Cooperativity in dimeric hemoglobins

Abstract

A number of homo- and heterodimeric hemoglobins has been identified in several phyla of invertebrates, lower vertebrates and even in unicellular organisms (bacteria, algae, fungi). These hemoglobins are characterized by different modes of assembly of the typical globin fold and hence represent ideal systems for testing thermodynamic models and mechanistic hypotheses aimed at explaining cooperative phenomena. In turn, the structural diversity of the subunit interfaces implies that in dimeric hemoglobins cooperativity in ligand binding can arise from quite diverse molecular mechanisms. The homodimeric hemoglobin fromScapharca inaequivalvis (HbI) is prototypic of systems where cooperative ligand binding originates within stable dimers and is thus mainly homotropic in nature. The availability of high resolution crystallographic data and the extensive characterization of the dynamic and equilibrium features of the ligand binding reactions allow a detailed description of the events that lead from the unliganded to the liganded form. InScapharca HbI cooperativity results primarily from tertiary rather than quaternary structural rearrangements. It can be ascribed to direct communication between the heme groups that are in close contact across the subunit interface, with the propionate groups providing a direct pathway for heme-heme interaction. The cooperative mechanism entails the ligand-triggered sinking of the heme into its pocket and the movement of key residues,e.g. the extrusion of Phe 97 from the proximal side of the heme into the subunit interface. In turn, the reorientation of the Phe 97 side chain brings about changes in the number of ordered interfacial water molecules. The oxygen binding isotherms of HbI can be described in terms of a simp le two-state MWC model under a symmetry constraint that imposes a constant KR/KT ratio. Kinetically, cooperative ligand binding manifests itself in the rate of oxygen dissociation and in the rate of carbonmonoxide combination. Several independent specroscopic probes indicate that the structural rearrangements accompanying ligand binding take place in a concerted fashion in the μsecond time-regime.