Plastic degradation by insect hexamerins: Near-atomic resolution structures of the polyethylene-degrading proteins from the wax worm saliva

Abstract

Plastic waste management is a pressing ecological, social, and economic challenge. The saliva of the lepidopteran Galleria mellonella larvae is capable of oxidizing and depolymerizing polyethylene in hours at room temperature. Here, we analyze by cryo–electron microscopy (cryo-EM) G. mellonella ’s saliva directly from the native source. The three-dimensional reconstructions reveal that the buccal secretion is mainly composed of four hexamerins belonging to the hemocyanin/phenoloxidase family, renamed Demetra, Cibeles, Ceres, and a previously unidentified factor termed Cora. Functional assays show that this factor, as its counterparts Demetra and Ceres, is also able to oxidize and degrade polyethylene. The cryo-EM data and the x-ray analysis from purified fractions show that they self-assemble primarily into three macromolecular complexes with striking structural differences that likely modulate their activity. Overall, these results establish the ground to further explore the hexamerins’ functionalities, their role in vivo, and their eventual biotechnological application.

Improved reference quality genome sequence of the plastic-degrading greater wax moth, Galleria mellonella

“…Seven lepidopteran enzymes exhibited a particularly high match with the HMM and conserved hemocyanin domains, producing an e -value of zero and a bit score cut-off of 1,100 ( Supplementary Table 2 ). Three of the 7 enzymes originated from G. mellonella , including 2 arylphorins and 1 acidic juvenile hormone-suppressible protein corresponding to Demetra, Cibeles, and Ceres, respectively ( Spínola-Amilibia et al . 2023 ).…”

“…Recently, several enzymes from the saliva of the larvae of the greater wax moth have been shown to possess PE-degrading capabilities ( Sanluis-Verdes et al . 2022 ; Spínola-Amilibia et al . 2023 ).…”

Toward a Circular Bioeconomy: Designing Microbes and Polymers for Biodegradation

“…155 Microbes from the guts of worms have also displayed a propensity to degrade synthetic plastics. 14,[51][52][53][54]57,58 Certain organisms and enzymes have displayed the ability to utilize both lignocellulose and synthetic plastics, but there are still many obstacles that must be overcome before these lignocellulose-degrading systems can efficiently be translated to plastic biodegradation. The general lack of hydrolyzable bonds in petroleum-based plastics (PE, PS, PP, etc.)…”

“…Instead, enzymes from the saliva of G. mellonella were identified as the key agents, including two enzymes named Demetra and Cibeles that self-associate into a heterocomplex (which could oxidize and break down PE) and two hexamerins, Ceres (only able to oxidize PE) and Cora (able to degrade PE). , …”

“…Instead, enzymes from the saliva of G. mellonella were identified as the key agents, including two enzymes named Demetra and Cibeles that selfassociate into a heterocomplex (which could oxidize and break down PE) and two hexamerins, Ceres (only able to oxidize PE) and Cora (able to degrade PE). 57,58 Fungi have also shown the ability to degrade polyethylene microplastics, 59 polypropylene, 60 and other recalcitrant materials. 61−63 Aspergillus terreus achieved 25.29% degradation when grown on a UV-pretreated PP metallized film, while Engyodontium album achieved 27.08% degradation on a metallized PP film pretreated with heat over a period of 90 days.…”