THE X-RAY STRUCTURE OF URIDINE PHOSPHORYLASE FROM E. Coli AT 2.5 Å RESOLUTION. A. M. Mikhailov, E. Yu. Morgunova, Institute of Crystallography, Moscow, Russia, S. E. Ealick, Ch. Mao, Sh. R. Armstrong, Cornell University, NY 14853, USA; A. S. Mironov, A. A. Komissarov Gniigenetica, Moscow, Russia

Uridine phosphorylase (EC 2.4.2.3; UPhase) from E.coli catalyzes the reversible phosphorolysis of uridine with the formation of riboso-1-phosphate and uracil. Uphase has been identified as the enzyme which is responsible for the cleavage of some pyrimidine nucleoside analogs possessing the antitumor activity. The enzyme molecule is hexamer with point symmetry 32. The molecular weight is 165 KDa. The primary structure of the subunits is known and includes 253 aminoacid residues. Uphase structure was solved at 2.5 Å resolution by molecular replacement method. The crystallographic refinement was performed by the program X-PLOR to R-factor 18.6% .Rms deviations from bond length of 0.012 Å and bond angles 2.095deg.. The 150 water molecules were picked in the structure. The UPhase monomer is an [[alpha]]/ß protein. Six [[alpha]]-helixes and two twisted ß-sheets form the main core of the monomer. There are two flexible loops which play the important role in the enzyme catalyze. So, the flexible loop (residues Tyr163-Phe180) extends into the neighboring subunit. This loop is hydrophobic and five hydrogen bonds were found between this loop and neighboring subunit. The active site is located near the interface of two subunits. This region includes residues 26-30, 68-70, 91-96, 162-163, 166-168, 195-198 and 220-223 of one subunit and residues 5-8 of another. Recent studies by selective chemical modification have indicated that Asp-5, Cys-136 and Tyr-169 are present near or in the UPhase active site.