and non-proteinogenic amino acids also play critical non-protein roles within the body. For example: in the human brain, glutamate (standard glutamic acid) and gamma-amino-butyric acid("GABA", non-standard gamma-amino acid) are respectively the main excitatory and inhibitory neurotransmitters; hydroxyproline (a major component of the connective tissue collagen) is synthesised fromproline; the standard amino acid glycine is used to synthesise porphyrins used in red blood cells; and the non-standard carnitine is used in lipid transport.
9 of the 20 standard amino acids are called "essential" for humans because they cannot be created from othercompounds by the human body, and so must be taken in as food. Others may be conditionally essential for certain ages or medical conditions. Essential amino acids may also differ between species.
Because of their biological significance, amino acids are important in nutrition and are commonly used innutritional supplements, fertilizers, and food technology. Industrial uses include the production of drugs,biodegradable plastics and chiral catalysts.