The importance of molecular associations in biological signaling processes was mentioned in the preceding chapter. That chapter concentrated on the physical aspects of the association process and paid little attention to the signaling events that are initiated by ligand binding. This chapter will accept the binding event as given, and go on to look at what consequences this has on the biological function of a protein.

Powerful theories to explain this kind of signaling can be developed by combining the concepts of molecular associations from Chapter 4 with the concepts of global states and transitions from Chapter 1. In putting these two ideas together, a key point to remember is that both processes are governed primarily by the kinds of noncovalent forces covered in Chapter 1. As a result the energies for global transitions and binding events are often in the same range. This enables an association reaction to trigger a conformational transition in a protein, and this is what makes allosteric interactions possible. Here, we will develop this theory, known as allosteric theory, and illustrate its use with examples.

The word allosteric is quite popular in molecular biology. The word was introduced as a combination of the Greek words allo and steric to mean other-site. A classical usage in this sense is when a ligand binds to a regulatory site of an enzyme and alters the enzyme's effectiveness as a catalyst.