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Unusual Heme Iron-Lipid Acyl Chain Coordination in Escherichia coli Flavohemoglobin

Paola D'Angelo ^*, Debora Lucarelli ^*, Stefano della Longa , Maurizio Benfatto , Jean Louis Hazemann , Alessandro Feis ^¶, Giulietta Smulevich ^¶, Andrea Ilari ^||, Alessandra Bonamore ^|| and Alberto Boffi ^||

^* Department of Chemistry University "La Sapienza", Rome, and Istituto Nazionale per la Fisica della Materia UdF, Camerino, Italy; Department of Experimental Medicine, University of L'Aquila, L'Aquila, Italy; Laboratori Nazionali di Frascati, LFN-INFN, Frascati, Italy; Laboratoire Crystallographie CNRS, Grenoble, France; ^¶ Department of Chemistry, University of Florence, Sesto Fiorentino, Italy; and ^|| Department of Biochemical Sciences and CNR Institute of Molecular Biology and Pathology, University "La Sapienza", Rome, Italy

Correspondence: Address reprint requests to Alberto Boffi, Fax: 39-06-44-40062; E-mail: alberto.boffi{at}uniroma1.it.

Escherichia coli flavohemoglobin is endowed with the notable property of binding specifically unsaturated and/or cyclopropanated fatty acids both as free acids or incorporated into a phospholipid molecule. Unsaturated or cyclopropanated fatty acid binding to the ferric heme results in a spectral change observed in the visible absorption, resonance Raman, extended x-ray absorption fine spectroscopy (EXAFS), and x-ray absorption near edge spectroscopy (XANES) spectra. Resonance Raman spectra, measured on the flavohemoglobin heme domain, demonstrate that the lipid (linoleic acid or total lipid extracts)-induced spectral signals correspond to a transition from a five-coordinated (typical of the ligand-free protein) to a hexacoordinated, high spin heme iron. EXAFS and XANES measurements have been carried out both on the lipid-free and on the lipid-bound protein to assign the nature of ligand in the sixth coordination position of the ferric heme iron. EXAFS data analysis is consistent with the presence of a couple of atoms in the sixth coordination position at 2.7 Å in the lipid-bound derivative (bonding interaction), whereas a contribution at 3.54 Å (nonbonding interaction) can be singled out in the lipid-free protein. This last contribution is assigned to the CD1 carbon atoms of the distal LeuE11, in full agreement with crystallographic data on the lipid-free protein at 1.6 Å resolution obtained in the present work. Thus, the contributions at 2.7 Å distance from the heme iron are assigned to a couple of carbon atoms of the lipid acyl chain, possibly corresponding to the unsaturated carbons of the linoleic acid.

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