Subunit dissociation and denaturation of Fasciolaria tulipa hemocyanin

1. The hemocyanin from the marine snail, Fasciolaria tulipa has a molecular weight of 8.6 +/- 0.6 x 10(6) determined by light-scattering and a sedimentation constant of (105.9 +/- 1.1)S. 2. The dissociated subunits at pH 11 and in 8.0 M urea (pH 7.4) had molecular weights of 4.4 x 10(5) and 4.7 x 10(5), close to one-twentieth of the parent didecameric assembly. 3. The pH dependence of the molecular weight profile exhibited bell-shaped transitions in both the presence and absence of Ca2+ and Mg2+ ions. In the physiological pH range of about 7.5-8.2 in divalent ion-containing buffers neither the molecular weight behavior nor the sedimentation patterns suggest any significant dissociation. 4. Both the urea and the Hofmeister salt series were found to dissociate the didecameric hemocyanin assembly. The ureas exhibit increasing effectiveness as dissociating agents with the higher alkyl substituted members of the series, suggesting hydrophobic stabilization of the subunit assembly. 5. Denaturation of the hemocyanin subunits by the urea series follows the same trend in effectiveness as the dissociation reaction; the reagent concentrations required to cause unfolding of the globular domains of the hemocyanin chains were, however, much higher than those needed for dissociation.