Tamoxifen is an example of an antagonist that binds to the same binding regions as the natural ligand, estrogen.

Tamoxifen

Tamoxifen metabolite

$($a$)$ Suggest how it might bind to the receptor in order to do this. Explain the reason why tamoxifen acts as an antagonist of the receptor and not as an agonist.

$($b$)$ The tamoxifen metabolite $($on the right$)$ acts as an estrogen agonist rather than an antagonist. Why?