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 acid to form a single water molecule (H2O). The condensation product leaves a covalent peptide bond (O=C-N-H).

(B) Because succesive 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 NC direction.
Each of the repeating ( -N-C(R)-C- ) subunits remaining after the dehydration reaction is an amino acid residue.

Figure © 2012 TA Brown, Introduction to Genetics (1st ed.); additional text © 2014 by Steven M. Carr