Mechanism and regulation of thiamine pyrophosphokinase from parsely leaf

Thiamine pyrophosphokinase (EC 2.7.6.2) from parsely leaf showed an absolute requirement for divalent cation such as Mg2+, Mn2+ and Co2+. The activation effect varied with the species and concentrations of such cations. When Mn2+ or Co2+ was used as cofactor, maximal activation was found at a lower level than ATP concentration, whereas the activation by Mg2+ increased hyperbolically with the concentration. Studies of initial velocity and product inhibition led to conclude that the kinase reaction obeys a sequential ordered Bi Bi mechanism; i.e. the enzyme combines in turns with MgATP and thiamine, followed by release of TPP and AMP. The inhibition type revealed for inorganic pyrophosphate was competitive with respect to thiamine with Ki of approximately 2.8 mM. On the other hand, thiamine monophosphate exhibited noncompetitive inhibition with Ki of 0.2 mM. The plots of the reaction rate against MgATP concentrations gave a sigmoidal curve. Addition of either AMP or GMP resulted in restoration of a depressed activity at low concentration of MgATP. The "allosteric" inhibition was also relieved by the addition of an excess amount of magnesium ions. These findings suggest that transphosphorylation is regulated by subcellular concentrations of metal ions relative to ATP or of the products involved in the thiamine biosynthesis.