Plastic waste management and recycling became a serious global issue as it affects living beings from all the ecosystems. Researchers investigated biodegradation of polyethylene (PE) by measuring changes in various physico-chemical and structural characteristics using techniques like as fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), etc. However, these evidences are not enough to prove the exact biodegradation of PE. In this review, we summarized microbial biodegradation of polyethylene and discussed recent developments for the candidate microbial enzymes and their possible roles in PE degradation. In addition, we conversed the advanced technologies correctly used for measuring PE degradation using isotope-labeled PE to figure out its metabolism into the end products like as 13CO2.
In the present study, the gene encoding a multicopper oxidase, more precisely a laccase from the thermoalkaliphilic aerobic bacterium Caldalkalibacillus thermarum strain TA2.A1 (CtLac), was cloned and expressed in Escherichia coli. CtLac is a monomeric protein with a molecular weight of 57 kDa as determined by native polyacrylamide gel electrophoresis. The optimum pH and temperature for 2,6-dimethoxyphenol (2,6-DMP) oxidation were 8.0 and 70 °C, respectively. The kinetic constants K and k for 2,6-DMP were of 200 μM and 23 s, respectively. The enzyme was highly thermostable at 80 °C and retained more than 80% of its activity after 24 h preincubation under thermoalkaliphilic conditions. Remarkably, it showed a half-life of about 12 h at 90 °C. The enzyme activity was significantly enhanced by Cu and Mn and was not affected in the presence of most of the other metal ions. CtLac activity was stimulated in the presence of halides, organic solvents, and surfactants. Furthermore, the activity of CtLac on a dimeric lignin model compound, guaiacylglycerol-β-guaiacyl ether (GGGE) was investigated. Liquid chromatography-mass spectrometry analysis indicated that CtLac catalyzes dimerization of GGGE to form a C5-C5 biphenyl tetramer. The stability and activity of CtLac characterized herein under thermoalkaliphilic conditions make it a highly suitable biocatalyst for various biotechnological and industrial applications.
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