Amino acids

Amino acids are molecules containing an amine group, a carboxylic acid group and a side chain that varies between different amino acids. These molecules contain the key elements of carbon, hydrogen,oxygen, and nitrogen. These molecules are particularly important in biochemistry, where this term usually refers to alpha-amino acids with the general formula H₂NCHRCOOH, where R is an organicsubstituent. In an alpha amino acid, the amino group is attached to the carbon atom immediately adjacent to the carboxylate group (the α–carbon), but note that other types of amino acid exist when the amino group is attached to another carbon atom (for example, in gamma-amino acids such as gamma-amino-butyric acid the carbon atom to which the amino group attaches is separated from the carboxylate group by two other carbon atoms). The various alpha amino acids differ in which side chain (R group) is attached to their alpha carbon and these can vary in size from just one hydrogen atom in glycine or a methyl group in alanine, through to a large heterocyclic group in tryptophan.

Amino acids are critical to life, and have many functions inmetabolism. One particularly important function is to serve as the building blocks of proteins, which are just linear chains of amino acids, or more precisely, amino acid residues (the remaining part of an amino acid once a water molecule has been lost due to peptide bonding). Every protein is chemically defined by the order of amino acid residues, their primary structure and this, in turn, determines their secondary structure (for example well-described features such as alpha-helices or beta-pleated sheets), tertiary structure (the shape of the individual protein, for example globular as in mostenzymes or linear as in collagen) or quaternary structure (the overall shape of the protein complex if several protein monomers are grouped together, as in the hemoglobin complex, which consists of four hemoglobin monomers held together by hydrogen bonds that are capable of reacting together allosterically when one or more of them binds another molecule for example oxygen). Just as the letters of the alphabet can be combined to form an almost endless variety of words, amino acids can be linked together in varying sequences to form a vast variety of proteins. Amino acids are also important in many other biological molecules, for example they play vital roles in coenzymes such as S-adenosylmethionine. Due to their central role in biochemistry, amino acids are very important innutrition and are commonly used in food technology and industry. For example, monosodium glutamate is a common flavor enhancer that gives foods the taste called umami. They are also used in industry where applications include the production of biodegradable plastics, drugsand chiral catalysts.

In human nutrition

When taken up into the human body from the diet, the twenty-one 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 acidthat enters the citric acid cycle. Glucogenic amino acids can also be converted into glucose, through gluconeogenesis.

Pyrrolysine trait is restricted to several microbes, and only one organism has both Pyl and Sec. Of the twenty-two standard amino acids, eight 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 quite complicated since cysteine, taurine,tyrosine, histidine and arginine are semiessential amino acids in children, because the metabolic pathways that synthesize these amino acids are not fully developed.^[3[35] 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.

Uses in technology

Amino acids are used for a variety of applications in industry but their main use is as additives to animal feed. This is necessary since many of the bulk components of these feeds, such as soybeans, either have low levels or lack some of the essential amino acids: lysine, methionine, threonine, and tryptophan are most important in the production of these feeds. The food industry is also a major consumer of amino acids, particularly glutamic acid, which is used as a flavor enhancer, and Aspartame (aspartyl-phenylalanine-1-methyl ester) as a low-calorie artificial sweetener.The remaining production of amino acids is used in the synthesis of drugs and cosmetics.

Amino acid derivative	Pharmaceutical application
5-HTP (5-hydroxytryptophan)	Experimental treatment for depression.
L-DOPA (L-dihydroxyphenylalanine)	Treatment for Parkinsonism.
Eflornithine	Drug that inhibits ornithine decarboxylase and is used in the treatment of sleeping sickness.

Expanded genetic code

Since 2001, 40 non-natural amino acids have been added into protein by creating a unique codon (recoding) and a corresponding transfer-RNA:aminoacyl – tRNA-synthetase pair to encode it with diverse physicochemical and biological properties in order to be used as a tool to exploring protein structure and function or to create novel or enhanced proteins.

Chemical building blocks

Amino acids are important as low-cost feedstocks. These compounds are used in chiral pool synthesis as enantiomerically-pure building blocks.

Amino acids have been investigated as precursors chiral catalysts, e.g. for asymmetric hydrogenation reactions, although no commercial applications exist.

Biodegradable plastics

Amino acids are under development as components of a range of biodegradable polymers. These materials have applications as environmentally-friendly packaging and in medicine in drug delivery and the construction of prosthetic implants. These polymers include polypeptides, polyamides, polyesters, polysulfides and polyurethanes with amino acids either forming part of their main chains or bonded as side chains. These modifications alter the physical properties and reactivities of the polymers.An interesting example of such materials is polyaspartate, a water-soluble biodegradable polymer that may have applications in disposable diapers and agriculture. Due to its solubility and ability to chelate metal ions, polyaspartate is also being used as a biodegradeable anti-scaling agent and a corrosion inhibitor.In addition, the aromatic amino acid tyrosine is being developed as a possible replacement for toxic phenols such as bisphenol A in the manufacture of polycarbonates.