Human 11b-hydroxysteroid dehydrogenase type I (11b-HSD1) is an ER-localized membrane protein that catalyzes the interconversion of cortisone and cortisol. In adipose tissue, excessive cortisol production via 11b-HSD1 activity has been implicated in the pathogenesis of type II diabetes and obesity. We report here biophysical, kinetic, mutagenesis, and structural data on two ternary complexes of 11b-HSD1. The combined results reveal flexible active site interactions relevant to glucocorticoid recognition and demonstrate how four 11b-HSD1 C-termini converge to form an as yet uncharacterized tetramerization motif. A C-terminal Pro-Cys motif is localized at the center of the tetramer and forms reversible enzyme disulfides that alter enzyme activity. Conformational flexibility at the tetramerization interface is coupled to structural changes at the enzyme active site suggesting how the central Pro-Cys motif may regulate enzyme activity. Together, the crystallographic and biophysical data provide a structural framework for understanding 11b-HSD1 activities and will ultimately facilitate the development of specific inhibitors