The experimental upper limit for room temperature (RT) deuterium kinetic isotope effects (KIEs) has been creeping up for decades, beyond a semi-classical limit of kH/kD~7. While rate ratios between 10 and 15 had been easily accepted and rationalized, via the incorporation of a role for hydrogen tunneling beneath the top of the reaction barrier, values for kH/kD in the range of 100 are also no longer considered “out of bounds”. The enzyme soybean lipoxygenase(SLO), which has played an important role in the acceptance of highly inflated RT KIEs, continues to astound us with unprecedented behavior. This talk will focus on the recent evidence for an average KIE of 661±27 between 5 and 30 oC for a variant of SLO in which two hydrophobic side chains have been reduced in size. The data are well reproduced within a vibronically nonadiabatic proton-coupled electron transfer model in which the active site has been simultaneously expanded and rigidified compared to wild type enzyme and single site mutants.

Enormous, temperature independent kinetic deuterium isotope effects in the proton-coupled electron transfer reaction catalyzed by soybean lipoxygenase

254th National Meeting (2017)

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