6.
The three-dimensional structures of thymidine phosphorylase from
E. coli containing the bound sulfate ion in the phosphate-binding site and of the complex of thymidine phosphorylase with sulfate in the phosphate-binding site and the inhibitor 3′-azido-2′-fluoro-2′,3′-dideoxyuridine (N
3F-ddU) in the nucleoside-binding site were determined at 1.55 and 1.50 Å resolution, respectively. The amino-acid residues involved in the ligand binding and the hydrogen-bond network in the active site occupied by a large number of bound water molecules are described. A comparison of the structure of thymidine phosphorylase in complex with N
3F-ddU with the structure of pyrimidine nucleoside phosphorylase from
St. Aureus in complex with the natural substrate thymidine (PDB_ID: 3H5Q) shows that the substrate and the inhibitor in the nucleoside-binding pocket have different orientations. It is suggested that the position of N
3F-ddU can be influenced by the presence of the azido group, which prefers a hydrophobic environment. In both structures, the active sites of the subunits are in the open conformation.
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