Electron transfer and redox chemistry in hexa-coordinate hemoglobins
The heme prosthetic group can be held into proteins in a variety of ways. Most often amino acid side chains coordinate one or both of the two available axial coordination sites of the heme iron. Coordination of both sites, such as in cytochrome b5, produces a good electron transfer protein but excludes the binding of exogenous ligands. In hemoglobins, coordination can occur at a single site (as in the “pentacoordinate” hemoglobins associated with oxygen transport), or at both sites (as in the “hexacoordinate” hemoglobins found in a wider distribution of organisms and functions). Surprisingly, hexacoordination in hemoglobins is usually reversible and a variety of exogenous ligands can bind most hexacoordinate hemoglobins. Reversible coordination brings a variety of chemical features to hexacoordinate hemoglobins by affecting their affinity for ligands, redox equilibrium, and the kinetics and extent of electron transfer. These reactions are reviewed for hexa- and pentacoordinate hemoglobins with the goal of using these characteristics for understanding potential functions of hexacoordinate hemoglobins in different species.