Crystal structure of human thymidylate synthase: a structural mechanism for guiding substrates into the active site
Department of Biochemistry and Molecular Pharmacology
Medical Subject Headings
Amino Acid Sequence; Binding Sites; Conserved Sequence; Crystallography; DNA Transposable Elements; Deoxyuracil Nucleotides; Eukaryotic Cells; Humans; Hydrogen Bonding; Models, Molecular; Molecular Sequence Data; *Protein Structure, Tertiary; Sequence Alignment; Structure-Activity Relationship; Synchrotrons; Thymidine Monophosphate; Thymidylate Synthase
The crystal structure of human thymidylate synthase, a target for anti-cancer drugs, is determined to 3.0 A resolution and refined to a crystallographic residual of 17.8%. The structure implicates the enzyme in a mechanism for facilitating the docking of substrates into the active site. This mechanism involves a twist of approximately 180 degrees of the active site loop, pivoted around the neighboring residues 184 and 204, and implicates ordering of external, eukaryote specific loops along with the well-characterized closure of the active site upon substrate binding. The highly conserved, but eukaryote-specific insertion of twelve residues 90-101 (h117-128), and of eight residues between 156 and 157 (h146-h153) are known to be alpha-helical in other eukaryotes, and lie close together on the outside of the protein in regions of disordered electron density in this crystal form. Two cysteines [cys 202 (h199) and 213 (h210)] are close enough to form a disulfide bond within each subunit, and a third cysteine [cys 183 (h180)] is positioned to form a disulfide bond with the active site cysteine [cys 198 (h195)] in its unliganded conformation. The amino terminal 27 residues, unique to human TS, contains 8 proline residues, is also in a region of disordered electron density, and is likely to be flexible prior to substrate binding. The drug resistance mutation, Y6H, confers a 4-fold reduction in FdUMP affinity and 8-fold reduction in kcat for the dUMP reaction. Though indirectly connected to the active site, the structure suggests a mechanism of resistance that possibly involves a change in structure. This structure offers a unique opportunity for structure-based drug design aimed at the unliganded form of the human enzyme.
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Citation: Biochemistry. 1995 Dec 19;34(50):16279-87.
Schiffer, Celia A.; Clifton, Ian J.; Davisson, V. Jo; Santi, Daniel V.; and Stroud, Robert M., "Crystal structure of human thymidylate synthase: a structural mechanism for guiding substrates into the active site" (1995). Biochemistry and Molecular Pharmacology Publications and Presentations. Paper 96.