Heavy Atom Isotope Effects on the Reaction Catalyzed by the Oxalate Decarboxylase from Bacillus subtilis

Oxalate decarboxylase (OxDC) catalyzes a remarkable transformation in which the C−C bond in oxalate is cleaved to give carbon dioxide and formate. Like the native OxDC isolated from Aspergillus niger, the recombinant, bacterial OxDC from Bacillus subtilis contains Mn(II) in its resting state and requires catalytic dioxygen for activity. The most likely mechanism for OxDC-catalyzed C−C bond cleavage involves the participation of free radical intermediates, although this hypothesis remains to be unequivocally demonstrated. Efforts to delineate the catalytic mechanism have been placed on a firm foundation by the high-resolution crystal structure of recombinant, wild type B. subtilis OxDC (Anand et al., Biochemistry 2002, 41, 7659−7669). We now report the results of heavy-atom kinetic isotope effect measurements for the OxDC-catalyzed decarboxylation of oxalate, in what appear to be the first detailed studies of the mechanism employed by OxDC. At pH 4.2, the OxDC-catalyzed formation of formate and CO2 have normal 13C isotope effects of 1.5% ± 0.1% and 0.5% ± 0.1%, respectively, while the 18O isotope effect on the formation of formate is 1.1% ± 0.2% normal. Similarly at pH 5.7, the production of formate and CO2 exhibits normal 13C isotope effects of 1.9% ± 0.1% and 0.8% ± 0.1%, respectively, and the 18O isotope effect on the formation of formate is 1.0% ± 0.2% normal. The 18O isotope effect on the formation of CO2, however, 0.7% ± 0.2%, is inverse at pH 5.7. These results are consistent with a multistep model in which a reversible, proton-coupled, electron transfer from bound oxalate to the Mn-enzyme gives an oxalate radical, which decarboxylates to yield a formate radical anion. Subsequent reduction and protonation of this intermediate then gives formate.