The significance of the 20-carbonyl group of progesterone in steroid receptor binding: a molecular dynamics and structure-based ligand design study

Polar functional groups in the A- and D-ring (positions 3 and 17beta or 20) are common to all natural and synthetic steroid hormones. It was assumed that these pharmacophoric groups are involved in strong hydrogen bonding interactions with the respective steroid receptors. High resolution X-ray structures of the estrogen and androgen receptors have confirmed these assumptions. Also site-directed mutagenesis studies of the human progesterone receptor (hPR) suggest an important role for Cys891 in the recognition of the progesterone 20-carbonyl group. Surprisingly, the crystal structure of the hPR ligand binding domain (LBD) in complex with progesterone suggests that the carbonyl oxygen in position 20 (020) is not involved in hydrogen bond contacts. To investigate these surprising and contradicting results further, we performed a molecular dynamics simulation of the hPR-progesterone complex in an aqueous environment. The simulation revealed hPR-Cys891 as the sole but weak hydrogen bonding partner of progesterone in the D-ring. In contrast to the site-directed mutagenesis data a major role of hPR-Cys891 in progesterone recognition could not be confirmed. Isolated hydrogen bond acceptors, such as the prosterone 020 group, in a relatively lipophilic environment of the receptor led to a decrease in affinity of the ligand. Based on this consideration and the structure of the PR, we designed compounds lacking such an acceptor function. If the X-ray structure and the calculations were right, these compounds should bind with comparable or higher affinity versus that of progesterone. E-17-Halomethylene steroids were synthesized and pharmacologically characterized in vitro and in vivo. Although the compounds are unable to form hydrogen bonds with the hPR in the D-ring region, they bind with superior affinity and exert stronger in vivo progestational effects than progesterone itself. Our investigations have confirmed the results of the X-ray structure and disproved the old pharmacophore model for progestogenic activity, comprising two essential polar functional groups on both ends of the steroid core. The 20-carbonyl group of progesterone is likely to play a role beyond PR-binding, e.g. in the context of other functions via the androgen and mineralocorticoid receptors and as a site of metabolic inactivation. (C) 2003 Elsevier Inc. All rights reserved.