The major toxins of the Amanitas are cyclic and bicyclic peptides. Alpha amanitin is a particularly nasty one; it binds only to RNA polymerase that’s actively involved in transcription, so it’s technically a non-competitive inhibitor. It essentially rams itself through the “back” of the polymerase, since the active site of the enzyme is already busy. Phalloidin, another Amanita toxin, locks actin into a polymerized state, so it freezes cells in their tracks.

There’s a quick and dirty overview here and a paper on how the Amanitas make their toxic peptides here.