Enzyme catalyzes ester bond synthesis and hydrolysis: The key step for sustainable usage of plastics

Abstract

Petro-plastic wastes cause serious environmental contamination that require effective solutions. Developing alternatives to petro-plastics and exploring feasible degrading methods are two solving routes. Bio-plastics like polyhydroxyalkanoates (PHAs), polylactic acid (PLA), polycaprolactone (PCL), poly (butylene succinate) (PBS), poly (ethylene furanoate) s (PEFs) and poly (ethylene succinate) (PES) have emerged as promising alternatives. Meanwhile, biodegradation plays important roles in recycling plastics (e.g., bio-plastics PHAs, PLA, PCL, PBS, PEFs and PES) and petro-plastics poly (ethylene terephthalate) (PET) and plasticizers in plastics (e.g., phthalate esters, PAEs). All these bio- and petro-materials show structure similarity by connecting monomers through ester bond. Thus, this review focused on bio-plastics and summarized the sequences and structures of the microbial enzymes catalyzing ester-bond synthesis. Most of these synthetic enzymes belonged to α/β-hydrolases with conserved serine catalytic active site and catalyzed the polymerization of monomers by forming ester bond. For enzymatic plastic degradation, enzymes about PHAs, PBS, PCL, PEFs, PES and PET were discussed, and most of the enzymes also belonged to the α/β hydrolases with a catalytic active residue serine, and nucleophilically attacked the ester bond of substrate to generate the cleavage of plastic backbone. Enzymes hydrolysis of the representative plasticizer PAEs were divided into three types (I, II, and III). Type I enzymes hydrolyzed only one ester-bond of PAEs, type II enzymes catalyzed the ester-bond of mono-ester phthalates, and type III enzymes hydrolyzed di-ester bonds of PAEs. Divergences of catalytic mechanisms among these enzymes were still unclear. This review provided references for producing bio-plastics, and degrading or recycling of bio- and petro-plastics from an enzymatic point of view.

Keywords: bio-plastics; enzyme; ester bond; plasticizers; plastics; recycling.

Figure 1

Sequence alignment and phylogenetic tree of PHA synthases. (A) Sequence alignment of PHA synthases. “*” Conservative amino acid. (B) Phylogenetic tree of PHA synthase.

Figure 2

Homologous modeling of PHA synthases. (A) Class I PHA synthase (Cupriavidus necator H16), (B) Class II PHA synthase (Pseudomonas sp. 61-3) (C) Class III PHA synthase (Allochromatium vinosum DSM 180) (D) Class IV PHA synthase (Bacillus sp. INT005) (E) structure alignment of these PHA synthase.

Figure 3

Residues involved in substrate binding pocket and substrate entrance channel of PHA synthases.

Figure 4

Catalytic mechanism of PHA synthase.

Figure 5

Mechanism of enzyme-catalyzed polymerization of lactones into polymers. , Enzymes; EAM, enzyme-activated monomer.

Figure 6

Sequence alignment of enzymes catalyzing the hydrolysis of PCL, PES, PBS, PBSA, and PBAT. (A) The phylogenetic tree of PCL, PES, PBS, PBSA, and PBAT depolymerase. The fan with light purple represents group1; the fan with light red represents group2; the fan with light green represents group3; the fan with light bule represents group4. (B) Sequence alignment of Group 1 enzymes, (C) Sequence alignment of Group 2 enzymes. (D) Sequence alignment of Group 3 enzymes. “*” The catalytic active site.

Figure 7

Chemical structure of PET and monomers, and PETase structure. (A) Chemical structure of PET and monomers. (B) PETase structures (PDB ID 5XJH).

Figure 8

Sequence analysis of PAEs hydrolases. (A) The phylogenetic tree of PAEs hydrolases. (B) Sequence alignment of PAEs hydrolases, the blue line marks the conserved sequence of the enzyme.

Figure 9

Enzyme-catalyzed ester bond hydrolysis of PAEs. (A) When the lid region opened, PAEs entered into the substrate binding pocket to form a tetrahedral transition state. (B) The Ser hydroxyl oxygen atom nucleophilic attacked the carboxyl carbon atom of the substrate to form a covalent bond, and the covalent bond was broken between the carboxyl carbon atom and the oxygen atom of the substrate. (C) One water molecule entered into the substrate binding pocket to form a tetrahedron transition state. (D) The carboxyl carbon atom of the substrate formed a covalent bond with the oxygen atom of water, and the covalent bond was broken between Ser hydroxyl oxygen atom with the carbon atom of the substrate. Finally, the product escaped from the catalytic pocket to complete the ester bond hydrolysis reaction.