Oxygen binding by hemocyanin from Levantina hierosolima. II. Interpretation of cooperativity in terms of ligand-ligand linkage.

Oxygen binding by hemocyanin from Levantina hierosolima was studied at pH 7.30, in solutions containing calcium in the concentration range 0-1 M. The binding was found to be cooperative, the degree of cooperativity being calcium concentration dependent. The dependence on calcium concentration of the affinity toward oxygen for both deoxygenated and oxygenated hemocyanin was interpreted in terms of two oxygen-linked calcium ions, one promoting and the other opposing oxygen binding. The results show that cooperativity may be fully explained on the basis of a coupling of the free energy of binding between calcium and oxygen.