Two amino acids in a protein are joined together by a peptide bond, or amide bond. The amino acids are bonded by the alpha-carboxyl group of one with the alpha-amino group of the other. Proteins are composed of a series of peptide bonds linked together to form linear polypeptide chains.
When two amino acids are joined together in a peptide bond, the reaction causes the loss of one water molecule. A polypeptide chain is polar because has different ends, with an alpha-carboxyl group on one end and an alpha-amino group at the other. Proteins consist of a polypeptide chain of between 50 and 2,000 amino acids, joined by peptide bonds.