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J. Biol. Chem., Vol. 283, Issue 20, 13897-13904, May 16, 2008

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An Oxidized Tryptophan Facilitates Copper Binding in Methylococcus capsulatus-secreted Protein MopE^*

Ronny Helland, Anne Fjellbirkeland, Odd Andre Karlsen, Thomas Ve, Johan R. Lillehaug, and Harald B. Jensen¹

From the Norwegian Structural Biology Centre, Faculty of Science, University of Tromso, N-9073 Tromso, Norway and the Department of Molecular Biology, University of Bergen, N-5020 Bergen, Norway

Proteins can coordinate metal ions with endogenous nitrogen and oxygen ligands through backbone amino and carbonyl groups, but the amino acid side chains coordinating metals do not include tryptophan. Here we show for the first time the involvement of the tryptophan metabolite kynurenine in a protein metal-binding site. The crystal structure to 1.35Å of MopE^* from the methane-oxidizing Methylococcus capsulatus (Bath) provided detailed information about its structure and mononuclear copper-binding site. MopE^* contains a novel protein fold of which only one-third of the structure displays similarities to other known folds. The geometry around the copper ion is distorted tetrahedral with one oxygen ligand from a water molecule, two histidine imidazoles (His-132 and His-203), and at the fourth distorted tetrahedral position, the N1 atom of the kynurenine, an oxidation product of Trp-130. Trp-130 was not oxidized to kynurenine in MopE^* heterologously expressed in Escherichia coli, nor did this protein bind copper. Our findings indicate that the modification of tryptophan to kynurenine and its involvement in copper binding is an innate property of M. capsulatus MopE^*.

Received for publication, January 14, 2008 , and in revised form, March 4, 2008.

The atomic coordinates and structure factors (codes 2VOV, 2VOW, and 2VOX) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/).

^* The present study was supported by the national Functional Genomics Program (FUGE) and other grants from The Research Council of Norway. 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 Figs. S1–S4.

¹ To whom correspondence should be addressed: Dept. of Molecular Biology, HIB, University of Bergen, Thormøhlensgate 55, N-5020 Bergen, Norway. Tel.: 47-55-58-64-22; Fax: 47-55-58-96-83; E-mail: harald.jensen{at}mbi.uib.no.

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