This carbon center ionizes to form a carbanion, which adds to the carbonyl group of oxalyl-CoA. This addition is followed by the decarboxylation of oxalyl-CoA, and then the oxidation and removal of formyl-CoA to regenerate the carbanion form of TPP. While the reaction mechanism is shared with other TPP-dependent enzymes, the residues found in the active site of OXC are unique, which has raised questions about whether TDP must be deprotonated by a basic amino acid at a second site away from the carbanion-forming site to activate the cofactor. Oxalyl-CoA decarboxylase is tetrameric, and each monomer consists of three α/β-type domains. The thiamine diphosphate-binding site rests on the subunit-subunit interface between two of the domains, which is commonly seen in its class of enzymes. Oxalyl-CoA decarboxylase is structurally homologous to acetolactate synthase found in plants and other microorganisms, but OXC binds ADP in a region that is similar to the FAD-binding site in acetolactate synthase.