Crystal structure of a purine/pyrimidine phosphoribosyltransferase-related protein from Thermus thermophilus HB8.

Adenine phosphoribosyltransferase (APRTase) is a widely distributed enzyme involved in the salvage of adenine to form an adenine nucleotide. We crystallized and determined the X-ray crystallographic structure of a purine/pyrimidine phosphoribosyltransferase-related protein from the thermophilic bacterium, Thermus thermophilus HB8. The crystal space group was C2 with unit cell dimensions of a = 167.42 A, b = 61.41 A, c = 102.39 A, beta = 94.0 degrees . Initial phases were determined to 2.6 A using the multiple wavelength anomalous dispersion method and selenomethionine substituted protein (Se-MAD), and refined using a 1.9 A "native" data set. The asymmetric unit contains two pairs of identical dimers, each related by noncrystallographic two-fold symmetry. The fifth monomer forms a similar dimer across a crystallographic two-fold axis. These dimers appear to be the biological unit with both monomers contributing to an unusual highly charged arginine-rich bridge region separating the two active sites. Comparison with distantly related APRTases reveal similarities and differences of the active site.