Heterolytic OO bond cleavage: Functional role of Glu113 during bis-Fe(IV) formation in MauG.

The diheme enzyme MauG utilizes H2O2 to perform oxidative posttranslational modification on a protein substrate. A bis-Fe(IV) species of MauG was previously identified as a key intermediate in this reaction. Heterolytic cleavage of the OO bond of H2O2 drives the formation of the bis-Fe(IV) intermediate. In this work, we tested a hypothesis that a glutamate residue, Glu113 in the distal pocket of the pentacoordinate heme of MauG, facilitates heterolytic OO bond cleavage, thereby leading to bis-Fe(IV) formation. This hypothesis was proposed based on sequence alignment and structural comparison with other H2O2-utilizing hemoenzymes, especially those from the diheme enzyme superfamily that MauG belongs to. Electron paramagnetic resonance (EPR) characterization of the reaction between MauG and H2O2 revealed that mutation of Glu113 inhibited heterolytic OO bond cleavage, in agreement with our hypothesis. This result was further confirmed by the HPLC study in which an analog of H2O2, cumene hydroperoxide, was used to probe the pattern of OO bond cleavage. Together, our data suggest that Glu113 functions as an acid-base catalyst to assist heterolytic OO bond cleavage during the early stage of the catalytic reaction. This work advances our mechanistic understanding of the H2O2-activation process during bis-Fe(IV) formation in MauG.