Thermodynamics of information transfer between subunits in oligomeric enzymes and kinetic cooperativity

The theory and the methods that have been described in the two preceding papers in this journal have been used to analyze the kinetic properties of chloroplast fructose bisphosphatase. The enzyme is a tetramer made up of apparently identical subunits and displays a sigmoidal kinetics with respect to its substrate, fructose bis-phosphate. The free ionic species, magnesium and fructose bisphosphate bind to the enzyme and the chelate fructose-bisphosphate-magnesium does not affect the sigmoidicity of the rate curves. The Hill coefficient with respect to free fructose bisphosphate is equal to 2.3, which is indeed incompatible with the view that the enzyme behaves as a dimer of dimers. This conclusion is confirmed by direct analysis of the rate curve. On the basis of the sum of the residuals, their sum of squares, the standard error of the kinetic parameters of the equation, the kinetic scheme associated with a dimer of dimers may be ruled out. On the basis of the same criteria, the fit of an Adair equation to the rate data cannot be retained as satisfactory. This is a direct proof that neither the Monod nor the Koshland model can correctly fit these kinetic data. In fact the model that fits these data best is a structural kinetic scheme where information transfer occurs between each subunit and its three neighbors (‘tetrahedral’ mode of information transfer). The fit of these models to a large number of kinetic data allows one to compute the free energy profile during the successive binding processes of the four substrate molecules to the enzyme. Whereas the first two steps are associated with an increase of free energy, all the other subsequent steps are associated with a decrease of free energy.