Recombinant Burkholderia xenovorans 4-hydroxy-2-oxovalerate aldolase 4 (bphI)

Catalyzes the retro-aldol cleavage of both 4-hydroxy-2-oxopentanoate (HOPA) and 4-hydroxy-2-oxohexanoate (HOHA) to pyruvate and acetaldehyde or propanaldehyde, respectively. The aldehydes produced by this reaction are directly channeled from BphI to the dehydrogenase BphJ, ensuring that these toxic aldehydes are sequestered from cellular components. Is involved in the meta-cleavage pathway for the degradation of polychlorinated biphenyls (PCBs). Appears to be stereospecific since it can cleave (4S)-4-hydroxy-2-oxopentanoate but not the (4R) isomer. Also exhibits a secondary oxaloacetate decarboxylase activity. Finally, is also able to catalyze the reverse reaction, albeit much less efficiently, i.e. the condensation of aldehyde acceptors of two to three carbons in length with pyruvate. This aldol addition reaction is stereospecific; the condensation of acetaldehyde and pyruvate with BphI produces only the (4S)-4-hydroxy-2-oxopentanoate isomer. Aldehyde channeling in the BphI-BphJ complex can occur in reverse, from the dehydrogenase to the aldolase active sites, and the BphJ reductive deacylation reaction increases 4-fold when BphI is catalyzing the aldol addition reaction. Therefore, the BphI-BphJ enzyme complex exhibits unique bidirectionality in substrate channeling and allosteric activation.