CRYSTAL STRUCTURE OF HUMAN LIVER ∆-3-KETOSTEROID 5β-REDUCTASE (AKR1D1) AND IMPLICATIONS FOR SUBSTRATE BINDING AND CATALYSIS* Luigi Di Costanzo, Jason E. Drury, Trevor M. Penning, and David W. Christianson From the Roy and Diana Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, and Center of Excellence in Environmental Toxicology an...