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J. Biol. Chem., Vol. 280, Issue 3, 2275-2281, January 21, 2005

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A Structural Basis of Equisetum arvense Ferredoxin Isoform II Producing an Alternative Electron Transfer with Ferredoxin-NADP⁺ Reductase^*

Genji Kurisu, Daisuke Nishiyama, Masami Kusunoki, Shinobu Fujikawa¶, Midori Katoh¶, Guy Thomas Hanke||, Toshiharu Hase||, and Keizo Teshima¶**

From the Research Center for Structural and Functional Proteomics, Institute for Protein Research, Osaka University, Suita, Osaka 565-0871, ^¶Faculty of Integrated Arts and Sciences, Hiroshima University, 1-7-1 Kagamiyama, Higashi-Hiroshima 739-8521, and ^||Division of Enzymology, Institute for Protein Research, Osaka University, Suita, Osaka 565-0871, Japan

We have determined the crystal structure, at 1.2-Å resolution, of Equisetum arvense ferredoxin isoform II (FdII), which lacks residues equivalent to Arg³⁹ and Glu²⁸ highly conserved among other ferredoxins (Fds). In other Fds these residues form an intramolecular salt bridge crucial for stabilization of the [2Fe-2S] cluster, which is disrupted upon complex formation with Fd-NADP⁺ oxidoreductase (FNR) to form two intermolecular salt bridges. The overall structure of FdII resembles the known backbone structures of E. arvense isoform I (FdI) and other plant-type Fds. Dramatically, in the FdII structure a unique, alternative salt bridge is formed between Arg²² and Glu⁵⁸. This results in a different relative orientation of the -helix formed by Leu²³-Glu²⁹ and eliminates the possibility of forming three of the five intermolecular salt bridges identified on formation of a complex between maize FdI and maize FNR. Mutation of FdII, informed by structural differences with FdI, showed that the alternative salt bridge and the absence of an otherwise conserved Tyr residue are important for the alternative stabilization of the FdII [2Fe-2S] cluster. We also investigated FdI and FdII electron transfer to FNR on chloroplast thylakoid membranes. The K_m and V_max values of FdII are similar to those of FdI, contrary to previous measurements of the reverse reaction, from FNR to Fd. The affinity between reduced FdI and oxidized FNR is much greater than that between oxidized FdI and reduced FNR, whereas this is not the case with FdII. The pH dependence of electron transfer by FdI, FdII, and an FdII mutant with FdI features was measured and further indicated that the binding mode to FNR differs between FdI and FdII. Based on this evidence, we hypothesize that binding modes with other Fd-dependent reductases may also vary between FdI and FdII. The structural differences between FdI and FdII therefore result in functional differences that may influence partitioning of electrons into different redox metabolic pathways.

Received for publication, August 4, 2004 , and in revised form, October 13, 2004.

^* This work was supported in part by Grant-in-aid for Scientific Research 15GS0320 (to T. H.). 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.

Present address: Dept. of Life Sciences, Graduate School of Arts and Sciences, the University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan.

^** To whom correspondence should be addressed: Faculty of Integrated Arts and Sciences, Hiroshima University, 1-7-1 Kagamiyama, Higashi-Hiroshima 739-8521, Japan. Tel.: 81-82-424-6529; Fax: 81-82-424-0757; E-mail: teshi{at}hiroshima-u.ac.jp.

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