 The α-subunits of the oxaloacetate and methylmalonyl-CoA decarboxylases are homologous to many biotin-containing enzymes including (1) pyruvate carboxylases, (2) homocitrate synthases, (3) biotin carboxyl carrier proteins, (4) isopropylmalate synthases and (5) acyl-CoA carboxylase.The α-subunit of the glutaconate decarboxylase is homologous to propionyl-CoA carboxylase.The crystal structure of the carboxyltransferase at 1.7 Å resolution shows a dimer of TIM barrels with an active site metal ion, identified spectroscopically as Zn2+.