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Conformational flexibility in a small globular hormone: X‐ray analysis of avian pancreatic polypeptide at 0.98‐Å resolution

The 36‐amino acid avian pancreatic polypeptide has been studied by x‐ray analysis at 0.98‐Å resolution and refined using a restrained least‐squares technique to an agreement factor of 15.6%. The polypeptide, which has a compact globular structure with a hydrophobic core, comprises a polyproline–like helix (residues 2–8) and an α‐helix (residues 14–32). The molecule forms symmetrical dimers linked through zinc atoms in the crystal lattice. The high‐resolution analysis defines sequence‐dependent distortions in the α‐helical parameters due to hydrogen bonding of water molecules and side chains. The thermal parameters indicate an increased flexibility of the main chain at the turn between the helices and in the C‐terminal residues. For the first time, six‐parameter anisotropic thermal ellipsoids have been refined for each atom; these define the directions of the molecular motions in the polypeptide, indicating concerted vibrations. The physiological roles of conformation, flexibility, and dynamics of this polypeptide hormone are discussed.