Involved in cholesterol degradation. Catalyzes the retro-aldol cleavage of 4-hydroxy-2-oxohexanoate (HOHA) to pyruvate and propanal. Can also catalyze the cleavage of 4-hydroxy-2-oxopentanoate (HOPA) to pyruvate and acetaldehyde. The aldehydes produced by this reaction are directly channeled to the dehydrogenase HsaG. (S)-4-hydroxy-2-oxohexanoate = propanal + pyruvate (S)-4-hydroxy-2-oxopentanoate = acetaldehyde + pyruvate Requires a divalent metal cation. Activity is highest in the presence of Mn(2+), followed by Co(2+) and Ni(2+). Mg(2+) and Ca(2+) are poor metal cofactors. No activity with Cd(2+), Zn(2+) or Cu(2+). Shows aldolase activity only when expressed and copurified with HsaG. This arrangement probably prevents the deleterious formation and release of toxic aldehydes in the absence of the partner dehydrogenase. kcat is 0.38 sec(-1) with 4-hydroxy-2-oxohexanoate as substrate. kcat is 0.41 sec(-1) with 4-hydroxy-2-oxopentanoate as substrate. Homodimer. Forms an heterotetramer composed of two aldolase (HsaF) and two dehydrogenase (HsaG) subunits. Belongs to the 4-hydroxy-2-oxovalerate aldolase family.