Deciphering the structural role of Histidine 83 for Heme binding in Hemophore HasA

Heme carrier HasA has a unique type of histidine/ tyrosine heme iron ligation in which the iron ion is in a thermally driven two spin states equilibrium. We recently suggested that the H-bonding between Tyr(75) and the invariantly conserved residue His(83) modulates the strength of the iron-Tyr(75) bond. To unravel the role of His(83), we characterize the iron ligation and the electronic properties of both wild type and H83A mutant by a variety of spectroscopic techniques. Although His(83) in wild type modulates the strength of the Tyr-iron bond, its removal causes detachment of the tyrosine ligand, thus giving rise to a series of pH-dependent equilibria among species with different axial ligation. The five coordinated species detected at physiological pH may represent a possible intermediate of the heme transfer mechanism to the receptor.