ABSTRACT
Enzymatic polymerizations are a powerful and versatile approach that can compete with chemical and physical techniques to produce known materials such as “commodity plastics” and also to synthesize novel macromolecules so far not accessible via traditional chemical approaches. Enzymatic polymerizations can prevent waste generation by using catalytic processes with high stereo-and regioselectivity; prevent or limit the use of hazardous organic reagents by, for instance, using water as a green solvent; design processes with higher energy efficiency and safer chemistry by conducting reactions at room temperature under ambient atmosphere; and increase atom efficiency by avoiding extensive protection and deprotection steps in synthetic processes. For this reason, enzymatic polymerizations can provide an essential contribution to achieving industrial sustainability in the future. Biotechnology therefore holds tremendous opportunities for realizing unique new functional polymeric materials.Several investigations have been performed into the use of im-mobilized enzymes as catalysts for polymer synthesis (Ienczak and de Aragão Falcão, 2011). Among these efforts, those that use immo-bilized enzymes as catalysts for polymerization reactions are prominent. Enzyme-catalyzed polymerization not only is environmentally
desirable (as it replaces the use of toxic catalysts) but also offers a number of technical and economical benefits. Among these benefits are the following: (a) synthesis of new types of monomers, oligomers, and polymers that are not possible to obtain using chemi-cal routes; (b) elimination of synthesis steps that require protection and deprotection of functional groups; (c) production of chemical species through high stereo-, and regio-selectivity; (d) performing synthesis at milder temperatures (60-110°C); and (e) synthesis of biodegradable or recyclable polymers (Gross et al., 2001). 30.2.1 Lipase for Polymer Synthesis
Lipases (triacylglycerol acylhydrolases, E.C. 3.1.1.3) are ubiquitous enzymes of considerable physiological significance and industrial potentiality. Lipases catalyze the hydrolysis of triacylglycerols to glycerol and free fatty acids (Sharma et al., 2001). In contrast to esterases, lipases are activated only when adsorbed within an oil-water interface (Martinelle et al., 1995), and do not hydrolyze dissolved substrates in the bulk fluid. A true lipase will split emulsified esters of glycerin and long-chain fatty acids such as triolein and tripalmitin. Lipases are classified as serine hydrolases (Sharma et al., 2001). Lipases are considered hydrolases, a family commonly used at industrial scale to achieve various biotransformations (Pandey et al., 1999).Lipases can be found in diverse sources, such as plants, animals, and micro-organisms. More abundantly, they are found in bacteria, fungi, and yeasts. In most lipases, a part of the enzyme molecule covers the active site with a short amphiphilic α-helix, called the lid. The side of the α-helical lid facing the catalytic site, as well as the protein chains surrounding the catalytic site are mostly composed of hydrophobic side chains. The lid in its closed conformation (i.e., in the absence of an interphase or organic solvent) prevents access of the substrate to the catalytic triad. Opening of the lid twists and exposes a large hydrophobic surface, and the previously exposed hydrophilic domain becomes buried inside the protein (Schmid and Dordick, 2001).All lipases show structural and functional similarities, regardless of the organism from which they were isolated; all of them have a α-β-hydrolase fold structure containing a catalytic triad. However, small variations in the substrate binding site may
have a strong effect on the catalytic properties and the enzyme stability (Schmid and Dordick, 2001). Most lipases contain a Ser-His-Aspartate/Glutamate catalytic triad (Ser105-His224-Asp187) in the active site, and share (at least in part) the common structural framework of the α-β-hydrolase fold. This fold is mainly composed of parallel β-sheet, flanked on both sides by α-helices (Uppenberg et al., 1995). Also, water is critical for enzymes behavior as it influences enzyme structure via noncovalent bonding. Too-low water content generally reduces enzymic activity, whereas too-high content can also reduce the reaction rates by causing the aggregation of enzyme particles and hence diffusional limitations. The optimum water content is often within a narrow range.Lipases that have been reported for polyester synthesis are from mammalian (porcine pancreatic lipase, PPL), fungal (Candida antartica, CA; Candida rugosa, CR; Candida cylindracea, CC; Aspergillus niger, AN; Penicillium requeforti, PR; Rhizopus delemar, RD; Rhizomucor miehei, RM; Yarrowia lipolytica, YLL) or bacterial origin (Pseudomonas cepacia, PC; Pseudomonas fluorescens, PF; Pseudomonas species lipase, PS). 30.3 Polyester SynthesisPolyesters are in fourth place in abundance in living systems, following the three major biomacromolecules, namely nucleic acids (DNA and RNA), proteins (polypeptides), and polysaccharides. On the other hand, polyesters such as poly(ethylene terephthalate) (PET) [an aromatic polyester], poly(butylene succinate), poly(ε-caprolactone) (poly(ε-CL)), and poly(lactic acid) (PLA) [aliphatic polyesters] are very important materials widely used in diverse commercial applications. Industrially speaking, the former two are produced via a polycondensation procedure, whereas the latter two are produced via ring-opening polymerization.Aliphatic polyesters are one of the most commercially important biodegradable polymers. They found widespread use in biomedical applications and have been extensively investigated. Ester linkages are frequently encountered in nature and hence it is expected that synthetic polymers containing such linkages and an appropriate structure would be environmentally degradable.
