Amino Acid

Amino Acid

Amino acids ( /əˈmiːnoʊ/, /əˈmaɪnoʊ/, or /ˈæmɪnoʊ/) are biologically important organic compounds made from 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. About 500 amino acids are known which 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, acid/base/neutral, and side chain group type (including: aliphatic, acyclic, hydroxyl or sulphur-containing, aromatic). In the form of proteins, amino acids comprise the second largest component other than water of human muscles, cells and other tissues. Outside proteins, amino acids also perform critical biological roles including neurotransmitters, transport, and in synthesis.

Amino acids having both the amine and carboxylic acid groups attached to the first, or 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. 20 of the 22 proteinogenic amino acids are known as "standard" amino acids-those found in human beings and other eukaryotes, and which are encoded directly within the universal genetic code. The 2 exceptions are the "non-standard" or "non-canonical" pyrrolysine — found only in some methanogenic organisms but not humans — and selenocysteine; both of these are encoded via variant codons signaled by mRNA instead. Codon–tRNA combinations not found in nature can also be used to "expand" the genetic code and create novel proteins known as "alloproteins" incorporating non-proteinogenic amino acids.

Many important proteinogenic and non-proteinogenic amino acids also play critical non-protein roles within the body. For example the standard glutamic acid (glutamate) and the non-standard gamma-amino acid gamma-amino-butyric acid (GABA) are respectively the brain's main excitatory and inhibitory neurotransmitters, hydroxyproline-a major component of the connective tissue collagen-is synthesised from proline, 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" amino acids for humans because they cannot be created from other compounds by the human body, and so must be taken in as food. Others may be conditionally essential for some 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 in nutritional supplements, fertilizers, and food technology. Industrial uses include the production of biodegradable plastics, drugs, and chiral catalysts.