Proline
Also known as: Pro, P
Proline is one of the twenty standard amino acids and the only one whose side chain loops back to bond with its own backbone nitrogen, forming a rigid five-membered ring. That structure makes proline a disruptor of alpha helices and beta sheets.
Proline is a nonpolar, nonessential amino acid abbreviated Pro or P. Its distinguishing feature is structural: the three-carbon side chain connects back to the nitrogen of the amino group, creating a pyrrolidine ring. Because that nitrogen is part of the ring, proline is technically a secondary amine — often called an imino acid — and it has no free hydrogen on its backbone nitrogen once incorporated into a chain.
That missing hydrogen has direct consequences for protein folding. Alpha helices and beta sheets are held together by backbone hydrogen bonds in which the amide N–H acts as a donor. Proline cannot donate one, and the rigid ring also restricts rotation around the backbone. As a result, proline is a helix breaker: it introduces a kink where it appears in a helix and commonly shows up in turns, loops, and the ends of secondary structure elements. Proline residues also make the preceding peptide bond unusually likely to adopt the cis configuration, whereas nearly all other peptide bonds are trans, and cis-trans isomerization at proline is often the slow step in protein folding.
Proline is abundant in collagen, whose repeating Gly-X-Y motif places glycine at every third residue and frequently fills the X and Y positions with proline and hydroxyproline. Hydroxylation of proline residues requires vitamin C as a cofactor, which is why deficiency causes scurvy — unstable collagen leads to poor wound healing, bleeding gums, and fragile blood vessels. In the body proline can be synthesized from glutamate, so it is not required in the diet.
The MCAT tests proline within amino acid structure and protein folding, including its effect on secondary structure and its role in collagen. USMLE Step 1 covers it in translation and collagen synthesis, and the CSCS exam includes amino acids more generally in its sports nutrition and macronutrients material.
Key takeaways
- Proline's side chain bonds back to its own backbone nitrogen, forming a rigid five-membered ring.
- It cannot donate a backbone hydrogen bond, so it disrupts alpha helices and beta sheets.
- Proline is commonly found in turns and loops and at the ends of secondary structure elements.
- Peptide bonds preceding proline can adopt the cis configuration, and their isomerization often limits folding rate.
- Proline and hydroxyproline are abundant in collagen, and proline hydroxylation requires vitamin C.