Mitochondrial
thymidine kinase 2 (TK2) is a nuclear gene-encoded
protein, synthesized in the cytosol and subsequently translocated into the mitochondrial matrix, where it catalyzes the phosphorylation of
thymidine (dT) and
deoxycytidine (dC). The kinetics of dT phosphorylation exhibits negative cooperativity, but dC phosphorylation follows hyperbolic Michaelis-Menten kinetics. The two substrates compete with each other in that dT is a competitive inhibitor of dC phosphorylation, while dC acts as a noncompetitive inhibitor of dT phosphorylation. In addition, TK2 is feedback inhibited by
dTTP and
dCTP. TK2 also phosphorylates a number of
pyrimidine nucleoside analogues used in
antiviral and anticancer
therapy and thus plays an important role in mitochondrial toxicities caused by
nucleoside analogues. Deficiency in TK2 activity due to genetic alterations causes devastating
mitochondrial diseases, which are characterized by
mitochondrial DNA (
mtDNA) depletion or multiple deletions in the affected tissues. Severe TK2 deficiency is associated with early-onset fatal
mitochondrial DNA depletion syndrome, while less severe deficiencies result in late-onset phenotypes. In this review, studies of the
enzyme kinetic behavior of
TK2 enzyme variants are used to explain the mechanism of
mtDNA depletion caused by TK2 mutations,
thymidine overload due to
thymidine phosphorylase deficiency, and mitochondrial toxicity caused by
antiviral thymidine analogues.