Add Allosteric modulator example

[cthoyt]

Allosteric modulators do not bind to the agonist-binding site of the receptor but instead on specific allosteric binding sites, through which they modify the effect of the agonist. For example, benzodiazepines (BZDs) bind to the BZD site on the GABAA receptor and potentiate the effect of endogenous GABA.

a(BZSs) -> complex(GABAA, a(BZSs))
complex(GABAA, a(BZSs)) -> act(endogenous ligand)
act(endogenous ligand) -> act(GABAA)

a(BZSs) -> complex(GABAA, a(BZSs))
complex(GABAA, a(BZSs)) -> complex(GABAA, a(BZSs), a(endogenous ligand))
complex(GABAA, a(BZSs), a(endogenous ligand)) -> act(GABAA)