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J. Biol. Chem., Vol. 280, Issue 29, 26645-26648, July 22, 2005

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Crystal Structure of the NAD Biosynthetic Enzyme Quinolinate Synthase^*

Haruhiko Sakuraba, Hideaki Tsuge, Kazunari Yoneda, Nobuhiko Katunuma, and Toshihisa Ohshima¶

From the Department of Biological Science and Technology, Faculty of Engineering, The University of Tokushima, 2-1 Minamijosanjima-cho, Tokushima 770-8506, and the Institute for Health Science, Tokushima Bunri University, Yamashiro-cho, Tokushima 770-8514, Japan

A gene encoding a quinolinate synthase has been identified in the hyperthermophilic archaeon Pyrococcus horikoshii via genome sequencing. The gene was overexpressed in Escherichia coli, and the crystal structure of the produced enzyme was determined to 2.0 Å resolution in the presence of malate, a substrate analogue. The overall structure exhibits a unique triangular architecture composed of a 3-fold repeat of three-layer () sandwich folding. Although some aspects of the fold homologous to the each domain have been observed previously, the overall structure of quinolinate synthase shows no similarity to any known protein structure. The three analogous domains are related to a pseudo-3-fold symmetry. The active site is located at the interface of the three domains and is centered on the pseudo-3-fold axis. The malate molecule is tightly held near the bottom of the active site cavity. The model of the catalytic state during the first condensation step of the quinolinate synthase reaction indicates that the elimination of inorganic phosphate from dihydroxyacetone phosphate may precede the condensation reaction.

Received for publication, May 9, 2005 , and in revised form, June 2, 2005.

^* This work was supported by the National Project on Protein Structural and Functional Analysis promoted by the Ministry of Education, Science, Sports, Culture, and Technology of Japan. 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 Fig. 1 and Table I.

The atomic coordinates and structure factors (code 1WZU) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/).

^¶ To whom correspondence should be addressed. Tel.: 81-88-656-7518; Fax: 81-88-656-9071; E-mail: ohshima{at}bio.tokushima-u.ac.jp.

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