Correct Option: C

Proline is unique in that it is the only amino acid where the side chain is connected to the protein backbone twice, forming a five-membered nitrogen-containing ring. Proline is unable to occupy many of the main chain conformations easily adopted by all other amino acids. Proline can often be found in very tight turns in protein structures (i.e. where the polypeptide chain must change direction). It can also function to introduce kinks into alpha helices, since it is unable to adopt a normal helical conformation. Proline plays important roles in molecular recognition, particularly in intracellular signalling. The Proline side chain is very non-reactive and it does not undergo phosphorylation. ​