Three-dimensional structure of thymidine phosphorylase from Escherichia coli at 2.8 A resolution.

The three-dimensional structure of thymidine phosphorylase from Escherichia coli has been determined at 2.8 A resolution using multiple-isomorphous-replacement techniques. The amino acid sequence deduced from the deoA DNA sequence is also reported. Thymidine phosphorylase exists in the crystal as an S-shaped dimer in which the subunits are related by a crystallographic 2-fold axis. Each subunit is composed of a small alpha-helical domain of six helices and a large alpha/beta domain. The alpha/beta domain includes a six-stranded mixed beta-sheet and a four-stranded antiparallel beta-sheet. The active site has been identified by difference Fourier analyses of the binding of thymine and thymidine and lies in a cavity between the small and large domains. The central beta-sheet is splayed open to accommodate a putative phosphate-binding site which is probably occupied by a sulfate ion in the crystal.