Explain why Pro residues can occupy the N-terminal turn of...

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Key Concepts

N-Terminal Turn and Helix Capping

The N-terminal turn of an ?-helix is more flexible in accommodating irregularities in structure. Here, the rigid conformation and the inability of proline to donate a hydrogen bond are less disruptive, allowing proline residues to be accommodated at the helix start where the full hydrogen-bonding pattern is not yet established. This helix capping can help stabilize the initiation of the helical structure.

Alpha Helix Conformation

The ?-helix is a prevalent protein secondary structure characterized by a right-handed coil where each backbone carbonyl oxygen forms a hydrogen bond with the amide hydrogen of the amino acid four residues ahead. This regular pattern of hydrogen bonding, along with specific backbone dihedral angles (phi and psi), defines its stability and shape.

Proline's Structural Properties

Proline is distinct among amino acids due to its cyclic structure, which links the side chain to the backbone nitrogen. This constrains the phi angle, reducing backbone flexibility, and it lacks an amide hydrogen, which limits its ability to participate in typical ?-helical hydrogen bonding. These features often make proline a helix breaker when found in the middle of an ?-helix.

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