[Modeling of the activity kinetics of oligomer enzymes using phosphofructokinase as an example. III. A polyligand model].

A general statistical-thermodynamical theory of the oligomeric enzymes was applied for description of the complex regulatory properties of human erythrocyte phosphofructokinase (PFC, EC 2.7.1.11), experimental data were obtained by Wenzel et al. (Eur. J. Biochem. 1976. V. 61. P. 181). Simple two-ligand and hierarchical three-ligand models for polymeric enzymes were analysed. In hierarchical model structural PFK unit (protomer) was treated as tetramer so both interactions between subunits inside the protomer and between protomers themselves were considered. It was shown that except active center of fructoso-6-phosphate (F6P) binding and two or more inhibiting centers of ATP binding. F6P binding by active centers of different subunits inside the protomer caused negative cooperative interactions but F6P binding by activating centers of the same subunits causes positive interactions. The F6P dissociation constant for activating centers is essentially higher than for active centers. ATP binding by inhibiting centers of the protomer causes very high positive cooperative interactions between neighbouring protomers leading to distinguish additional "quaternary" level of the enzyme structure. Numerical non-linear identification for the parameters of these two models was given and five experimental curves were quantitatively described in the wide range of ligand concentrations (0.01-15 mM F6P and 1-5 mM ATP) for high enzyme concentration (greater than 0.5 mkg/ml).