Catalyzes the aldol cleavage of 4-hydroxy-4-methyl-2-oxoglutarate (HMG) into 2 molecules of pyruvate. Also contains a secondary oxaloacetate (OAA) decarboxylase activity due to the common pyruvate enolate transition state formed following C-C bond cleavage in the retro-aldol and decarboxylation reactions. 4-hydroxy-4-methyl-2-oxoglutarate = 2 pyruvate H(+) + oxaloacetate = CO2 + pyruvate Divalent metal cation. Has preference for nickel ions for the HMG aldolase activity. Has preference for cobalt and zinc ions for the OAA decarboxylase activity. Competitively inhibited by oxalate, a pyruvate enolate analog. kcat is 0.0276 sec(-1) with 4-hydroxy-4-methyl-2-oxoglutarate as substrate and 3.91 sec(-1) with oxaloacetate as substrate. Homotrimer. Forms a ring-like structure with a central cavity. Present with 846 molecules/cell in log phase SD medium. Belongs to the class II aldolase/RraA-like family.