Biomaterial Database

Reversible ATP-dependent transition between two forms of human cytosolic thymidine kinase with different enzymatic properties. 1993

B Munch-Petersen, and G Tyrsted, and L Cloos

Department of Biology and Chemistry, Roskilde University, Denmark.

Human cytosolic thymidine kinase, subunit molecular mass about 24 kDa, is a tetramer in the presence of ATP but a dimer in the presence of thymidine or without substrates. The pure, substrate-free enzyme showed complex, non-hyperbolic thymidine substrate kinetics with an apparent Km of 15 microM. Incubation with ATP at 4 degrees C induced a time-dependent transition to an enzyme form with hyperbolic kinetics and a 20-fold lower Km value for thymidine (0.7 microM) but the same maximal velocity as for cytosolic thymidine kinase (TK1) without ATP. Removal of the ATP by carboxymethyl chromatography reestablished the non-hyperbolic kinetics with the low affinity for thymidine (Km(app) = 12 microM), and this enzyme form could be reversed once more by ATP incubation to the high affinity enzyme form. Similar shifts could not be induced by thymidine. The activating effect of ATP depended on the concentration of enzyme protein in a linear manner. These results indicate that ATP is a positive effector of cytosolic thymidine kinase, controlling a kinetically slow transition between two molecular forms of the enzyme. A hypothetical reaction mechanism is presented to explain the complex kinetic behavior.

UI	MeSH Term	Description	Entries
D007527	Isoenzymes	Structurally related forms of an enzyme. Each isoenzyme has the same mechanism and classification, but differs in its chemical, physical, or immunological characteristics.	Alloenzyme,Allozyme,Isoenzyme,Isozyme,Isozymes,Alloenzymes,Allozymes
D007700	Kinetics	The rate dynamics in chemical or physical systems.
D008970	Molecular Weight	The sum of the weight of all the atoms in a molecule.	Molecular Weights,Weight, Molecular,Weights, Molecular
D003600	Cytosol	Intracellular fluid from the cytoplasm after removal of ORGANELLES and other insoluble cytoplasmic components.	Cytosols
D006801	Humans	Members of the species Homo sapiens.	Homo sapiens,Man (Taxonomy),Human,Man, Modern,Modern Man
D000255	Adenosine Triphosphate	An adenine nucleotide containing three phosphate groups esterified to the sugar moiety. In addition to its crucial roles in metabolism adenosine triphosphate is a neurotransmitter.	ATP,Adenosine Triphosphate, Calcium Salt,Adenosine Triphosphate, Chromium Salt,Adenosine Triphosphate, Magnesium Salt,Adenosine Triphosphate, Manganese Salt,Adenylpyrophosphate,CaATP,CrATP,Manganese Adenosine Triphosphate,MgATP,MnATP,ATP-MgCl2,Adenosine Triphosphate, Chromium Ammonium Salt,Adenosine Triphosphate, Magnesium Chloride,Atriphos,Chromium Adenosine Triphosphate,Cr(H2O)4 ATP,Magnesium Adenosine Triphosphate,Striadyne,ATP MgCl2
D013379	Substrate Specificity	A characteristic feature of enzyme activity in relation to the kind of substrate on which the enzyme or catalytic molecule reacts.	Specificities, Substrate,Specificity, Substrate,Substrate Specificities
D013937	Thymidine Kinase	An enzyme that catalyzes the conversion of ATP and thymidine to ADP and thymidine 5'-phosphate. Deoxyuridine can also act as an acceptor and dGTP as a donor. (From Enzyme Nomenclature, 1992) EC 2.7.1.21.	Deoxythymidine Kinase,Deoxypyrimidine Kinase,Kinase, Deoxypyrimidine,Kinase, Deoxythymidine,Kinase, Thymidine