ABSTRACT
The control of homeostasis and the signals accelerating or reducing various functions are mediated by chemical interactions at the molecular level, often between small molecules and macromolecules. There are two rather different types of interaction involved. Combination of various substances, e.g. transmitters with receptors, is usually by a reversible interaction, whereas the control of the rate of some enzymic processes takes place by the attachment of a chemical grouping by a covalent bond, e.g. as in phosphorylation of many proteins (TIBS, 1994). These two forms of interaction (Figure 3.1) are also involved in the action of therapeutic and toxic agents. The degree and rate of interaction are important determinants of their activity.
