Structure of tropinone reductase-II complexed with NADP(+) and pseudotropine at 1.9 angstrom resolution: Implication for stereospecific substrate binding and catalysis

Tropinone reductase-II (TR-II) catalyzes the NADPH-dependent reduction of t he carbonyl group of tropinone to a beta-hydroxyl group. The crystal struct ure of TR-II complexed with NADP(+) and pseudotropine (psi-tropine) has bee n determined at 1.9 Angstrom resolution. A seven-residue peptide near the a ctive site, disordered in the unliganded structure, is fixed in the ternary complex by participation of the cofactor and substrate binding. The psi-tr opine molecule is bound in an orientation which satisfies the product confi guration and the stereochemical arrangement toward the cofactor. The substr ate binding site displays a complementarity to the bound substrate (psi-tro pine) in its correct orientation. In addition, electrostatic interactions b etween the substrate and Glu156 seem to specify the binding position and or ientation of the substrate. A comparison between the active sites in TR-II and TR-I shows that they provide different van der Waals surfaces and elect rostatic features. These differences likely contribute to the correct bindi ng mode of the substrates, which are in opposite orientations in TR-II and TR-I, and to different reaction stereospecificities. The active site struct ure in the TR-II ternary complex also suggests that the arrangement of the substrate, cofactor, and catalytic residues is stereoelectronically favorab le for the reaction.