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There is considerable interest in how "second-shell" interactions between protein side chains and metal ligands might modulate Mn(II) ion redox properties and reactivity in metalloenzymes. One such Mn-dependent enzyme is oxalate decarboxylase (OxDC), which catalyzes the disproportionation of oxalate monoanion into formate and CO 2 . Electron paramagnetic resonance (EPR) studies have shown that a mononuclear Mn(III) ion is formed in OxDC during catalytic turnover and that the removal of a hydrogen bond between one of the metal ligands (Glu101) and a conserved, second-shell tryptophan residue (Trp132) gives rise to altered zero-field splitting parameters for the catalytically important Mn(II) ion. We now report heavy-atom kinetic isotope effect measurements on the W132F OxDC variant, which test the hypothesis that the Glu101/Trp132 hydrogen bond modulates the stability of the Mn(III) ion during catalytic turnover. Our results suggest that removing the Glu101/Trp132 hydrogen bond increases the energy of the oxalate radical intermediate from which decarboxylation takes place. This finding is consistent with a model in which the Glu101/Trp132 hydrogen bond in WT OxDC modulates the redox properties of the Mn(II) ion.

Second-Shell Hydrogen Bond Impacts Transition-State Structure in Bacillus subtilis Oxalate Decarboxylase