Figure 17: Many membrane receptors, including the Fas receptor, are activated by an extracellular protein ligand that dimerizes or oligomerizes the receptor, causing the intracellular tails to have a proximal relationship. As these tails, and sometimes tail-associated proteins, often have an enzyme-substrate relationship, ligand-induced dimerization achieves a high effective molarity of the enzyme and substrate and thus a large reaction rate61.