title: Histidine pK(a) shifts and changes of tautomeric states induced by the binding of gallium-protoporphyrin IX in the hemophore HasA(SM).
abstract: The HasA(SM) 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 HasA(SM), 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 HasA(SM) in solution. Holo-HasA(SM) was formed with gallium-protoporphyrin IX (GaPPIX), giving rise to a diamagnetic protein. By use of heteronuclear correlation NMR spectroscopy, the imidazole side-chain (15)N and (1)H resonances of the six HasA(SM) 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.
authors.affiliation: Unité de Résonance Magnétique Nucléaire des Biomolécules, CNRS URA 2185, Institut Pasteur, Paris, France. wolff@pasteur.fr
authors.forename: Nicolas
authors.initials: N
authors.lastname: Wolff
authors.forename: Clarisse
authors.initials: C
authors.lastname: Deniau
authors.forename: Sylvie
authors.initials: S
authors.lastname: Létoffé
authors.forename: Catherine
authors.initials: C
authors.lastname: Simenel
authors.forename: Veena
authors.initials: V
authors.lastname: Kumar
authors.forename: Igor
authors.initials: I
authors.lastname: Stojiljkovic
authors.forename: Cécile
authors.initials: C
authors.lastname: Wandersman
authors.forename: Muriel
authors.initials: M
authors.lastname: Delepierre
authors.forename: Anne
authors.initials: A
authors.lastname: Lecroisey
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism; D001665:Binding Sites; D002352:Carrier Proteins; D018360:Crystallography, X-Ray; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism; D005708:Gallium / Q000378:metabolism*; D006418:Heme / Q000378:metabolism*; D006639:Histidine / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D009682:Magnetic Resonance Spectroscopy; D008565:Membrane Proteins / Q000737:chemistry* / Q000378:metabolism; D008958:Models, Molecular; D011485:Protein Binding; D011487:Protein Conformation; D011524:Protoporphyrins / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D002352:Carrier Proteins; C093956:HasA protein, Serratia marcescens; D008565:Membrane Proteins; D011524:Protoporphyrins; D006418:Heme; D006639:Histidine; C028025:protoporphyrin IX; D005708:Gallium