Non-Michaelian kinetics of rabbit muscle uridine diphosphoglucose pyrophosphorylase.
The kinetic properties of rabbit muscle uridine diphosphoglucose (UDP-Glc) pyrophosphorylase have been studied, in both directions, with respect to substrate saturation, product inhibition, and cation requirement for activity. The results demonstrate that UDP-Glc pyrophosphorylase is a non-Michaelian enzyme: the synthetic reaction is characterized by a marked inhibition by glucose-1-phosphate (at concentrations higher than 0.3 mM) and by an hyperbolic saturation for UTP. In the reverse reaction, instead, the saturation function for UDP-Glc is hyperbolic and that for inorganic pyrophosphate is sigmoid, with a high Hill coefficient of (nH) 2.5. The study of the metal requirement indicates a distinctive ability of cations to stimulate the reactions of synthesis and degradation of the sugar nucleotide and a different stoichiometry of the metal chelates involved. The reaction mechanism is of the ordered-sequential type and the data of product inhibition allowed the identification of glucose-1-phosphate as the first substrate bound and UDP-Glc as the last product released. The inhibition pattern by UDP-Glc gives evidence for cooperativity also in the binding of this molecule.
D000818:Animals; D066298:In Vitro Techniques; D007700:Kinetics; D009132:Muscles / Q000201:enzymology*; D009713:Nucleotidyltransferases / Q000378:metabolism*; D011817:Rabbits; D005957:UTP-Glucose-1-Phosphate Uridylyltransferase / Q000037:antagonists & inhibitors / Q000378:metabolism*
D009713:Nucleotidyltransferases; D005957:UTP-Glucose-1-Phosphate Uridylyltransferase