Biotechnological Plastic Degradation and Valorization Using Systems Metabolic Engineering

Abstract

Various kinds of plastics have been developed over the past century, vastly improving the quality of life. However, the indiscriminate production and irresponsible management of plastics have led to the accumulation of plastic waste, emerging as a pressing environmental concern. To establish a clean and sustainable plastic economy, plastic recycling becomes imperative to mitigate resource depletion and replace non-eco-friendly processes, such as incineration. Although chemical and mechanical recycling technologies exist, the prevalence of composite plastics in product manufacturing complicates recycling efforts. In recent years, the biodegradation of plastics using enzymes and microorganisms has been reported, opening a new possibility for biotechnological plastic degradation and bio-upcycling. This review provides an overview of microbial strains capable of degrading various plastics, highlighting key enzymes and their role. In addition, recent advances in plastic waste valorization technology based on systems metabolic engineering are explored in detail. Finally, future perspectives on systems metabolic engineering strategies to develop a circular plastic bioeconomy are discussed.

Keywords: bio-upcycling; biodegradation; circular plastic bioeconomy; plastic waste; systems metabolic engineering.

Figure 1

Schematic diagram of microbial and enzymatic biodegradation and bio-upcycling of plastic waste.

Figure 2

Enzymatic degradation of various types of plastics. Plastic-degrading microorganisms colonize the plastic surface and release enzymes for biodeterioration and biofragmentation.

Figure 3

Metabolic pathways for bio-upcycling of hexadecanoic and lactic acids. Each metabolic pathway for bio-upcycling is indicated by the color of the respective substrate box. Single reactions are denoted with a solid line, while multiple reactions are represented with a dashed line. 1: acyl-CoA synthetase; 2: acyl-CoA dehydrogenase; 3: enoyl-CoA hydratase; 4: hydroxyacyl-CoA dehydrogenase; 5: ketoacyl-CoA thiolase; 6: alcohol-forming fatty acyl-CoA reductase; 7: aldehyde-forming fatty acyl-CoA reductase; 8: fatty aldehyde reductase; 9: wax ester synthase/acyl-CoA:diacylglycerol acyltransferase; 10: acetyl-CoA carboxylase; 11. malonyl-CoA transacylase; 12: β-ketoacyl ACP synthase; 13: β-ketoacyl ACP reductase; 14: β-hydroxyacyl-ACP dehydrase; 15: enoyl-ACP reductase; 16: 3-(3-hydroxyalkanoyloxy)alkanoate (HAA) synthase; 17: rhamnosyltransferase; 18: lactate dehydrogenase; 19: propionyl-CoA transferases; 20: PHA synthase; 21: β-ketothiolase; 22: acetoacetyl-CoA reductase; 23: acetate-CoA transferase.

Figure 4

Metabolic pathways for bio-upcycling of ethylene glycol (EG), styrene, 1,4-butanediol (1,4-BDO), and adipic acid. Each metabolic pathway for bio-upcycling is indicated by the color of the respective substrate box. Single reactions are denoted with a solid line, while multiple reactions are represented with a dashed line. 1: glycoaldehyde reductase; 2: glycolaldehyde dehydrogenase; 3: glycolate oxidase; 4: malate synthase; 5: styrene monooxygenase; 6: epoxide hydrolase; 7: alcohol dehydrogenase; 8: phenylacetaldehyde dehydrogenase; 9: styrene oxide isomerase; 10: phenylacetaldehyde dehydrogenase; 11: 4HB-CoA:CoA transferase; 12: PHA synthase; 13: acetyl-CoA acetyltransferase; 14: adipyl-CoA transferase; 15: carboxylic acid reductase; 16: transaminase.

Figure 5

Metabolic pathways for bio-upcycling of TPA. Single reactions are denoted with a solid line, while multiple reactions are represented with a dashed line. 1: TPA dioxygenase; 2: DCD dehydrogenase; 3: PCA 3,4-dioxygenase; 4: 3-carboxy-cis,cis-muconate cycloisomerase; 5: 4-carboxymuconolactone decarboxylase; 6: 3-oxoadipate enol-lactonase; 7: p-hydroxybenzoate hydroxylase; 8: PCA decarboxylase; 9: catechol hydrolase; 10: O-methyltransferase; 11: TPA decarboxylase and benzoic acid CoA ligase; 12: cyclohexa-1,5-dienecarbonyl-CoA hydratase, 6-hydroxycyclohex-1-ene-1-carboxyl-CoA dehydrogenase, 6-oxo-cyclohex-1-ene-carbonyl-CoA hydrolase; 13: crotonase; 14: β-hydroxybutyryl-CoA dehydrogenase, acetyl-CoA acetyltransferase; ND: not determined.