Ethylene glycol metabolism in the poly(ethylene terephthalate)-degrading bacterium Ideonella sakaiensis

Abstract

Poly(ethylene terephthalate) (PET)-degrading bacterium Ideonella sakaiensis produces hydrolytic enzymes that convert PET, via mono(2-hydroxyethyl) terephthalate (MHET), into the monomeric compounds, terephthalic acid (TPA), and ethylene glycol (EG). Understanding PET metabolism is critical if this bacterium is to be engineered for bioremediation and biorecycling. TPA uptake and catabolism in I. sakaiensis have previously been studied, but EG metabolism remains largely unexplored despite its importance. First, we identified two alcohol dehydrogenases (IsPedE and IsPedH) and one aldehyde dehydrogenase (IsPedI) in I. sakaiensis as the homologs of EG metabolic enzymes in Pseudomonas putida KT2440. IsPedE and IsPedH exhibited EG dehydrogenase activities with Ca²⁺ and a rare earth element (REE) Pr³⁺, respectively. We further found an upregulated dehydrogenase gene when the bacterium was grown on EG, whose gene product (IsXoxF) displays a minor EG dehydrogenase activity with Pr³⁺. IsPedE displayed a similar level of activity toward various alcohols. In contrast, IsPedH was more active toward small alcohols, whereas IsXoxF was the opposite. Structural analysis with homology models revealed that IsXoxF had a larger catalytic pocket than IsPedE and IsPedH, which could accommodate relatively bulkier substrates. Pr³⁺ regulated the protein expression of IsPedE negatively; IsPedH and IsXoxF were positively regulated. Taken together, these results indicated that the combination of IsPedH and IsXoxF complements the function of IsPedE in the presence of REEs. IsPedI exhibited dehydrogenase activity toward various aldehydes with the highest activity toward glycolaldehyde. This study demonstrated a unique alcohol oxidation pathway of I. sakaiensis, which could be efficient in EG utilization. KEY POINTS: • IsPedH and IsXoxF complement IsPedE function in the presence of REEs. • IsPedI displayed the highest dehydrogenase activity toward glycolaldehyde. • Unique alcohol oxidation pathway of I. sakaiensis identified for EG utilization.

Keywords: Alcohol dehydrogenase; Aldehyde dehydrogenase; Ethylene glycol metabolism; Ideonella sakaiensis; Poly(ethylene terephthalate).