Catalyzes the reversible retro-aldol cleavage of 4-hydroxy-2-ketoheptane-1,7-dioate (HKHD) to pyruvate and succinic semialdehyde. 4-hydroxy-2-oxoheptanedioate = pyruvate + succinate semialdehyde Binds 1 divalent metal cation per subunit. Aromatic compound metabolism; 4-hydroxyphenylacetate degradation; pyruvate and succinate semialdehyde from 4-hydroxyphenylacetate: step 7/7. Homohexamer; trimer of dimers. Belongs to the HpcH/HpaI aldolase family.