Peptide Bonds

 Amino acids are joined together to form a protein by peptide bonds.

 Peptide bonds are created by the condensation of the carboxyl (COO^-) group of one amino acid with the amino (NH₃⁺) group of the next (Fig. 1). Condensation involves the removal of one water molecule.

 Since peptide bonds are covalent, they are relatively stable, and in the living cell are broken only rarely, usually as the result of a specific enzymatic action. Peptide bonds are broken in a process which is functionally the opposite of the condensation reaction. This process involves the addition of H₂O and is termed hydrolysis (Fig. 2).

 Electrons are shared by the O,C, & N atoms in such a way the bond has a stiffness which resists rotation. As a result, each peptide bond lies in a flat plane (Fig. 3). Rotations must occur at the alpha carbon around the Phi and Psi bonds (Fig. 4).

 A peptide consisits of a small number of amino acids connected by peptide bonds.  These form an articulated chain of flat plates (Fig. 4).

 A longer chain of amino acids joined in this manner is called a polypeptide.

 We can define the direction of a polypeptide chain according to the orientation of the peptide bonds. The amino acid at one end of the chain must have a free NH2 group and thus defines the amino- or N terminal end (Fig. 4).

 The amino acid at the other end must have a free COOH group and thus defines the carboxy- or C terminal end (Fig. 4).

 Protein sequences are conventionally written from N-terminus (at the left) to C-terminus (at the right).

 The peptide bonds form a zig-zag backbone, from which the side-groups (denoted R) protrude (Fig. 5). The R group is different for each amino acid and determines the nature of its contribution to the overall protein structure.