title: Yeast hexokinase. A fluorescence temperature-jump study of the kinetics of the binding of glucose to the monomer forms of hexokinases P-I and P-II.
abstract: The binding of glucose to the monomeric forms of hexokinases P-I and P-II in Tris and phosphate buffers at pH 8.0 in the presence of 1 mol l-1 KCl has been studied using the fluorescence temperature-jump technique. For both isozymes only one relaxation time was observed; values of tau-1 increased linearly with increasing concentration of free reacting partners. The apparent second-order rate constant for association was about 2 X 10(6) 1 mol-1 s-1 for both isozymes; the differences in the stabilities of the complexes with P-I and P-II are entirely attributable to the fact that glucose dissociates more slowly from its complex with P-I than P-II (approximately 300 s-1 and 1100 s-1 respectively). Although the kinetic data are compatible with a single-step mechanism for glucose binding the association rate constant was much lower than that expected for a diffusion-limited rate of encounter. Other mechanisms for describing an induced-fit are discussed. It is shown that the data are incompatible with a slow 'prior-isomerization' pathway of substrate binding, but are consistent with a 'substrate-guided' pathway involving isomerization of the enzyme-substrate complex.
mesh_terms: D001665:Binding Sites; D005947:Glucose; D006593:Hexokinase / Q000378:metabolism*; D007527:Isoenzymes / Q000378:metabolism*; D007700:Kinetics; D008433:Mathematics; D011485:Protein Binding; D012441:Saccharomyces cerevisiae / Q000201:enzymology*; D013050:Spectrometry, Fluorescence; D013696:Temperature
chemical_list: D007527:Isoenzymes; D006593:Hexokinase; D005947:Glucose