Definitions

# amino acid

amino acid, any one of a class of simple organic compounds containing carbon, hydrogen, oxygen, nitrogen, and in certain cases sulfur. These compounds are the building blocks of proteins. They are characterized by the presence of a carboxyl group (COOH) and an amino group (NH2) attached to the same carbon at the end of the compound. The 20 amino acids commonly found in animals are alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, serine, threonine, tryptophan, tyrosine, and valine. More than 100 less common amino acids also occur in biological systems, particularly in plants. Every amino acid except glycine can occur as either of two optically active stereoisomers, d or l; the more common isomer in nature is the l-form. When the carboxyl carbon atom of one amino acid covalently binds to the amino nitrogen atom of another amino acid with the release of a water molecule, a peptide bond is formed. Amino acids are released in the intestinal tract by the digestion of food proteins and are then carried in the bloodstream to the body cells, where they are used for growth, maintenance, and repair. Cellular catabolism breaks amino acids down into smaller fragments. Many of the amino acids necessary in metabolism can be synthesized in the human or animal body when needed; these are called nonessential. Others cannot be synthesized in sufficient quantities; these are termed essential and must be provided in the diet.

Any of a class of organic compounds in which a carbon atom has bonds to an amino group (singlehorzbondNH2), a carboxyl group (singlehorzbondCOOH), a hydrogen atom (singlehorzbondH), and an organic side group (called singlehorzbondR). They are therefore both carboxylic acids and amines. The physical and chemical properties unique to each result from the properties of the R group, particularly its tendency to interact with water and its charge (if any). Amino acids joined linearly by peptide bonds (see covalent bond) in a particular order make up peptides and proteins. Of over 100 natural amino acids, each with a different R group, only 20 make up the proteins of all living organisms. Humans can synthesize 10 of them (by interconversions) from each other or from other molecules of intermediary metabolism, but the other 10 (essential amino acids: arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine) must be consumed in the diet.

In enzymology, a nonpolar-amino-acid-transporting ATPase is an enzyme that catalyzes the chemical reaction

ATP + H2O + nonpolar amino acidout $rightleftharpoons$ ADP + phosphate + nonpolar amino acidin

The 3 substrates of this enzyme are ATP, H2O, and nonpolar amino acid, whereas its 3 products are ADP, phosphate, and nonpolar amino acid.

This enzyme belongs to the family of hydrolases, specifically those acting on acid anhydrides acting on acid anhydrides to catalyse transmembrane movement of substances. The systematic name of this enzyme class is ATP phosphohydrolase (nonpolar-amino-acid-transporting).

## References

• Kuan G, Dassa E, Saurin W, Hofnung M, Saier MH Jr "Phylogenetic analyses of the ATP-binding constituents of bacterial extracytoplasmic receptor-dependent ABC-type nutrient uptake permeases". Res. Microbiol. 146 271–8.
• Saier MH Jr "Molecular phylogeny as a basis for the classification of transport proteins from bacteria, archaea and eukarya". Adv. Microb. Physiol. 40 81–136.
• Griffiths JK and Sansom CE "The Transporter Factsbook, Academic Press, San Diego, 1998".