Wax worm saliva and the enzymes therein are the key to polyethylene degradation by Galleria mellonella

Abstract

Plastic degradation by biological systems with re-utilization of the by-products could be a future solution to the global threat of plastic waste accumulation. Here, we report that the saliva of Galleria mellonella larvae (wax worms) is capable of oxidizing and depolymerizing polyethylene (PE), one of the most produced and sturdy polyolefin-derived plastics. This effect is achieved after a few hours’ exposure at room temperature under physiological conditions (neutral pH). The wax worm saliva can overcome the bottleneck step in PE biodegradation, namely the initial oxidation step. Within the saliva, we identify two enzymes, belonging to the phenol oxidase family, that can reproduce the same effect. To the best of our knowledge, these enzymes are the first animal enzymes with this capability, opening the way to potential solutions for plastic waste management through bio-recycling/up-cycling.

Enzymes’ Power for Plastics Degradation

“…Namely, the animal enzymes include arylphorin, renamed Demetra (NCBI accession number XP_026756396.1), and an hexamerin renamed Ceres (NCBI accession number XP_026756459.1), both belonging to the phenol oxidase family. 632 Using very high concentration of purified enzymes (∼10−15 g per g PE ), the M w value was only very slightly decreased (from 207 to 199 kg•mol −1 for film and from 4.0 to 3.9 kg•mol −1 for PE4000), eventually questioning the efficacy of the enzymatic degradation despite the authors' claims. These were in fact based on the appearance of carbonyl groups after treatment, but carbonyl index proved rather low (at most 0.1).…”

Microbial Enzyme Biotechnology to Reach Plastic Waste Circularity: Current Status, Problems and Perspectives

“…The latter can produce intermediate acid oligomers that are intracellularly taken-up by microorganisms and further metabolized [ 176 , 177 ]. Several microbial strains capable of degrading PE with this proposed system have been reported, including the gut microbiota of invertebrates [ 106 , 171 ]; animal phenol oxidase (EC 1.14.15.3) enzymes contained in the saliva of wax worm ( Galleria mellonella ) larvae were also demonstrated to be involved in the oxidative degradation of PE [ 107 ]. No system studied to date can achieve complete biodegradation without abiotic intervention.…”

Why have we not yet solved the challenge of plastic degradation by biological means?

“…So what means do insect larvae use to exert their effect on the most resilient synthetic polymers? Although we do not have the answer to this key question yet, a new discovery in the larva of G. mellonella (the wax worm) is offering an alternative to the metabolic paradigm [ 39 ]. Enzymes from the saliva of the larvae are able to break down PE within a few hours of exposure.…”

“…These enzymes have been identified as phenol-oxidases that can introduce oxygen into the polymer and cause depolymerization ( Fig 1D ). In this case, plastic is not used to produce energy, and no CO 2 is released [ 39 ]. Oxidation of PE by biological means at room temperature and in aqueous solution represents a new paradigm within plastic degradation, opening up new paths to be explored in the quest for a sustainable way to dispose of plastic residues, and pointing towards unanswered questions that urgently need to be addressed.…”

“…Insights in this direction came from recent data on the larvae of G. mellonella. These invertebrates had already shown a remarkably rapid ability to oxidize untreated PE (within hours of contact) [ 39 ], but the modality and origin of this action was unknown. Until now.…”