CRYSTAL-STRUCTURE OF ESCHERICHIA-COLI XANTHINE PHOSPHORIBOSYLTRANSFERASE

Xanthine phosphoribosyltransferase (XPRT; EC 2.4.2.22) from Escherichi a coil is a tetrameric enzyme having 152 residues per subunit. XPRT ca talyzes the transfer of the phosphoribosyl group from 5-phospho-alpha- D-ribosyl l-pyrophosphate (PRib-PP) to the 6-oxopurine bases guanine, xanthine, and hypoxanthine to form GMP, XMP, and IMP, respectively. Cr ystals grown in the absence of substrate or product were used to deter mine the structure of XPRT at a resolution of 1.8 Angstrom by multiple isomorphous replacement. The core structure of XPRT includes a five-s tranded parallel B-sheet surrounded by three or-helices, which is simi lar to that observed in other known phosphoribosyltransferase (PRTase) structures. The XPRT structure also has several interesting features. A glutamine residue in the purine binding site may be responsible for the altered 6-oxopurine base specificity seen in this enzyme compared to other 6-oxopurine PRTases. Also, we observe both a magnesium ion a nd a sulfate ion bound at the PRib-PP binding site of XPRT. The sulfat e ion interacts with Arg-37 which has a cis-peptide conformation, and the magnesium ion interacts with Asp-89, a highly conserved acidic res idue in the PRib-PP binding site motif. The XPRT structure also incorp orates a feature which has not been observed in other PRTase structure s. The C-terminal 12 residues of XPRT adopt an unusual extended confor mation and make interactions with a neighboring subunit. The very last residue, Arg-152, could form part of the active site of a symmetry-re lated subunit in the XPRT tetramer.