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Probing the Origins of Enzymatic Catalysis Through Physical Organic Chemistry Graeme Howe

Probing the Origins of Enzymatic Catalysis Through Physical Organic Chemistry Graeme Howe

Probing the origins of enzymatic catalysis through physical organic Graeme Howe

Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States Email: [email protected]

Enzymes are extraordinary catalysts that can achieve rate accelerations as high as 1026-fold over the corresponding spontaneous reactions. Despite decades of effort, the mechanisms by which achieve these accelerations remain hotly contested. While understanding how enzymes function has always been of great fundamental interest, the advent of de novo design has imbued these efforts with practical significance: an understanding of the origins of enzymatic catalysis will greatly facilitate the design and production of increasingly efficient enzymes with useful functions and properties. In this seminar, I will present two approaches that use physical to probe the origins of catalysis in members of two of the most catalytically efficient enzyme families: decarboxylases and phosphoryl transferases.

Studies of the of a model of an enzymatic pre-decarboxylation intermediate revealed that the decarboxylation was general catalyzed. Using a combination of and kinetic isotope effects and linear free energy relationships, a novel mechanism of decarboxylation was uncovered in which bicarbonate is lost in place of CO2. Subsequent computation showed that this mechanism avoids recombination of the carbanion-CO2 pair by releasing a less electrophilic leaving group. Re-examination of available literature reveals that this unexpected mechanism is employed by a number of decarboxylases and gives rise to the catalytic efficiency of these enzymes.

Phosphite dehydrogenase (PTDH) catalyzes an incredibly unusual phosphoryl transfer with displacing a hydride leaving group. Traditional enzymology has been unable to determine how PTDH catalyzes this unusual reaction. Recently, a combination of kinetic isotope effects and quantum mechanical calculations was used to determine the structure for the PTDH-catalyzed reaction. This structure provides incredibly detailed insights into the origins of catalysis by PTDH, illustrating the relative timing of bond forming and breaking, the mode of nucleophilic and the interactions within the active site that stabilize the phosphoryl unit. This work demonstrates the power of isotope effects and computation to probe the origins of enzymatic catalysis and the general utility of physical organic techniques to elucidate the catalytic mechanisms of enzymes.