Recently, enzymatic depolymerisation of synthetic polyesters has emerged as an attractive complement to current plastic recycling methods. However, the arsenal of robust enzymes available for these applications remains limited. Here, we identified and biochemically characterised three novel carboxylesterases (TA21, TA26 and TA29) with polyesterase activity from the genome of the thermophilic, obligately alkane-degrading bacterium Thermoleophilum album YS-3 (ATCC 35264). The purified proteins hydrolysed model para-nitrophenyl monoesters, favouring short-chain substrates (C2-C6), with TA21 showing the highest carboxylesterase activity. All three enzymes displayed maximal activity at 55-60 °C, with 20-25% activity remaining at 75 °C. Notably, they also retained substantial activity at moderate temperatures (15 °C), which is uncommon for thermophilic enzymes. In agarose-plate screens with emulsified polyesters, the enzymes hydrolysed amorphous poly(ethylene terephthalate) (aPET), bis(benzoyloxyethyl) terephthalate (3PET), polylactic acid (PLA) and polycaprolactone (PCL). HPLC analysis identified terephthalic acid (TA) and mono(2-hydroxyethyl) terephthalate (MHET) as the major degradation products of 3PET and aPET, with TA21 efficiently converting MHET to TA. TA21 was active on PLA and PCL, hydrolysing them to lactic acid and 6-hydroxyhexanoic acid, respectively, and preferred 3PET and PLA over aPET and PCL (0.25-1.5 mm products formed after overnight incubation). Structural analysis revealed the presence of a medium-sized lid domain in all three enzymes. In TA21, this domain contributes two aromatic residues and an arginine for coordinating MHET in the active site, which may account for its higher MHET-degrading activity. These findings introduce novel thermophilic polyesterases that may serve as promising candidates for further optimisation and protein engineering research on enzymatic depolymerisation of synthetic polyesters.
Chernikova et al. (Mon,) studied this question.