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Histidine pK a shifts and changes of tautomeric states induced by the binding of gallium‐protoporphyrin IX in the hemophore HasA SM
Author(s) -
Wolff Nicolas,
Deniau Clarisse,
Létoffé Sylvie,
Simenel Catherine,
Kumar Veena,
Stojiljkovic Igor,
Wandersman Cécile,
Delepierre Muriel,
Lecroisey Anne
Publication year - 2002
Publication title -
protein science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.353
H-Index - 175
eISSN - 1469-896X
pISSN - 0961-8368
DOI - 10.1110/ps.3630102
Subject(s) - tautomer , histidine , protoporphyrin ix , gallium , chemistry , crystallography , protoporphyrin , computational chemistry , stereochemistry , biochemistry , porphyrin , organic chemistry , enzyme , photodynamic therapy
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.