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Abstract

This invited essay outlines how the idea of directed evolution of stereoselective enzymes was born and implemented experimentally at the Max-Planck-Institut für Kohlenforschung in Mülheim/Germany during the period 1994–1998, a time when Andreas Pfaltz was present in the Institute. As the new and sole director of the MPI, I initiated new research projects, and also started to restructure the Institute with the establishment of five departments, all dedicated to some form of catalysis and to be led by five independent directors. Andreas Pfaltz was the first director whom I hired, heading the Department of Homogeneous Catalysis. During his stay in Mülheim until 1998, the Pfaltz group invented effective chiral ligands for a number of particularly challenging enantioselective transformations. During this period, Andreas Pfaltz witnessed the birth and development of directed evolution of stereoselective enzymes as a new research direction in the Reetz group. Indeed, he was one of the very few organic chemists who realized at the time that a door to a novel research area had been opened. The widespread application of enzymes was hampered for decades due to limited enantio-, diastereo-, and regioselectivity, which was the reason why organic chemists were not interested in biocatalysis. This attitude slowly changed with the advent of directed evolution of stereoselective enzymes in 1997, in a publication from the Reetz group. Methodology development with emphasis on stereo- and regioselectivity as well as activity followed, the techniques and strategies becoming more and more rational. Today, semi-rational approaches and so-called rational enzyme design have merged, as evidenced, inter alia, by the development of focused rational iterative site-specific mutagenesis (FRISM). The toolbox of organic chemists now includes enzymes, primarily because the possibility of controlling stereoselectivity by protein engineering has ensured reliability when facing synthetic challenges.