Figure 8.

Graphical representation of a c-CN. A_i and A_j are the same molecular species but contained in different compartments i and j, respectively. We allow for an activated form of A, denoted by A*, which binds to the promotor site of h and activates its expression. H is an active transporter molecule for A. Once exported to the extracellular space, an A_i molecules become a molecule of B. We assume that each compartment has a transmembrane protein E with m extracellular binding sites. If all m binding sites are occupied by B then the internal site becomes active (indicated by green) and can catalyze the activation of A.