Two-Site Systems: Direct and Indirect Cooperativity

Abstract

In this chapter we begin to study cooperative systems. We start by defining the term correlation between any two events. The term “cooperativity” is then identified with the term “correlation” when applied to a particular event such as “sites i ₁,... i _n are occupied.” We examine two fundamental sources of cooperativity: direct and indirect. The first is due to direct ligand-ligand interaction. The second can arise from various sources: (1) effect of the ligand on the translational and rotational PF of the molecule; (2) effect of the solvent; (3) effect of the ligand on the conformational state of the molecule. The first possibility is usually negligible since the ligands are very small compared with the adsorbent molecule (Appendix B). The binding of proteins to DNA, discussed in Section 5.10, is an exception. The second is important and should be considered whenever a solvent is present. We shall discuss solvent effects in Chapter 9. What remains is the effect of the ligand on the conformational state of the adsorbent molecule. This is discussed beginning in Section 4.5 and is shown to be one of the most interesting aspects of cooperativity. In Chapters 5 and 6, we shall see that the mechanism of transmitting information by the adsorbent molecule is, in some formal sense, essentially the same as through the solvent. We shall see that in order to transmit information through the adsorbent molecule, which in general consists of several subunits, three conditions must be fulfilled. First, the ligand must discriminate between the different conformations.