Murex trunculus haemocyanin. I. Physical properties and pH-induced dissociation.

Abstract

The native or whole molecule of the haemocyanin from the whelk, Murex trunculus had a sedimentation coefficient of 102.7 S, and combination of this with the diffusion coefficient of 1.03 × 10−7 cm2 s−1 led to a molecular weight of 8.85 × 106. The parameter β of Scheraga and Mandelkern was 2.14 × 106, and the frictional ratio f/f0 was 1.5 indicating that the molecule did not deviate markedly from the spherical. Electron microscopy showed that the molecule was a short cylinder, about 35 nm in diameter.When the pH was raised dissociation occurred into halves and then into smaller subunits which were referred to as “twentieths”, but which were heterogeneous. This dissociation took place at pH values greater than 7.6 in the absence of Mg2+, or at pH values near 10.0 in presence of 0.01 M MgCl2. Whole molecules were re‐formed from halves upon lowering the pH, but “twentieths”, once formed, did not re‐associate. The products of dissociation were separable by preparative ultracentrifugation and by zone electrophoresis. The proportion of wholes, halves, and twentieths formed was determined by the pH and Mg2+ concentration and was not influenced by rotor speed or by protein concentration. It was considered that there was not a dynamic equilibrium between wholes, halves and twentieths.In 6 M guanidine‐HCl in the presence of 2‐mercaptoethanol a single species of molecular weight about 220000 was formed. In detergent‐gel electrophoresis there was rather poor evidence for a component of molecular weight about 50000, which whould correspond with one oxygen‐binding site containing 2 copper atoms.