Enzyme

2.7.1.21 - Thymidine kinase

Alternative Name(s)

There are no alternative names for this Enzyme

Catalytic Activity

ATP + thymidine = ADP + dTMP + H(+)

Cofactors

There are no Cofactors for this Enzyme

Reaction Mechanism

    Thymidine kinases (TK) are key enzymes in the pyrimidine salvage pathway catalyzing the phosphate transfer from ATP to thymidine (dT) in the presence of Mg2+ and thus, yielding thymidine monophosphate (dTMP) and ADP. Herpesviridae, such as Herpes simplex virus type 1, encode for their own, multifunctional TK. Unlike the very specific human cytosolic TK (TK1), it is able to phosphorylate pyrimidine as well as purine analogs and demands less stereochemical restrictions concerning the sugar moiety also accepting acyclic side chains as phosphate acceptors (e.g., aciclovir). Therefore, the difference in substrate specificity of human TK 1 and TKHSV1 is a crucial point in establishing a molecular basis for selective antiviral therapy, featuring TKHSV1 as the center of activation of antiviral drugs such as aciclovir (ACV), penciclovir, and ganciclovir (GCV). First being activated by phosphorylation by viral encoded TK, these nucleoside analogs in their triphosphate form block the viral replication by subsequently terminating DNA elongation at the viral DNA polymerase. In combination with GCV TKHSV1 is an established tool used as a prodrug-activating enzyme, so-called suicide enzyme, in gene therapy of cancer, AIDS, and in controlling graft-versus-host disease by allogenic bone marrow transplant (allo BMT).

    The catalytic mechanism proceeds as follows: Glu83 acts as a general base to activate the acceptor group, 5'-hydroxyl group of thymidine. The activated acceptor group makes a nucleophilic attack on the transferred group, gamma-phosphate of ATP. During the transition state, the transferred group and leaving group (beta- and alpha-phosphate groups of ATP) must be stabilised by stabiliser residues, Arg/Lys cluster, along with the magnesium ion as cofactor. It is proposed Asp162 can bind to ATP or ADP through the magnesium ion, although the exact position and interactions of magnesium are unclear from published protein structures.
    Catalytic Residues
    AA Uniprot Uniprot Resid PDB PDB Resid
    Asp P03176 162 1kim 152
    Glu P03176 225 1kim 215
    Arg P03176 220 1kim 210
    Arg P03176 222 1kim 212
    Arg P03176 163 1kim 153
    Lys P03176 62 1kim 52
    Glu P03176 83 1kim 73
    Step Components

    overall reactant used, proton transfer, overall product formed, bimolecular nucleophilic substitution, native state of enzyme regenerated, inferred reaction step

    Step 1.

    Glu83 acts as a base catalyst and deprotonates thymidines 5' OH group. This increases the OH group's nucleophilicity to attack the gamma phosphate on ATP.

    Step 2.

    Inferred deprotonation of Glu83 ready for the next round of catalysis.

    Products.

    The products of the reaction.

Reaction Parameters

There are no kinetic parameters information for this Enzyme

Associated Proteins

Protein name Organism
Thymidine kinase b Mouse-ear cress
Thymidine kinase VACV
Thymidine kinase a Mouse-ear cress
Thymidine kinase, cytosolic Human
Thymidine kinase 1 Slime mold

Citations