Amino acid residue

When two or more amino acids combine to form a peptide, the elements of water are removed, and what remains of each amino acid is called an amino-acid residue. It is basically an amino acid once inside the polypeptide chain, which means it is an amino acid but with its alpha-amino andcarboxyl group as part of the peptide bonds. (Residues of amino acids that contain two amino groups or two carboxyl groups may be joined by isopeptide bonds, and so may not have the formulas shown.)