Role of Dienelactone Hydrolases in PET Biodegradation by Flavobacteria Maribacter dokdonensis and Arenibacter palladensis

Dienelactone hydrolases (DLHs, EC 3.1.1.45) are enzymes that play a crucial role in the breakdown of cyclic esters and some have been found to act on substrates such as terephthalate esters, which are monomers of polyethylene terephthalate (PET). In the current study, we show that bacteria affiliated with the Bacteroidota (class Flavobacteria) harbor DLHs acting on PET foil and powder. We report on the isolation of two marine bacterial strains, Arenibacter palladensis UHH-Hm9b and Maribacter dokdonensis UHH-5R5, forming biofilms on PET foil and releasing µM amounts of terephthalic acid after 5-7 days. Genome sequencing and functional analyses identified two secreted DHLs designated PET93 and PET 94 involved in PET degradation. While their predicted active sites and substrates binding pockets were identical to previously published PETases, both enzymes differed largely in their structural features from known PETases and represent novel scaffolds. Further they lacked the typical porC-domain of the known PETases from the Flavobacteria. Biochemical characterization of the two recombinant enzymes confirmed activity on PET, the primary degradation products Bis(2-Hydroxyethyl) terephthalate (BHET) and Mono-(Hydroxyethyl) terephthalate (MHET). These are the first DLHs to be reported being active on plastics and our findings indicate that Flavobacteria harbor an unexpectedly wide range of PET-active promiscuous enzymes.