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J. Biol. Chem., Vol. 282, Issue 18, 13627-13636, May 4, 2007

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Structural and Functional Properties of a Truncated Hemoglobin from a Food-borne Pathogen Campylobacter jejuni^*

Changyuan Lu, Tsuyoshi Egawa, Laura M. Wainwright, Robert K. Poole, and Syun-Ru Yeh¹

From the Department of Physiology and Biophysics, Albert Einstein College of Medicine, Bronx, New York 10461 and the Department of Molecular Biology and Biotechnology, University of Sheffield, Sheffield S10 2TN, United Kingdom

Campylobacter jejuni contains two hemoglobins, Cgb and Ctb. Cgb has been suggested to perform an NO detoxification reaction to protect the bacterium against NO attack. On the other hand, the physiological function of Ctb, a class III truncated hemoglobin, remains unclear. By using CO as a structural probe, resonance Raman data show that the distal heme pocket of Ctb exhibits a positive electrostatic potential. In addition, two ligand-related vibrational modes, _Fe-O2 and _O-O, were identified in the oxy derivative, with frequencies at 542 and 1132 cm^-1, respectively, suggesting the presence of an intertwined H-bonding network surrounding the heme-bound ligand, which accounts for its unusually high oxygen affinity (222 µM^-1). Mutagenesis studies of various distal mutants suggest that the heme-bound dioxygen is stabilized by H-bonds donated from the Tyr(B10) and Trp(G8) residues, which are highly conserved in the class III truncated hemoglobins; furthermore, an additional H-bond donated from the His(E7) to the Tyr(B10) further regulates these H-bonding interactions by restricting the conformational freedom of the phenolic side chain of the Tyr(B10). Taken together, the data suggest that it is the intricate balance of the H-bonding interactions that determines the unique ligand binding properties of Ctb. The extremely high oxygen affinity of Ctb makes it unlikely to function as an oxygen transporter; on the other hand, the distal heme environment of Ctb is surprisingly similar to that of cytochrome c peroxidase, suggesting a role of Ctb in performing a peroxidase or P450-type of oxygen chemistry.

Received for publication, October 4, 2006 , and in revised form, February 16, 2007.

^* This work was supported by National Institutes of Health Grant HL65465 (to S.-R. Y.) and the Biotechnology and Biological Sciences Research Council (UK) Grant D18084 [GenBank] (to R. K. P.). 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.

¹ To whom correspondence should be addressed: Dept. of Physiology and Biophysics, Albert Einstein College of Medicine, Bronx, NY 10461. Tel.: 718-430-4234; Fax: 718-430-4230; E-mail: syeh{at}aecom.yu.edu.

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