Inversion of the Bohr effect upon oxygen binding to 24-meric tarantula hemocyanin

The Bohr effect describes the usually negative coupling between the binding of oxygen and the binding of protons to respiratory proteins. It was first described for hemoglobin and provides for an optimal oxygen supply of the organism under changing physiological conditions. Our measurements of both oxygen and proton binding to the 24-meric tarantula hemocyanin establish the unusual case where a respiratory protein binds protons at low degrees of oxygenation but releases protons at high degrees of oxygenation. In contrast to what is observed with hemoglobin and other respiratory proteins, this phenomenon amounts to the inversion of the Bohr effect in the course of an oxygen-binding curve at a given pH value. Therefore, protons in spider blood can act either as allosteric activators or as allosteric inhibitors of oxygen binding, depending on the degree of oxygenation of hemocyanin. These functional properties of tarantula hemocyanin, which cannot be explained by classical allosteric models, require at least four different conformational states of the subunits. Inspection of the known x-ray structures of closely related hemocyanins suggests that salt bridges between completely conserved histidine and glutamate residues located at particular intersubunit interfaces are responsible for the observed phenomena.