Role of pyrrolidine residues in the structure and stabilization of collagen

The physical properties of two neutral salt-soluble invertebrate collagens of markedly different total imino acid and hydroxyproline content have been compared. Measurements of sedimentation velocity, viscosity and optical rotation indicate that both Ascaris cuticle and earthworm cuticle collagens behave as rigid, asymmetric, rod-like particles in solution, with component polypeptide chains in the helical configuration of poly- L-proline II type and with molecular weights appreciably greater than those reported for the vertebrate tropo-collagens. Ascaris collagen (29% Pro; 2%Hypro) undergoes a thermal transition (collagen → gelatin) at 52°C ( T D ) whereas earthworm collagen (0·8% Pro; 17% Hypro) exhibits a T D of 22°C. When these results are considered with the thermal denaturation temperatures of other soluble collagens of known composition, it is concluded that the total pyrrolidine (Pro + Hypro) rather than the Hypro content alone, is the significant feature in the stabilization of the collagen structure. The distribution of pyrrolidine residues in various collagens has been estimated statistically and leads to the conclusion that triplets containing neighboring pyrrolidine residues are thermally stable. The re-naturation rates of Ascaris and earthworm gelatins have been measured at low temperature and their relationship to the pyrrolidine content and distribution considered.