Solution structure of Delta 5-3-ketosteroid isomerase complexed with the steroid 19-nortestosterone hemisuccinate.

The solution structure of the ketosteroid isomerase homodimer complexed with the product analogue 19-nortestosterone hemisuccinate (19-NTHS) was solved by heteronuclear multidimensional NMR methods using 1647 distance restraints, 77 dihedral angle (φ) restraints, and 67 hydrogen bond restraints per monomer. The refined secondary structure of each subunit consists of three α-helices, eight β-strands, four turns, and two β-bulges. The β-strands form a mixed β-sheet. One of the five proline residues, Pro-39, is cis and begins a nonclassical turn. A self-consistent ensemble of 15 tertiary/quaternary structures of the enzyme dimer−steroid complex, with no distance violations greater than 0.35 A, was generated by simulated annealing and energy minimization with the program X-PLOR. The mean pairwise RMSD of the secondary structural elements was 0.63 A for the average subunit and 1.25 A for the dimer. Within each subunit, the three α-helices are packed onto the concave surface of the β-sheet with a groove between...