Catalyzes the last step of the bacterial protocatechuate 4,5-cleavage pathway. The preferred substrates of the enzyme are 2-keto-4-hydroxy acids with a 4-carboxylate substitution. Catalyzes the conversion of 4-hydroxy-4-methyl-2-oxoglutarate (HMG) to pyruvate. Also catalyzes the conversion of 4-carboxy-4-hydroxy-2-oxoadipic acid (CHA) to pyruvate and oxaloacetate. 4-hydroxy-4-methyl-2-oxoglutarate = 2 pyruvate 2-hydroxy-4-oxobutane-1,2,4-tricarboxylate = oxaloacetate + pyruvate H(+) + oxaloacetate = CO2 + pyruvate Homohexamer. Belongs to the LigK/PcmE family.