Amino Acid

An amino acid is an organic molecule containing an amino group and a carboxyl group. The amino acids used to build proteins are primarily alpha-amino acids, in which these groups are attached to the same central carbon atom, called the alpha carbon. This carbon also carries a hydrogen atom and a variable side chain, or R group, that distinguishes one amino acid from another .^1,2

Proteins are constructed mainly from 20 standard amino acids. Their side chains differ in properties such as size, charge, polarity, and hydrophobicity. These differences influence how amino acids interact with water and with one another and therefore contribute to protein structure and function. At physiological pH, the amino and carboxyl groups of free amino acids are generally ionized, so amino acids commonly carry both positive and negative charges .^1,2

During protein synthesis, amino acids are joined by covalent peptide bonds between the carboxyl group of one amino acid and the amino group of the next. The resulting chain is called a polypeptide. The particular order of amino acids in the chain is its primary structure and influences the three-dimensional structures the polypeptide can adopt .¹

Strictly, an amino acid incorporated into a peptide or protein is called an amino acid residue. Amino acid is also a broader chemical term, and many amino acids occur in biology that are not among the standard amino acids directly incorporated into proteins during translation .^1,2