Active-site model Glutathione peroxidases

Figure 2-9. Reaction scheme for the complete catalytic cycle in glutathione peroxidase (left). Numbers represent calculated reaction barriers using the active-site model. The detailed potential energy diagram for the first elementary reaction, (E-SeH) + H2O2 - (E-SeOH) + H2O, calculated using both the active-site (dashed line) and ONIOM model (grey line) is shown to the right (Adapted from Prabhakar et al. [28, 65], Reprinted with permission. Copyright 2005, 2006 American Chemical Society.)...

When looking at the reaction mechanisms of glutathione peroxidase and isopenicillin N synthase, we did not find any reaction step where the transition state is significantly stabilized by long-range electrostatic interactions (i.e. electrostatic interactions outside the active-site model). However, it is should be added that most transition states have been calculated using ONIOM-ME. 

To assess the effect of protein binding on catalysis, we compared the initial rates of [1] oxidation by t-butylhydroperoxide in the presence of selenolsubtilisin and diphenyldiselenide, a nonenzymatic model compound. At low substrate concentrations the enzymatic reaction is roughly 70,000 times faster than the selenocystein catalyzed process. Clearly, the redox activity of the selenium group is considerably increased in the active site of subtilisin. Further characterization of the redox chemistry of selenolsubtilisin is in progress. We believe that the information gained in this study will provide a better understanding of how natural glutathione peroxidase works. 

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