A comparative study of galactose oxidase and active site analogs based on QM/MM Car Parrinello simulations

A parallel study of the radical copper enzyme galactose oxidase (GOase) and a low molecular weight analog of the active site was performed with dynami cal density functional and mixed quantum-classical calculations. This combi ned approach enables a direct comparison of the properties of the biomimeti c and the natural systems throughout the course of the catalytic reaction. In both cases, five essential forms of the catalytic cycle have been invest igated: the resting slate in its semi-reduced (catalytically inactive) and its oxidized (catalytically active) form, A(semi) and A(ox), respectively; a protonated intermediate B; the transition state for the rate-determining hydrogen abstraction step C, and its product D. For A and B the electronic properties of the biomimetic compound are qualitatively very similar to the ones of the natural target. However, in agreement with the experimentally observed difference in catalytic activity, the calculated activation energy for the hydrogen abstraction step is distinctly lower for GOase (16 kcal/m ol) than for the mimetic compound (21 kcal/mol). The enzymatic transition s tate is stabilized by a delocalization of the unpaired spin density over th e sulfur-modified equatorial tyrosine Tyr272, an effect that for geometric reasons is essentially absent in the biomimetic compound. Further differenc es between the mimic and its natural target concern the structure of the pr oduct of the abstraction step, which is characterized by a weakly coordinat ed aldehyde complex for the latter and a tightly bound linear complex for t he former.