Biotin-dependent multifunctional enzymes carry out metabolically important carboxyl group transfer reactions and are potential targets for the treatment of obesity and type 2 diabetes. These enzymes use a tethered biotin cofactor to carry an activated carboxyl group between distantly spaced active sites. The mechanism of this transfer has remained poorly understood. Here we report the complete structure of pyruvate carboxylase at 2.0 angstroms resolution, which shows its domain arrangement. The structure, when combined with mutagenic analysis, shows that intermediate transfer occurs between active sites on separate polypeptide chains. In addition, domain rearrangements associated with activator binding decrease the distance between active-site pairs, providing a mechanism for allosteric activation. This description provides insight into the function of biotin-dependent enzymes and presents a new paradigm for multifunctional enzyme catalysis.

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 CO(2) have normal (13)C isotope effects of 1.5% +/- 0.1% and 0.5% +/- 0.1%, respectively, while the (18)O isotope effect on the formation of formate is 1.1% +/- 0.2% normal. Similarly at pH 5.7, the production of formate and CO(2) exhibits normal (13)C isotope effects of 1.9% +/- 0.1% and 0.8% +/- 0.1%, respectively, and the (18)O isotope effect on the formation of formate is 1.0% +/- 0.2% normal. The (18)O isotope effect on the formation of CO(2), 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.