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.