Structure and properties of hemocyanins: XIII. Dissociation of Helix pomatia α-hemocyanin at alkaline pH

Abstract

Helix pomatia α-hemocyanin can dissociate stepwise into 1 2 -size, 1 10 -size and 1 20 -size molecules. Both 1 10 -size and 1 20 -size molecules can occur in two isomeric forms, differing considerably in sedimentation behaviour. The effects of pH, ionic strength, temperature, Ca 2+ concentration and oxygen pressure on these dissociation and isomerization steps were investigated systematically by sedimentation analysis. Dissociation and isomerization are favoured by increasing pH or temperature. Changes in ionic strength affect each step differently. Calcium ions are extremely effective in preventing dissociation and isomerization at low ionic strength, but this stabilizing ability diminishes at higher ionic strengths. Oxygen binding shifts the pH-dependent dissociation of whole into 1 2 -size molecules to higher pH. Oxygen has no effect on the other dissociation steps. Intermolecular interactions appear to be predominantly electrostatic.