Biochemistry - Chemical Reactions in Living Cells, Volume 1 - D. Metzler 1980

The Molecules We Are Made Of
Proteins

Proteins are built from 20 α-amino acid monomer units. These small molecules share a property common to all polymerization-capable molecules: they contain at least two distinct chemical groups capable of reacting with each other to form a covalent bond. In Amino Acids, these groups are the amino group (—NH₂) and the carboxyl group (—COOH), while the bond that defines The formation of the protein polymer is the peptide (amide) bond. The formation of a peptide bond can be viewed as the elimination of a Water molecule from the combining —COOH^- and —NH₂ groups [equation (2-7)]. In an aqueous environment, the equilibrium of such reactions lies in the direction of free amino acids rather than the peptide. Consequently, Peptide Synthesis (both in vivo and in vitro) proceeds via indirect pathways rather than a simple dehydration reaction.

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Since the dipeptide in equation (2-7) also retains reactive carboxyl and amino groups, additional amino acid monomers can attach to it via new peptide bonds, ultimately yielding a polypeptide (protein).