What is a notable feature of the Ramachandran plot for proline?

Proline is unique among the amino acids due to its distinctive cyclic structure, which creates constraints on its backbone conformation. The Ramachandran plot illustrates the conformational space available to amino acids based on their backbone dihedral angles, phi (φ) and psi (ψ).

In the case of proline, the presence of the ring significantly restricts the range of permissible dihedral angles because the nitrogen atom in proline is part of the ring, making its flexibility much lower compared to other amino acids. Consequently, the Ramachandran plot for proline shows that it occupies fewer regions, particularly avoiding areas where other residues might fall. As a result, proline typically fits into specific conformational states that are more limited compared to other amino acids, which can adopt a broader range of φ and ψ angles.

This characteristic explains why proline has a distinctly different profile on the Ramachandran plot compared to other amino acids, making it a significant factor in protein structure.