Cyclic, aliphatic amino acid. The only amino acid which has a side chain that also binds to the amino group. This gives it very unique characteristics in that it significantly disrupts alpha-helices in proteins.

Formula: C₅H₉NO₂
Molecular Weight: 115.13
Three-letter code: Pro
One-letter code: P

Name: Proline
Three Letter Code: Pro
One Letter Code: P
Molecular Weight: 115.13
Proline is a nonpolar, cyclic amino acid. It is unique among the 20 amino acids found in proteins in that it is secondary. Its cyclic nature makes it good for forming turns and bends. It is located in turns known as β-bends. Normal amino acids have two rotational degrees of freedom in a peptide, however because of its cyclic nature proline only has one degree of freedom which is severly limited. This causes it to interrupt secondary structures such as α-helicies and β-sheets.
Structure:

HOOC   H 
    \ /
     C
   /   \
 HN    CH2
  |    |
H2C----CH2