Oxidative degradation of polyethylene by two novel laccase-like multicopper oxidases from Rhodococcus opacus R7

The production of synthetic plastics, especially polyethylene, has reached a crucial level, making highly challenging plastic waste management. This paper aims to elucidate the role of two novel laccase-like multicopper oxidases in polyethylene oxidative degradation based on transcriptomic data from Rhodococcus opacus R7 grown on polyethylene. The purification of the recombinant proteins, LMCO2 and LMCO3 belonging respectively to the three- and two-domains laccase families, showed that they are endowed with different functional features. LMCO2 showed the highest enzyme activity at 65 °C with an optimal pH of 7.0, while LMCO3 exhibited the highest activity at 80 °C at acidic pH. LMCOs showed activity on both phenolic (i.e., 2,6-DMP) and non-phenolic (i.e., ABTS) compounds and they were thermostable. The oxidative activity of LMCO2 and LMCO3 on untreated low-density polyethylene was assessed by combining Fourier transform infrared spectroscopy and gas chromatography coupled with mass spectrometry analyses. Oxidation of PE was recorded within a short time range (24-48 h) and revealed previously not described patterns of alkyl compounds and oxygenated products including ketones, alcohols, and carboxylic acids. Structural analysis of LMCO2 and LMCO3 together with density functional theory calculations allowed to identify structural and electronic elements presumably involved in the oxidation of polyethylene.