**3.1.2 Additional examples**

The existence of a clash between antagonists and helix-12 was first demonstrated for ER by studies comparing the X-ray structures of Estradiol to Raloxifene (Brzozowski et al, 1997) and Diethylstilbestrol to Tamoxifen (Shiau et al, 1998). Numerous reviews of these two studies have been published (Hubbard et al, 2000;Kong, Pike, and Hubbard, 2003;Mueller-Fahrnow and Egner, 1999;Pike et al, 2000;Pike, Brzozowski, and Hubbard, 2000) as have many further studies on the X-ray structures of SERMs, full antagonists and full agonists bound to the ERs (Blizzard et al, 2005;Dykstra et al, 2007;Heldring et al, 2007;Kim et al, 2004;Renaud et al, 2003;Renaud et al, 2005;Tan et al, 2005;Vajdos et al, 2007).

Fig. 1. Binding of PR agonist Norethindrone (orange) from X-ray structure compared to PR antagonist Mifepristone (green) demonstrating clash between antagonists and Met909 in helix-12.

The same helix-12 clash has also been demonstrated for AR (Cantin et al, 2007) and GR (Schoch et al, 2010) in recent X-ray structure determination studies. It was also suggested for GR by a mutagenesis study (Hillmann et al, 2002) that showed that mutating Leu753 (equivalent to Met909 in PR) to a phenylalanine results in a receptor defective in transactivation. We can conclude that the reason for this loss of activation is that an increase in the size of the residue at this position prevents helix-12 from adopting its agonistic conformation due to a clash with the ligand.
