A steady‐state kinetic analysis of the coupled reactions catalysed by the three‐enzyme system, aldolase, glyceraldehyde‐3‐phosphate dehydrogenase and triosephosphate isomerase, was performed. The kinetic parameters of the progress curves of end‐product formation calculated for noninteracting enzymes were compared with those measured in the two‐enzyme and three‐enzyme systems. Changes in the fluorescence anisotrophy of labelled dehydrogenase upon addition of aldolase and/or isomerase were also measured. Glyceraldehyde‐3‐phosphate oxidation catalysed by glyceraldehyde‐3‐phosphate dehydrogenase in the presence of isomerase (which ensures rapid equilibration of the triosephosphates) follows single first‐order kinetics. The rate constant depends simply on the concentration of the dehydrogenase, indicating no kinetically significant isomerase‐dehydrogenase interaction. Fluorescence anisotropy measurements also fail to reveal complex formation between the two enzymes. The steady‐state velocity of 3‐phosphoglycerate formation from fructose 1,6‐bisphosphate in the reactions catalysed by aldolase and dehydrogenase is not increased twofold on addition of the isomerase, even though a 1:2 stoichiometry of fructose 1,6‐bisphosphate/glyceraldehyde 3‐phosphate is expected. In fact, by increasing the concentration of the isomerase, the steady‐state velocity actually decreases. This effect of the isomerase may be a kinetic consequence of an aldolase‐isomerase interaction, which results in a decrease of aldolase activity. Furthermore, the fluorescence anisotropy of labelled dehydrogenase, measured at different aldolase concentrations, is significantly lower when the sample contains isomerase. The decrease in the steady‐state velocity of the consecutive reactions caused by the elevation of isomerase concentration could be negated by increasing the dehydrogenase concentration in the three‐enzyme system. All of these observations fit the assumption that the amount of aldolase‐dehydrogenase complex is reduced due to competition of isomerase with dehydrogenase. The alternate binding of dehydrogenase and isomerase to aldolase may regulate the flux rate of glycolysis.
|Number of pages||5|
|Journal||European Journal of Biochemistry|
|Publication status||Published - Nov 1986|
ASJC Scopus subject areas