Proline

(CC) Image: David E. Volk
Proline, a common amino acid.

Proline, abbreviated Pro or P, is one of the twenty common $\alpha$-amino acids used by living organisms to build proteins. It is the only cyclic amino acid and because of its restricted allowed conformations, it often ends secondary structure elements in proteins and forms turns in protein structures. It is aliphatic, cyclic and nonpolar. It is the only amino acid that does not have an amide proton. Technically, because the proline side chain bonds to both the alpha carbon and the backbone nitrogen atom, the nitrogen is a secondary amine, making proline an imino acid rather than an amino acid. However, it is typically referred to as an amino acid.