Evaluating Cooperativity in Dimeric Hemoglobins

Abstract

A growing number of homo- and heterodimeric hemoglobins have been identified not only within several phyla of invertebrates and lower vertebrates, but also in unicellular micro-organisms, such as bacteria, algae, and fungi. Even though all these proteins share the typical globin fold (with the exception of truncated hemoglobins, in which two helices are missing), the quaternary assembly and the nature of the subunit interface differ widely. This observation, which implies that cooperative ligand binding arises from diverse molecular mechanisms, renders dimeric hemoglobins ideal molecular machineries for testing thermodynamic models and mechanistic hypotheses aimed at explaining the origin of cooperative phenomena. The homodimeric hemoglobin from the mollusk Scapharca inaequivalvis (HbI) is by far the most extensively investigated protein within this class of proteins. Thus, cooperativity is ascribed to the direct communication between the two heme groups that are in close contact across the subunit interface. The heme propionate groups provide a direct pathway for the heme–heme interaction and ligand-linked structural changes are localized to the heme proximal side. A completely different mechanism is observed in hemoglobins from lower vertebrates, such as Cyclostomata, in which cooperative oxygen binding is the result of ligand-linked subunit dissociation. The behavior of lamprey (Petromyzon marinus) and hagfish (Myxine glutinosa) hemoglobins is prototypic: the ligand-bound species are monomeric, whereas the deoxygenated proteins are able to form both homodimers and heterodimers (and eventually higher order heteropolymers). This multifaceted behavior indicates that there is no unique structural basis at the origin of cooperativity in dimeric hemoglobins, both from the thermodynamic and mechanistic points of view. Cooperative ligand binding thus appears to have been selected evolutionarily, despite the mutational pressure on the intersubunit contact regions (not conserved), as a consequence of a conserved tertiary fold endowed with sufficient conformational plasticity to allow ligand-linked, heme iron-driven structural rearrangements that ultimately lead to cooperative behaviors. In this chapter, it is envisaged that cooperativity in dimeric hemoglobins can arise from two completely different mechanisms: (1) co- operative ligand binding driven by ligand-linked subunit dissociation (mainly heterotropic), or (2) cooperative ligand binding within a stable dimer (homotropic). In real systems, the presence of one mechanism does not exclude the other.