Basics Enzymes as
Basics Enzymes as catalysts for bioplastic production and deconstruction E nzymes are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes act are called substrates or reactants and the enzyme converts the substrates into different molecules known as products. Almost all metabolic processes in the cell need enzyme catalysis to occur at rates fast enough to sustain life. Enzymes are known to catalyse more than 5,000 biochemical reactions. Enzymes are like any other catalyst and are not consumed in chemical reactions [1]. Enzymes are ubiquitous in nature and catalyse a plethora of useful reactions. Enzymes are involved in the breakdown of our food to generate the energy for our life. Some enzymes are used commercially in the synthesis of antibiotics. Some household products use enzymes to speed up chemical reactions. Enzymes in biological washing powder, for example, break down protein, starch, or fat stains on clothes. Enzymes in meat tenderizer break down proteins into smaller molecules making the meat easier to chew. Industrially, enzymes are used to make low-calorie beer, to clarify fruit juices, to remove proteins on contact lenses to prevent infections, to produce Camembert cheese and blue cheeses such as Roquefort – to name just a few [1]. Some enzymes can be used to produce or to degrade certain (bio) plastics. The following text by Alankar Vaidya and Marc Gaugler goes into detail about this topic. MT The bioplastic industry by obtaining its building blocks from bio-based feedstocks can eliminate fossil resources from its value-chain. The application of enzymes in the production, modification, and deconstruction of biobased feedstock will enable environmentally sustainable production and use of bioplastics. Enzyme biocatalysts Enzymes are biological catalysts with a high reaction specificity and selectivity. They can perform synthesis at ambient temperature and pressure unlike chemical catalysts. Enzymes are environmentally friendly, have low toxicity, and high catalytic precision developed over millions of years of evolutionary processes. Biopolymers with welldefined structures can be synthesized by enzyme-catalysed processes. In contrast, attempts to attain similar levels of polymer structure control by conventional chemical catalysis require undesirable protection-deprotection steps [2]. Enzymes can be applied in different forms to generate or degrade bioplastics – when inside the bacterial cells used in biopolymer (e.g. polyhydroxyalkanoates) synthesis, as a cell free catalyst in esterification, transesterification, and other chemical reactions to produce biopolymers, to generate biobased feedstocks which can be used in bioplastic production, and to degrade the bioplastic at the end-of-life. By: Alankar Vaidya and Marc Gaugler Scion (former Forest Research Institute), Rotorua, New Zealand As shown in Fig. 1a some enzyme reactions are reversible and the equilibrium can be regulated by altering the reaction conditions such as type of solvent used, enzyme water activity, reaction stoichiometry, etc. One can drive enzymatic reactions toward synthesis or hydrolysis of the products by engineering the reaction medium. For any biochemical reaction, a minimum activation energy barrier must be surmounted by the reactants to form the corresponding products. As shown in the cartoon (Fig. 1b), in the presence of enzymes, this activation energy barrier is remarkably reduced thereby more reactant molecules (dry frogs) can easily reach product stage (wet frogs) giving a high conversion rate [3]. Biopolymer Monomers Synthesis Hydrolysis Figure 1 (top) Enzyme is a reversible catalyst (bottom) Enzyme reduces activation energy of a reaction Medium Engineering 40 bioplastics MAGAZINE [01/21] Vol. 16
Basics Enzymes in bioplastic synthesis and hydrolysis Different classes of enzymes can be used in the synthesis and hydrolysis of various types of biopolymers (Table 1). This could have future applications in waste treatment and biodegradation of bioplastics. The most interesting bioplastics/biopolymers synthesized using enzymes are polyhydroxyalkanoates, polylactide, polycaprolactone, polyethylene furanoate, and other polyesters, polyamines, polysaccharides, proteins, etc. For more than 20 years, science, research, and development has looked at utilising the potential of enzyme-catalysed reactions (Table 2). Table 1: Different enzyme classes used in biopolymer synthesis Enzyme class Enzyme type Biopolymers Oxidoreductases Transferases Hydrolases Isomerases Lyases "peroxidase, oxygen reductasease," Carbohydrolase, cyclase Lipase, protease, esterase Racemase, epimerase Aldolase, decarboxylase "Polyphenols, polyanilines, vinyl polymers" "Polysaccharides, cyclic oligosaccharides" Polysaccharides, poly(amino acid) s‚ polyamides, polyesters, polycarbonates Ligases Ligase Polyesters For example, papain (enzyme class – hydrolases, typeprotease) catalysed regioselective synthesis of L-glutamic acid oligomers which are used in the therapeutics [4]. Later, Candida antarctica lipase B (enzyme class – hydrolases) immobilized on synthetic support [5] and selfimmobilised without support [6] were used in the synthesis and degradation of polycaprolactone, a biomedical bioresorbable polymer. In both these examples, a medium engineering approach was used to fine-tune reaction equilibrium towards synthesis or hydrolysis of the polymer product. A common misperception is that enzymes can only be used in aqueous reaction systems, limiting their range of applications. However, enzyme-catalysed reactive extrusion has been shown as a feasible solvent-free route for bioplastic polymerisation [7]. Owing to high specificity of enzymes they can be used in enzyme catalysed reactive extrusion for selective modification of polymers. One example is the selective esterification and amidation reactions in grafting of polycaprolactone on chitosan producing a biodegradable copolymer [8]. Table 2: Important reviews published in the past 20 years on enzyme catalysed biopolymer synthesis Sr No Reference Why to read for 1 2 3 4 5 6 Polymer Synthesis by In Vitra Enzyme Catalysis. Gross, R.A. Kumar, A. Kalra, B. Chem. Rev. 2001, 101:2097— 2124. Enzyme catalysed synthesis of polyesters. Varma, LK. Albertsson, A-C. Rajkhowa, R. Srivastava, R.K. Progress in Polymer Science 2005, 30:949-981. Lipase and peroxidase catalysed polymer synthesis, reaction kinetics and optimization Lipase-catalysed ringopening polymerization and copolymerization of oxiranes with dicarboxylic acid anhydrides Enzymatic Polymer Synthesis: An opportunityfor Enzymatic polymer synthesis green polymer chemistry. and modifications Kobayashi, S and Makino‚ A. 2009, 109:5288—5353. Polymer synthesis by enzymatic catalysis. Kadokawa, J-l. Kobayashi, S. Current Opinion in Chemical Biology 2010, 14:145- 153. Enzymatic synthesis of biobased polyesters and polyamides. Jiang, Y. and Loos, K. Polymers 2016, 8, 243. A review on enzymatic polymerization to produce polycondensation polymerszThe case of aliphatic polyesters, polyamidesand polyesteramides. Douka, A. Vouyiouka, S. Papaspyridi, L-M. Papaspyrides, C.D. Progress in Polymer Science 2018, 79:1-25. Enzyme catalysed biobased polyesters and polyamides Enzyme catalysed biobased polyesters and polyamides Lipase and cutinase catalysed synthesis of aliphaticpolyesters, polyamidesand polyesteramides and the reaction conditions Challenges and future perspectives in enzyme catalysed biopolymer synthesis Enzymatic polymerisation is a powerful and versatile tool for the production of biopolymers with different chemical compositions (aliphatic and semi-aromatic polymers) with varied architecture (linear, branched, and hyperbranched polymers) and diverse functionalities (pendant hydroxyl groups, carbon–carbon double bonds, epoxy groups, and so on). The enzyme catalysed processes offer novel routes that will enable future processing and materials required to transition towards a more sustainable bioeconomy. bioplastics MAGAZINE [01/21] Vol. 16 41
