In chemistry, an Amino Acids is a molecule containing both amine and carboxyl functional groups. In biochemistry, this term refers to alpha-amino acids with the general formula H2NCHRCOOH, where R is an organic substituent. In the alpha amino acids
, the amino and carboxylate groups are attached to the same carbon, which is called the á–carbon. The various alpha amino acids differ in which side chain (R group) is attached to their alpha carbon. They can vary in size from just a hydrogen atom in glycine through a methyl group in alanine to a large heterocyclic group in tryptophan.
Beyond the amino acids that are found in all forms of life, many non-natural amino acids have vital roles in technology and industry. For example, the chelating agents EDTA and nitrilotriacetic acid are alpha amino acids that are important in the chemical industry.
Alpha-amino acids are the building blocks of proteins. Amino acids
combine in a condensation reaction, that is, through dehydration synthesis, that releases water and the new "amino acid residue" that is held together by a peptide bond. Proteins are defined by their unique sequence of amino acid
residues; this sequence is the primary structure of the protein. Just as the letters of the alphabet can be combined to form an almost endless variety of words, amino acids can be linked in varying sequences to form a vast variety of proteins.
Twenty standard amino acids are used by cells in protein biosynthesis, and these are specified by the general genetic code. These 20 amino acids are biosynthesized from other molecules, but organisms differ in which ones they can synthesize and which ones must be provided in their diet. The ones that cannot be synthesized by an organism are called essential amino acids.
Functions in proteins:
A polypeptide is a chain of amino acids.Amino acids are the basic structural building units of proteins. They form short polymer chains called peptides or longer chains called either polypeptides or proteins. The process of such formation from an mRNA template is known as translation, which is part of protein biosynthesis. Twenty amino acids are encoded by the standard genetic code and are called proteinogenic or standard amino acids. Other amino acids contained in proteins are usually formed by post-translational modification, which is modification after translation in protein synthesis. These modifications are often essential for the function or regulation of a protein; for example, the carboxylation of glutamate allows for better binding of calcium
cations, and the hydroxylation of proline is critical for maintaining connective tissues and responding to oxygen starvation. Such modifications can also determine the localization of the protein, e.g., the addition of long hydrophobic groups can cause a protein to bind to a phospholipid membrane.
The 20 standard amino acids are either used to synthesize proteins and other biomolecules or oxidized to urea and carbon dioxide as a source of energy. The oxidation pathway starts with the removal of the amino group by a transaminase, the amino group is then fed into the urea cycle. The other product of transamidation is a keto acid that enters the citric acid cycle. Glucogenic amino acids can also be converted into glucose, through gluconeogenesis.
Hundreds of types of non-protein amino acids have been found in nature and they have multiple functions in living organisms. Microorganisms and plants can produce uncommon amino acids. In microbes, examples include 2-aminoisobutyric acid and lanthionine, which is a sulfide-bridged alanine dimer. Both these amino acids are found in peptidic lantibiotics such as alamethicin. While in plants, 1-aminocyclopropane-1-carboxylic acid is a small disubstituted cyclic amino acid that is a key intermediate in the production of the plant hormone ethylene.
In humans, non-protein amino acids also have important roles, such as the neurotransmitter gamma-aminobutyric acid. Many amino acids are used to synthesize other molecules, for example:
Tryptophan is a precursor of the neurotransmitter serotonin.
Glycine is a precursor of porphyrins such as heme.
Arginine is a precursor of nitric oxide
Carnitine is used in lipid transport within the cell.
Ornithine and S-adenosylmethionine are precursors of polyamines.
Homocysteine is an intermediate in S-adenosylmethionine recycling.
Some amino acids have even been detected in meteorites, especially in a type known as carbonaceous chondrites. This observation has prompted the suggestion that life may have arrived on earth from an extraterrestrial source. The Murchison meteorite is especially rich in amino acids and other organic matter.
Further information: Protein in nutrition
Of the 20 standard proteinogenic amino acids, 8 are called essential amino acids because the human body cannot synthesize them from other compounds at the level needed for normal growth, so they must be obtained from food.However, the situation is a little more complicated since cysteine, tyrosine, histidine and arginine
are semiessential amino acids in children, because the metabolic pathways that synthesize these amino acids are not fully developed. The amounts required also depend on the age and health of the individual, so it is hard to make general statements about the dietary requirement for some amino acids.
(*) Essential only in certain cases.
Several common mnemonics have evolved for remembering the ten amino acids often described as essential. PVT TIM HALL ("Private Tim Hall") uses the first letter of each of these amino acids. Another mnemonic that frequently occurs in student practice materials beneath "AH TV TILL Past Midnight", is "These ten valuable amino acids have long preserved life in man".
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