In a protein molecule, the number of amino acid molecules may be as few as 5 500 50 5,000

Answer is 50.

Protein is a macromolecules and is composed of its units called amino-acids. In a protein molecule amino acids are bound together with peptide bonds to form polypeptides. Thus protein is a polypeptide. The number of amino acids in a protein molecules varies. Some polypeptides contain only 3 amino-acids, but it is not regarded as proteins due to its structural and functional differences between a simple polypeptide chain and a protein. A polypeptide of 50 amino acids is biologically functional protein that possesses all the properties of a protein. Most of the protein made of thousands of several polypeptide chains with thousands of amino-acids organised in complex structures.