Peptide Bond formation between successive amino acids

(A) The amino end on the second (R2) amino acid is added to the carboxyl end of the first (R1) amino acid. In the in vitro reaction, an O from the carboxyl end combines with two protons (H) from the amino end to form a single water molecule (H2O). The dehydration reaction leaves a covalent peptide bond (O=C-N-H).

(B) Because successive amino acids are added to the carboxyl end of the polypeptide, and the amino terminus of the
R1 amino acid remains unchanged, the polypeptide is said to grow in the N C direction.
Each of the repeating ( -N-C(R)-C- ) subunits remaining after the dehydration reaction is an amino acid residue.

Note: The diagram above shows the formation of a peptide bond in vitro by a dehydration reaction that splits out an H20 molecule. The same bond is formed in vivo by a condensation reaction, in which no molecules are lost.

Figure © 2012 TA Brown, Introduction to Genetics (1st ed.); additional text © 2016 by Steven M. Carr