Ligand binding and stereochemical effects in hemocyanins.

The Bohr effect in the binding of O(2) and CO to Helix pomatia beta-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 O(2) 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 O(2) 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 O(2), which forms a bridge between the two copper atoms in a site, have to be considered essential for the onset of cooperativity in hemocyanins.