Steve Ealick's Research Group

Acetobacter aceti N5-carboxyaminoimidazole Ribonucleotide Mutase (AaPurE)

PDB files:



N5-carboxyaminoimidazole ribonucleotide mutase (PurE) catalyzes the only C-C bond forming reaction in de novo purine biosynthesis, the formation of 4-carboxy-5-aminoimidazole ribonucleotide (CAIR).Acetobacter aceti converts wine into vinegar and the organisms have developed a high tolerance for acidic conditions. In this paper we used the crystal structure of AaPurE, determined at 1.5 Å, to identify structural features that may help confer acid stability on this enzyme. This is necessary because N5-CAIR readily decarboxylates by a non-enzymatic mechanism.

The monomer of AaPurE consists of a central core containing a five stranded parallel β-sheet flanked by two a helices on one side and by three a helices on the other. Superposition of the Ca positions of AaPurE with those of E. coli PurE, also determined in our laboratory, indicated that their folds were nearly identical.


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AaPurE is an octamer with 422 point symmetry, also making it very to the quaternary structure of EcPurE. AaPurE has many more pairwise interactions between the subunits than EcPurE. This may account for its greater stability to acid and/or heat.


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The AaPurE active site is at the interface of three subunits. In this model of the active site of AaPurE with AIR (5-aminoimidizole ribonucleotide) modeled, key residues are drawn in ball and stick representation. Important loops are highlighted in different colors. According to the EcPurE naming system these loops are the P loop (yellow), the “40’s loop” (cyan) and the “70’s loop” (red). The gray loop is donated from a neighboring subunit. Residues from a neighboring subunit are denoted by a prime.

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Settembre EC, Chittuluru JR, Mill CP, Kappock TJ and Ealick SE. Acidophilic Adaptations in the Structure of Acetobacter aceti N5-carboxyaminoimidazole Ribonucleotide Mutase (PurE). Acta Crystallogr. D. 60:1753-1760 (2004).

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