Proline is a naturally occurring amino acid in proteins and is critical for the stability and strength of proteins. It is involved in a variety of biochemical reactions within cells, playing an important role in energy transduction pathways, signal transduction pathways, and metabolic pathways. Proline has a cyclic structure and is one of the only amino acids with a ring structure. Its side chain contains a primary amino group and an alkyl side chain with a propionaldehyde, and its ring is composed of an oxygen atom and two nitrogen atoms. Its side chain contains a hydrophobic methylene group, making it one of the most hydrophobic amino acids. Its general formula is C5H9NO2.