Histidine pKa shifts and changes of tautomeric states induced by the binding of gallium-protoporphyrin IX in the hemophore HasASM

The HasASM hemophore, secreted by Serratia marcescens, binds free or hemoprotein bound heme with high affinity and delivers it to a specific outer membrane receptor, HasR. In HasASM, heme is held by two loops and coordinated to iron by two residues, His 32 and Tyr 75. A third residue His 83 was shown recently to play a crucial role in heme ligation. To address the mechanistic issues of the heme capture and release processes, the histidine protonation states were studied in both apo- and holo-forms of HasASM in solution. Holo-HasASM was formed with gallium-protoporphyrin IX (GaPPIX), giving rise to a diamagnetic protein. By use of heteronuclear correlation NMR spectroscopy, the imidazole side-chain 15N and 1H resonances of the six HasASM histidines were assigned and their pKa values and predominant tautomeric states according to pH were determined. We show that protonation states of the heme pocket histidines can modulate the nucleophilic character of the two axial ligands and, consequently, control the heme binding. In particular, the essential role of the His 83 is emphasized according to its direct interaction with Tyr 75.