Solution Structure of Native Proteins with Irregular Folds from Raman Optical Activity

Raman optical activity (ROA) spectra have been measured for the proteins hen phosvitin, yeast invertase, bovine alpha-casein, soybean Bowman-Birk protease inhibitor, and rabbit Cd(7)-metallothionein, all of which have irregular folds in the native state. The results show that ROA is able to distinguish between two types of disorder. Specifically, invertase, alpha-casein, the Bowman-Birk inhibitor, and metallothionein appear to possess a "static" type of disorder similar to that in disordered states of poly(L-lysine) and poly(L-glutamic acid); whereas phosvitin appears to possess a more "dynamic" type of disorder similar to that in reduced (unfolded) lysozyme and ribonuclease A and also in molten globule protein states. In the delimiting cases, static disorder corresponds to that found in loops and turns within native proteins with well-defined tertiary folds that contain sequences of residues with fixed but nonrepetitive phi,psi angles; and dynamic disorder corresponds to that envisaged for the model random coil in which there is a distribution of Ramachandran phi,psi angles for each amino acid residue, giving rise to an ensemble of interconverting conformers. In both cases there is a propensity for the phi,psi angles to correspond to the alpha, beta and poly(L-proline) II (PPII) regions of the Ramachandran surface, as in native proteins with well-defined tertiary folds. Our results suggest that, with the exception of invertase and metallothionein, an important conformational element present in the polypeptide and protein states supporting the static type of disorder is that of the PPII helix. Long sequences of relatively unconstrained PPII helix, as in alpha-casein, may impart a plastic (rheomorphic) character to the structure.