A TEST OF THE ROLE OF THE PROXIMAL HISTIDINES IN THE PERUTZ MODEL FORCOOPERATIVITY IN HEMOGLOBIN

Human haemoglobin has long been a paradigm for cooperative ligand bind ing and allostery. Through analysis of the crystal structures of deoxy haemoglobin and liganded haemoglobin, Perutz proposed a model for coop erativity in which the bond between the proximal histidine and the pro tein couples haem rearrangements to protein structure rearrangements. Here we test this model by deleting the bonds between the histidine im idazole ride chain and the polypeptide, This detachment method allows us to determine directly the contribution of proximal histidine coupli ng to cooperativity of distal ligand binding, Proximal detachment sign ificantly increases ligand affinity reduces cooperativity, and prevent s quaternary structure switching, in accord with the Perutz model. Res idual cooperativity indicates that additional haem communication pathw ays exist that do not involve the proximal histidine coupling mechanis m.