Morita-Baylis-Hillman reaction: how do optimal enzyme active sites compare with organocatalysts

The Morita-Baylis-Hillman reaction attracts significant attention for the synthesis of highly functionalized compounds. It requires multiple catalytic elements for efficient catalysis, making it an appealing target for the design of a well-defined active site that can be grafted on a chiral scaffold. Herein, we investigate the contribution of various catalytic motifs to catalysis using theoretical active site models, and report for the first time that a bidentate oxyanion hole cooperating with an acid-base catalyst allows concurrent stabilization of multiple steps along the reaction pathway. Calculations also suggest the potential dual role of a guanidinium ion in stabilizing oxyanion intermediates and mediating the proton transfer. Computed energy profiles of optimal active sites compared to those of previously studied organocatalytic reactions suggest that a conformationally adaptable preorganized electrostatic environment is essential in promoting such multistep transformations. This study highlights the importance of building cooperativity between catalytic elements operating between multiple conformations, explains the origins of observed MBH activity of proteins with critically positioned histidine and arginine residues, and provides insights into how these activities could be enhanced.