Thermodynamics of Heme Binding to the HasASM Hemophore: Effect of Mutations at Three Key Residues for Heme Uptake

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Abstract
HasASM secreted by the Gram-negative bacterium Serratia marcescens belongs to the hemophore family. Its role is to take up heme from host heme carriers and to shuttle it to specific receptors. Heme is linked to the HasASM protein by an unusual axial ligand pair: His32 and Tyr75. The nucleophilic nature of the tyrosine is enhanced by the hydrogen bonding of the tyrosinate to a neighboring histidine in the binding site: His83. We used isothermal titration microcalorimetry to examine the thermodynamics of heme binding to HasASM and showed that binding is strongly exothermic and enthalpy driven: ΔH = −105.4 kJ·mol-1 and TΔS = −44.3 kJ·mol-1. We used displacement experiments to determine the affinity constant of HasASM for heme (Ka = 5.3 × 1010 M-1). This is the first time that this has been reported for a hemophore. We also analyzed the thermodynamics of the interaction between heme and a panel of single, double, and triple mutants of the two axial ligands His32 and Tyr75 and of His83 to assess the implication of each of these three residues in heme binding. We demonstrated that, in contrast to His32, His83 is essential for the binding of heme to HasASM, even though it is not directly coordinated to iron, and that the Tyr75/His83 pair plays a key role in the interaction.