title: Engineering of a single conserved amino acid residue of herpes simplex virus type 1 thymidine kinase allows a predominant shift from pyrimidine to purine nucleoside phosphorylation.
abstract: Studies of herpes simplex virus type 1 (HSV-1) thymidine (dThd) kinase (TK) crystal structures show that purine and pyrimidine bases occupy distinct positions in the active site but approximately the same geometric plane. The presence of a bulky side chain, such as tyrosine at position 167, would not be sterically favorable for pyrimidine or pyrimidine nucleoside analogue binding, whereas purine nucleoside analogues would be less affected because they are located further away from the phenylalanine side chain. Site-directed mutagenesis of the conserved Ala-167 and Ala-168 residues in HSV-1 TK resulted in a wide variety of differential affinities and catalytic activities in the presence of the natural substrate dThd and the purine nucleoside analogue drug ganciclovir (GCV), depending on the nature of the amino acid mutation. A168H- and A167F-mutated HSV-1 TK enzymes turned out to have a virtually complete knock-out of dThd kinase activity (at least approximately 4-5 orders of magnitude lower) presumably due to a steric clash between the mutated amino acid and the dThd ring. In contrast, a full preservation of the GCV (and other purine nucleoside analogues) kinase activity was achieved for A168H TK. The enzyme mutants also markedly lost their binding capacity for dThd and showed a substantially diminished feedback inhibition by thymidine 5'-triphosphate. The side chain size at position 168 seems to play a less important role regarding GCV or dThd selectivity than at position 167. Instead, the nitrogen-containing side chains from A168H and A168K seem necessary for efficient ligand discrimination. This explains why A168H-mutated HSV-1 TK fully preserves its GCV kinase activity (Vmax/Km 4-fold higher than wild-type HSV-1 TK), although still showing a severely compromised dThd kinase activity (Vmax/Km 3-4 orders of magnitude lower than wild-type HSV-1 TK).
authors.affiliation: Rega Institute for Medical Research, K. U. Leuven, B-3000 Leuven, Belgium. jan.balzarini@rega.kuleuven.be
authors.forename: Jan
authors.initials: J
authors.lastname: Balzarini
authors.forename: Sandra
authors.initials: S
authors.lastname: Liekens
authors.forename: Nicola
authors.initials: N
authors.lastname: Solaroli
authors.forename: Kamel
authors.initials: K
authors.lastname: El Omari
authors.forename: David K
authors.initials: DK
authors.lastname: Stammers
authors.forename: Anna
authors.initials: A
authors.lastname: Karlsson
mesh_terms: D000998:Antiviral Agents / Q000494:pharmacology; D001665:Binding Sites; D018259:Herpesvirus 1, Human / Q000201:enzymology*; D007700:Kinetics; D008956:Models, Chemical; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D010766:Phosphorylation; D011487:Protein Conformation; D015202:Protein Engineering / Q000379:methods*; D011684:Purine Nucleosides / Q000737:chemistry*; D011741:Pyrimidine Nucleosides / Q000737:chemistry*; D013937:Thymidine Kinase / Q000737:chemistry*
chemical_list: D000998:Antiviral Agents; D011684:Purine Nucleosides; D011741:Pyrimidine Nucleosides; D013937:Thymidine Kinase