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J. Biol. Chem., Vol. 283, Issue 7, 3718-3730, February 15, 2008

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Superoxide Generation by Chlorophyllide a Reductase of Rhodobacter sphaeroides^*

From the Department of Life Science and Interdisciplinary Program of Integrated Biotechnology, Sogang University, Seoul 121-742, Korea

Chlorophyllide a reductase of Rhodobacter sphaeroides, which were reconstituted with the purified subunits of BchX, BchY, and BchZ, reduced ring B of chlorophyllide a using NADH under anaerobic conditions. Interestingly, suppressor mutations rescuing the inability of R. sphaeroides Fe-SOD mutant to grow in succinate-based minimal medium were predominantly mapped to BchZ subunit of chlorophyllide a reductase. The enzyme is labile in the presence of O₂. However, it generates superoxide at low O₂. The enzymes reconstituted with BchX, BchY, and the mutein subunit of BchZ from suppressor mutants showed less activity not only for chlorophyllide a reduction but also for superoxide generation compared with the enzyme reconstituted with the wild-type subunits. BchX, which contains FMN, and BchY are iron-sulfur proteins, whereas BchZ is a hemoprotein containing b-type heme. Neither chlorophyllide a reduction nor superoxide generation was observed with the enzyme reconstituted with the wild-type subunits of BchX and BchY, and the apo-subunit of BchZ that had been refolded without heme, in which FMN of BchX was fully reduced. Thus, superoxide is generated not from FMN of BchX but from heme of BchZ. Consistently, the heme of BchZ muteins was half-reduced in its redox state compared with that of wild-type BchZ.

Received for publication, September 17, 2007 , and in revised form, December 12, 2007.

^* This work was supported by the 21st Century Frontier Microbial Genomics and Applications Center Program from Ministry of Science and Technology, Korea, Grant MG05-0309-1-0. 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 Both authors contributed equally to this work.

² To whom correspondence should be addressed: Dept. of Life Science and Interdisciplinary Program of Integrated Biotechnology, Sogang University, Mapo, Seoul 121-742, Korea. Tel.: 82-2-705-8459; Fax: 82-2-704-3601; E-mail: jgklee{at}sogang.ac.kr.

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