Catalyzes the conversion of 4-carboxy-4-hydroxy-2-oxoadipic acid (CHA) to pyruvate and oxaloacetate, the last step of gallate degradation pathway. 4-hydroxy-4-methyl-2-oxoglutarate = 2 pyruvate 2-hydroxy-4-oxobutane-1,2,4-tricarboxylate = oxaloacetate + pyruvate Homohexamer. Cells are unable to grow in gallate. Belongs to the LigK/PcmE family.