Crystal structure of Escherichia coli PurE, an unusual mutase in the purine biosynthetic pathway

Background: Conversion of 5-aminoimidazole ribonucleotide (AIR) to 4-carbox yaminoimidazole ribonucleotide (CAIR) in Escherichia coli requires two prot eins - PurK and PurE. PurE has recently been shown to be a mutase that cata lyzes the unusual rearrangement of N-5-carboxyaminoimidazole ribonucleotide (N-5-CAIR), the PurK reaction product, to CAIR. PurEs from higher eukaryot es are homologous to E. coli PurE, but use AIR and CO2 as substrates to pro duce CAIR directly. Results: The 1.50 Angstrom crystal structure of PurE reveals an octameric s tructure with 422 symmetry. A central three-layer (alpha beta alpha) sandwi ch domain and a kinked C-terminal helix form the folded structure of the mo nomeric unit. The structure reveals a cleft at the interface of two subunit s and near the C-terminal helix of a third subunit. Go-crystallization expe riments with CAIR confirm this to be the mononucleotide-binding site. The n ucleotide is bound predominantly to one subunit, with conserved residues fr om a second subunit making up one wall of the cleft. Conclusions: The crystal structure of PurE reveals a unique quaternary stru cture that confirms the octameric nature of the enzyme, An analysis of the native crystal structure, in conjunction with sequence alignments and studi es of cc-crystals of PurE with CAIR, reveals the location of the active sit e. The environment of the active site and the analysis of conserved residue s between the two classes of PurEs suggests a model for the differences in their substrate specificities and the relationship between their mechanisms .