<prompt> Which one of the following is a heterocyclic amino acid?LeucineMethionineAlanineProline  <response>: D Proline Proline is one of the nonessential amino acids, found in many proteins, especially collagen. Because the nitrogen atom of its amino group is part of a ring structure making it a heterocyclic compound, its chemical properties differ from those of the other amino acids in proteins. It is used in biochemical, nutritional, and microbiological research and as a dietary supplement. 