Sensing of Chemical Signals by Enzymes

Living systems are able to sense the intensity of chemical signals originating from the external milieu and are able to detect whether this intensity increases or decreases. Chemotaxis of bacteria represents a striking example of these sensory properties defined at a rather simple level (Koshland, l979, 1980ab, 1981). Bound enzyme systems in which diffusion is a limiting process may also display these sensory properties (Engasser & Horvath, 1974, 1976; Ricard, 1987). This implies that the system possesses a memory and that its response is different depending on whether the concentration of a ligand increases or decreases. In other words the response of the system is sensitive to its history. Nucleic acids may display metastable secondary structures upon their titration and therefore may exhibit hysteresis effects (Revzin et al., 1973, Neumann, 1973, Schneider, 1976).