Ligand binding and stereochemical effects in hemocyanins

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Abstract
The Bohr effect in the binding of O2 and CO to Helix pomatia β‐hemocyanin has been estimated by the potentiometric titration method. On the basis of the results, as well as other information available, we conclude that binding of O2 to hemocyanin is associated with a quaternary allosteric transition (T→R), while binding of CO is not. This conclusion is in agreement with previous results, which indicate that homotropic interactions are observed only for the binding of O2 to the active site of hemocyanins. This is in marked contrast to the behaviour of hemoglobins which, otherwise, share with hemocyanins a number of important features, in spite of basic chemical differences. The stereochemical requirements of O2, which forms a bridge between the two copper atoms in a site, have to be considered essential for the onset of cooperativity in hemocyanins.