aminoacid

Amino acids are organic molecules that serve as the building blocks of proteins. Each consists of an alpha carbon bearing an amino group (—NH2), a carboxyl group (—COOH), a hydrogen atom, and a distinctive side chain (R). The side chain determines the identity and chemical properties of each amino acid. Most amino acids are chiral, with the exception of glycine, which has two hydrogen atoms on the alpha carbon.

Twenty standard amino acids are normally encoded by the genetic code and incorporated into proteins; these are called proteinogenic amino acids. They can be classified by the chemical nature of their side chains (nonpolar, polar uncharged, acidic, basic) as well as by nutritional essentiality. Humans require nine essential amino acids in the diet: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine; cysteine and tyrosine are considered conditionally essential in certain circumstances.

In physiological conditions, amino acids exist predominantly as zwitterions, with a protonated amino group and a deprotonated carboxyl group. They participate in various interactions that influence protein folding and function, including hydrogen bonding and ionic interactions.

Amino acids link together through peptide bonds between the carboxyl group of one residue and the amino group of the next, forming polypeptides that fold into proteins. The sequence of amino acids determines a protein’s structure and function. Proteins can exhibit multiple levels of structure, including secondary structures such as α-helices and β-sheets, and higher-level three-dimensional conformations.

Most amino acids used in protein synthesis are the L-enantiomers. D-amino acids occur in some bacterial cells and certain peptides. Many amino acids may be modified after synthesis (post-translational modifications) to yield diverse biological functions.