Structural disorder in proteins. A comparison of myoglobin and erythrocruorin.

The refinement of X-ray structural data gives the mean square displacements, (chi 2), at each position in the protein molecule. In order to get information on the significance of such values different refinement methods have been compared. The metmyoglobin structure was determined at 300 K and (chi 2)-values were obtained with the restrained refinement procedure in reciprocal space of Konnert and Hendrickson. A comparison with the results of Frauenfelder et al. was used for an error estimation. The inclusion of surface bound water increases the accuracy of the results but does not change the general picture. For erythrocruorin (CTT3) a refinement was performed in reciprocal space and compared with a refinement in real space performed earlier. The (chi 2)-values obtained from both procedures are similar although the reciprocal space refinement gives results which are physically more reasonable. A comparison of the disorder in myoglobin and erythrocruorin showed that the structural similarity results in a similarity in the disorder. Contacts of molecules in the crystal do not dominate the disorder although they locally influence (chi 2)-values. CTT3 shows large disorder in the heme region in contrast to myoglobin. The differences in the rigidity of the F-helix can be correlated with the oxygen affinities supporting models for O2 binding developed by Frauenfelder et al.