BASIC FEATURES OF POLYPEPTIDE CHAINS

Two amino acids react with the elimination of a water molecule to form a dipeptide. Additional amino acids are added to form a polymeric structure, a polypeptide. Peptide Synthesis

As indicated in the diagram, this reaction is unfavorable. Hydrolysis is favored but the activation energy needed to initiate hydrolysis is large so peptides and proteins can be quite stable under physiological conditions.

The bottom part of the diagram illustrates several key features of polypeptides.

What is the difference between a polypeptide and a protein? For our purposes, it is the outlook of the person using the word: a chemist interested in the structural features of the molecule might naturally call it a polypeptide while a biologist interested in its role in a cell might naturally call it a protein.