Specific ligand-induced association of an enzyme. A new model of dissociating allosteric enzyme

The kinetic behavior of dissociative enzyme system of the type inactive monomer in equilibrium active dimer where dimeric form is stabilized by specific ligand (in particular by substrate) which is bound in the region of the contact of monomers has been analysed. It is assumed that the dissociation of dimer results in formation of monomers which retain the subsites for specific ligand binding. The shape of the dependences of enzyme reaction rate (v) on substrate concentration (S) has been characterized using the order of enzyme reaction rate with respect to substrate concentration: ns = d ln v/d ln S]. When the substrate concentrations are low the dependences of v on S] have S-shaped form (the maximum value of ns exceeds the unity) at the definite values of the parameters of the enzyme system. The value of ns approaches--2 at sufficiently high substrate concentrations (in the region where the substrate reveals the inhibitory effect due to blocking the association of inactive monomers into active dimer). The methods of calculation of the parameters of the dissociative enzyme system under discussion have been elaborated on the basis of the analysis of the experimental dependences of specific enzyme activity on enzyme concentration obtained at various fixed substrate concentrations.