Contributes to the degradation of lignin, being involved in the final step of the protocatechuate 4,5-cleavage pathway. Catalyzes the conversion of 4-carboxy-4-hydroxy-2-oxoadipate (CHA, also named 2-hydroxy-4-oxobutane-1,2,4-tricarboxylate) to pyruvate and oxaloacetate but also the decarboxylation of oxaloacetate to pyruvate and CO2. Is essential for catabolism and utilization of vanillate and syringate (PubMed:12486039). Is also able to catalyze the formation of 4-hydroxy-4-methyl-2-oxoglutarate (HMG) from two molecules of pyruvate (PubMed:29658701). 4-hydroxy-4-methyl-2-oxoglutarate = 2 pyruvate 2-hydroxy-4-oxobutane-1,2,4-tricarboxylate = oxaloacetate + pyruvate H(+) + oxaloacetate = CO2 + pyruvate Both CHA aldolase and oxaloacetate decarboxylase activity are activated in the presence of phosphate ion, and inhibited by N-ethylmaleimide. CHA aldolase activity is also inhibited by oxaloacetate with a Ki value of 23 uM; thus, the amount of oxaloacetate in the cells might control the production of oxaloacetate from CHA. Optimum pH is 8.0 for CHA aldolase activity, and 7.0 for oxaloacetate decarboxylase activity. Secondary metabolite metabolism; lignin degradation. Homohexamer. Cells lacking this gene lose the ability to grow on both vanillate and syringate. Belongs to the LigK/PcmE family.