Kinetics of the monomer-dimer reaction of yeast hexokinase PI.
Kinetic studies of the glucose-dependent monomer-dimer reaction of yeast hexokinase PI at pH 8.0 in the presence of 0.1 M-KCl have been carried out using the fluorescence temperature-jump technique. A slow-relaxation effect was observed which was attributed from its dependence on enzyme concentration to the monomer-dimer reaction; the reciprocal relaxation times tau-1 varied from 3 s-1 at low concentrations of glucose to 42 s-1 at saturating concentrations. Rate constants for association (kass.) and dissociation (kdiss.) were determined as a function of glucose concentration using values of the equilibrium association constant of the monomer-dimer reaction derived from sedimentation ultracentrifugation studies under similar conditions, and also from the dependence of tau-2 on enzyme concentration. kass. was almost independent of glucose concentration and its value (2 x 10(5) M-1.s-1) was close to that expected for a diffusion-controlled process. The influence of glucose on the monomer-dimer reaction is entirely due to effects on kdiss., which increases from 0.21 s-1 in the absence of glucose to 25 s-1 at saturating concentrations. The monomer and dimer forms of hexokinase have different affinities and Km values for glucose, and the results reported here imply that there may be a significant lag in the response of the monomer-dimer reaction to changes in glucose concentrations in vivo with consequent hysteretic effects on the hexokinase activity.
Hoggett
J G
JG
Department of Biology, University of York, U.K.
Kellett
G L
GL
D005947:Glucose / Q000378:metabolism / Q000494:pharmacology; D006593:Hexokinase / Q000378:metabolism*; D007527:Isoenzymes / Q000378:metabolism*; D007700:Kinetics; D046911:Macromolecular Substances; D012441:Saccharomyces cerevisiae / Q000201:enzymology*
D007527:Isoenzymes; D046911:Macromolecular Substances; D006593:Hexokinase; D005947:Glucose