[A model of an enzyme with mixed cooperation: 3 states of aspartate transcarbamylase].

Allosteric enzyme models on the basis of the known properties of aspartate transcarbamylase (ATCase) from Escherichia coli are suggested. In the first model molecules are supposed to equilibrate between two states. In contrast to the classical Monod-Wyman-Changeux model the symmetry of enzyme molecules changes during the conformational transition. It is shown that the number of binding sites of the enzyme defined from the Scatchard plots is sufficiently dependent on values of parameters of enzyme reaction. This fact results from the mixed (both positive and negative) cooperative effects. However the complex kinetic of ATCase is not completely simulated by this model. Therefore the model is complicated by taking into account the inactive third state of the enzyme. Thus the complex kinetic behaviour of ATCase is explained. The models may be also used for other enzymes.