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In proteins, amino acids are covalently connected to form polypeptides

المؤلف:  Zlatanova, J., & van Holde, K. E.

المصدر:  Molecular Biology: Structure and Dynamics of Genomes and Proteomes (2023)

الجزء والصفحة:  2nd Edition , p44-45

2026-10-13

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The bonds that connect amino acids together into a protein polymer are called peptide bonds, and the polymer is a polypeptide. These bonds are formed between two monomers by a process that effectively eliminates a water molecule (Figure1A), although in cells the reaction is actually much more complex and indirect. The remainder of the amino acid that remains within the polypeptide chain is called an amino acid residue. The reverse of this process, cleaving the peptide bond by adding a water molecule, is called proteolysis and is a specialized form of hydrolysis, or splitting by adding water. In aqueous solution, the hydrolysis of polypeptides occurs spontaneously, although the process is slow unless it is catalyzed by proteolytic enzymes. Consequently, the cell must use a special energy source to make proteins from amino acids. Spontaneous hydrolysis also explains why no protein lasts forever.

Fig1. The peptide bond. (A) A peptide bond is formed between two amino acids by elimination of a molecule of water. (B) The peptide bond is planar and rigid. Rotation is possible only around the N-Cα bond (angle φ), and around the Cα-C bond (angle ψ). The group of atoms about the peptide bond could exist in two possible configurations, cis and trans. The trans configuration, in which the adjacent Cα atoms are placed further apart, is favored in proteins, as it avoids steric clashes between bulky –R groups.

The peptide bond is rigid and planar because of the directionality of the N- and C-carboxyl orbitals (Figure 1B). However, rotation is relatively free about the C-carboxyl–Cα and Cα–N bonds. This allows a long polypeptide chain to have great flexibility and many possible conformations.