Proteins are linear polymers formed by linking the a-carboxyl group of one amino acid to the a-amino group of another amino acid with a peptide bond. The formation of a dipeptide from two amino acids is accompanied by the loss of a water molecule. The equilibrium of this reaction lies on the side of hydrolysis rather than synthesis. Hence, the biosynthesis of peptide bonds requires an input of free energy. Nonetheless, peptide bonds are quite stable kinetically; the lifetime of a peptide bond in aqueous solution in the absence of a catalyst approaches 1000 years.