Computational Insights on an Artificial Imine Reductase Based on the Biotin-Streptavidin Technology

We present a computational study that combines protein-ligand docking, quantum mechanical, and quantum mechanical/molecular mechanical calculations to scrutinize the mechanistic behavior of the first artificial enzyme able to enantioselectively reduce cyclic imines. We applied a novel strategy that allows the characterization of transition state structures in the protein host and their associated reaction paths. Of the most striking results of our investigation is the identification of major conformational differences between the transition state geometries of the lowest energy paths leading to (R)- and (S)-reduction products. The molecular features of (R)- and (S)-transition states highlight distinctive patterns of hydrophobic and polar complementarities between the substrate and the binding site. These differences lead to an activation energy gap that stands in very good agreement with the experimentally determined enantioselectivity. This study sheds light on the mechanism by which transfer hydrogenases operate and illustrates how the change of environment (from homogeneous solution conditions to the asymmetric protein frame) affect the reactivity of the organometallic cofactor. It provides novel insights on the complexity in integrating unnatural organometallic compounds into biological scaffolds. The modeling strategy that we pursued, based on the generation of pseudo transition state structures, is computationally efficient and suitable for the discovery and optimization of artificial enzymes. Alternatively, this approach can be applied on systems for which a large conformational sampling is needed to identify relevant transition states. KEYWORDS: artificial enzymes, asymmetric catalysis, imine reduction, computational chemistry, QM/MM, protein-ligand dockings