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J. Biol. Chem., Vol. 283, Issue 9, 5960-5970, February 29, 2008

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Deciphering the Structural Role of Histidine 83 for Heme Binding in Hemophore HasA^*

From the Unité de RMN des Biomolécules, CNRS URA 2185, Institut Pasteur, 75015 Paris, France

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 Tyr⁷⁵ and the invariantly conserved residue His⁸³ modulates the strength of the iron-Tyr⁷⁵ bond. To unravel the role of His⁸³, we characterize the iron ligation and the electronic properties of both wild type and H83A mutant by a variety of spectroscopic techniques. Although His⁸³ 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.

Received for publication, May 8, 2007 , and in revised form, November 19, 2007.

^* This work was supported by funds from Ministère de l'Education Nationale de la Recherche et de la Technologie and the Caisse Nationale du Régime Social des Indépendants (to C. C.-S.); Marie Curie Host Fellowship MEST-CT-2004-504391 from the European Union (to C. C.-S.); and NIAID, National Institutes of Health Grants 1R15AI072719-01 and NCRR P20RR15556 (to K. R. R.). This work was also supported in part by EU-NMR Contract RII3-026145 and by funds from Progetti di Ricerca di Rilevante Interesse Nationalé 2005, CNRS, Institut Pasteur, and the Egide-Galileo Project. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Table S1 and supplemental Figs. S1-S4.

¹ To whom correspondence should be addressed: Unité de RMN des Biomolécules, CNRS URA 2185, Institut Pasteur, Paris, France. Tel.: 33-1-40-61-88-73; Fax: 33-1-45-68-89-29; E-mail: ccaillet{at}pasteur.fr or alecrois{at}pasteur.fr.

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