Catalyzes the reversible retro-aldol cleavage of 4-hydroxy-2-ketoheptane-1,7-dioate (HKHD) to pyruvate and succinate semialdehyde. Is also able to catalyze the aldol cleavage of 4-hydroxy-2-ketopentanoate and 4-hydroxy-2-ketohexanoate. Is not stereospecific since it can cleave both substrate enantiomers. Also exhibits significant oxaloacetate decarboxylase activity in vitro. In the reverse direction, is able to condense a range of aldehyde acceptors (from two to five carbons in length) with pyruvate or 2-oxobutanoate. Unlike with BphI from Burkholderia xenovorans, the aldol addition reaction lacks stereospecificity, producing a racemic mixture. 4-hydroxy-2-oxoheptanedioate = pyruvate + succinate semialdehyde Binds 1 divalent metal cation per subunit. Has the highest activity with Co(2+) and Mn(2+), but can also use Zn(2+), Fe(2+) and Mg(2+). Competitively inhibited by oxalate. The catalytic efficiency measured with 3-deoxy-D-manno-oct-2-ulosonate (KDO) is 37800-fold lower than that with 4-hydroxy-2-ketohexanoate. Aromatic compound metabolism; 4-hydroxyphenylacetate degradation; pyruvate and succinate semialdehyde from 4-hydroxyphenylacetate: step 7/7. Homohexamer; trimer of dimers. The aldol addition reaction proceeds via a rapid equilibrium random order mechanism. Belongs to the HpcH/HpaI aldolase family.