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J. Biol. Chem., Vol. 280, Issue 12, 10881-10887, March 25, 2005

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A Methanocaldococcus jannaschii Archaeal Signature Gene Encodes for a 5-Formaminoimidazole-4-carboxamide-1--D-ribofuranosyl 5'-Monophosphate Synthetase

A NEW ENZYME IN PURINE BIOSYNTHESIS^*

Katie Ownby, Huimin Xu, and Robert H. White

From the Department of Biochemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0308

We have identified and characterized a new member of the ATP-grasp enzyme family that catalyzes the ATP- and formate-dependent formylation of 5-aminoimidazole-4-carboxamide-1--D-ribofuranosyl 5'-monophosphate (AICAR) to 5-formaminoimidazole-4-carboxamide-1--D-ribofuranosyl 5'-monophosphate (FAICAR) in the absence of folates. The enzyme, which we designate as PurP, is the product of the Methanocaldococcus jannaschii purP gene (MJ0136), which is a signature gene for Archaea. As is characteristic of reactions catalyzed by this family of enzymes, the other products of the reaction, ADP and P_i, were produced stoichiometrically with the amount of ATP, formate, and AICAR used. Formyl phosphate was found to substitute for ATP and formate in the reaction, yet the methylene analog, phosphonoacetaldehyde, was not an inhibitor or substrate for the reaction. The enzyme, along with PurO, which catalyzes the cyclization of FAICAR to inosine 5'-monophosphate, catalyzes the same overall transformation in purine biosynthesis as is accomplished by PurH in bacteria and eukaryotes. No homology exists between PurH and either PurO or PurP. ¹H NMR and gas chromatography-mass spectrometry analysis of an M. jannaschii cell extract showed the presence of free formate that can be used by the enzyme for purine biosynthesis. This formate arises by the reduction of CO₂ with hydrogen; this was demonstrated by incorporating ¹³C into the formate when M. jannaschii cell extracts were incubated with and hydrogen gas. The presence of this signature gene in all of the Archaea indicates the presence of a purine biosynthetic pathway proceeding in the absence of folate coenzymes.

Received for publication, December 10, 2004 , and in revised form, December 23, 2004.

^* This work was supported by United States National Science Foundation Grant MCB0231319 (to R. H. W.). 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.

To whom correspondence should be addressed: Dept. of Biochemistry, 103 Engel Hall (0308), Virginia Polytechnic Institute and State University, Blacksburg, VA 24061. Tel.: 540-231-6605; Fax: 540-231-9070; E-mail: rhwhite{at}vt.edu.

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