Amino Acids and Peptides

Twenty α-amino acids are the monomers from which proteins are made. These amino acids share with other biochemical monomers a property essential to their role in polymer formation: They contain at least two different chemical groups able to react with each other to form a covalent linkage. In the amino acids these are the protonated amino (NH3 +) and carboxylate (COO–) groups. The characteristic linkage in the protein polymer is the peptide (amide) linkage whose formation can be imagined to occur by the splitting out of water between the carboxyl of one amino acid and the amino group of another.
This equation is not intended to imply a mechanism for peptide synthesis. The equilibrium position for this reaction in an aqueous solution favors the free amino acids rather than the peptide. Therefore, both biological and laboratory syntheses of peptides usually do not involve a simple splitting out of water. Since the dipeptide still contains reactive carboxyl and amino groups, other amino acid units can be joined by additional peptide linkages to form polypeptides. These range from short-chain oligomers to polymers of from ∼50 to several thousand amino acid units, the proteins.