The proline molecule in 3-D

The proline molecule - rotatable in 3 dimensions

3 letter code:     pro        1 letter code:    P
Proline is an imino acid.

It has an imino group (-NH) sometimes known as a secondary amine - blue with one white ball - within a ring structure, and a carboxylic acid group (-COOH) - grey, connected to red, and red and white - at the other end of the molecule. These groups are used to link onto other amino acids by peptide bonds. There is not really the equivalent of an R group.

Other information

Proline is the only imino acid amongst 19 amino acids which are found in common proteins. Its nitrogen has 1 not 2 hydrogen atoms attached, and when this is used in bonding there is no other hydrogen atom to form hydrogen bonds. The presence of proline therefore distorts the shape of an alpha helix, although it does not do so if it is the first or second in the polypeptide chain (counting from the N-terminal end). It also prevents the formation of a beta-pleated sheet.
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