Although it is the most unlikely amino acids in a beta-sheet and an alpha-helix, it is plentiful in adjacent "turn" or "loop" regions.
This amino acid is highly abundant in motifs that bind to SH3 protein domains.  It is synthesized from glutamate, and some of it may be further converted to a hydroxyl- form common in collagen. Although it is the most unlikely amino acids in a beta-sheet and an alpha-helix, it is plentiful in adjacent "turn" or "loop" regions. The most sterically hindered amino acid, FTP, name this compound, unique in that its R group forms a 5-membered ring with its amino group, and whose one-letter abbreviation is "P".
This amino acid is highly abundant in motifs that bind to SH3 protein domains.  
proline