Progesterone Receptor Ligand Binding Pocket Flexibility: Crystal Structures of the Norethindrone and Mometasone Furoate Complexes

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Abstract
Although progesterone, the natural ligand of the progesterone receptor (PR), has a hydrogen atom at the 17alpha position, other potent steroid agonists such as norethindrone and mometasone furoate have larger substituents at this position that are accommodated by the PR ligand binding pocket. Crystallographic analysis of PR ligand binding domain complexes clearly demonstrated that these moieties were accommodated by local shifts of the protein main chain and by adoption of alternative side chain rotamer conformations of ligand-proximal amino acids. These conformational changes imparted a ligand-specific volume to the binding pocket, from 490 Angstrom(3) in the metribolone complex to 520 Angstrom(3) in the norethindrone complex, 565 Angstrom(3) in the progesterone complex, and 730 Angstrom(3) in the mometasone furoate complex. Despite these marked alterations in binding pocket volume, critical interactions essential for establishment of an active AF2 conformation were maintained.