Polymer Biocatalysis and Biomaterials - ACS Publications - American

Chapter 1

Polymer Biocatalysis and Biomaterials 1

H . N. Cheng and Richard A. Gross

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Hercules Incorporated Research Center, 500 Hercules Road, Wilmington, DE 19808-1599 NSF I/UCRC for Biocatalysis and Bioprocessing of Macromolecules, Polytechnic University, 6 Metrotech Center, Brooklyn, N Y 11201

Downloaded by CLARKSON UNIV on October 10, 2013 | http://pubs.acs.org Publication Date: February 15, 2005 | doi: 10.1021/bk-2005-0900.ch001

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This overview briefly surveys the use of enzymatic and whole -cell approaches in polymers. Three types of reactions are covered: polymer syntheses, polymer modifications, and polymer hydrolyses. Thus far, most of the enzyme-related R&D activities involve hydrolases, oxidoreductases, and transferases, with occasional use of lyases and isomerases. Whole-cell methods continue to be valuable in both academic and industrial labs. All these research areas display continued vitality and creativity, as evidenced by the large number of publications. Advances in biotechnology have provided new and improved enzymes and additional tools. Also included in this overview is the related topic of biomaterials.

The use of enzymes and whole-cell approaches in polymers is now fairly widespread. A large number of reactions and processes has been developed, and new developments continue to appear in both the open and the patent literatures. Several excellent books (1,2) and reviews (3) are available. Biomaterials comprise an equally exciting field of research that finds many applications in dental, surgical, and medical areas (4). Both fields are highly interdisciplinary, requiring (at various times) knowledge and expertise in

© 2005 American Chemical Society In Polymer Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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organic and polymer chemistry, material science, biochemistry, molecular biology, and chemical engineering. This overview covers some of the recent developments in these fields, with a particular emphasis on the articles included in this book (5-33).


Enzyme Biocatalysis Enzymes are commonly classified, via a system of Enzyme Commission (EC) numbers, into six divisions: oxidoreductases, transferases, hydrolases, lyases, isomerases, and ligases (34). In this work, we are concerned with three types of polymer reactions: polymer syntheses, polymer modifications, and polymer degradation and hydrolyses. For these reactions, only hydrolases, oxidoreductases, and transferases are being used extensively in polymers and biomaterials. A summary is given in Table 1. Table 1. The Use of Biocatalysts in Polymer Reactions Biocatalyst Type

EC

Oxidoreductase Transferase Hydrolase Lyase Isomerase Ligase Whole-cell

1 2 3 4 5 6 -

Polymer Syntheses X X X

Polymer Modifications X X X

Polymer Hydrolyses X X X

X X

X

X

Hydrolases (EC 3) As a group, hydrolases are used more often in polymers than all other enzymes. Many hydrolases can accept different substrates and have utility for a variety of reactions. Their popularity is assisted by the commercial availability of many hydrolases and their relatively lower prices. Hydrolase-Catalyzed Polymerizations A very active and fruitful area of research is the use of lipases for the synthesis of polyesters, polylactones, and polycarbonates (35). Many creative reactions have been devised. In this book, lipase-catalyzed polycondensation of diols/diacids was reported separately by Mahapatro et al (25), Kulshrestha et al

In Polymer Biocatalysis and Biomaterials; Cheng, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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(26), and Mei et al (27). The synthesis of several chiral substituted poly(eeaprolactone) was achieved by Bisht et al (29). Nakaoki et al (30) described the enzymatic polymerization of poly(e-caprolactone) in supercritical C 0 . Frey et al (28) reported the synthesis of hyperbranched poly(e-caprolactone) using concurrent ring-opening polymerization and polycondensation. Kalra et al (31) copolymerized L-lactide and ω-pentadecalactone using lipase and an organometallic catalyst. Another class of polymers where hydrolases are used successfully comprises polysaccharides. This approach has been pioneered by Kobayashi et al (3c,36) and is exemplified by their article (18) where hyaluronan and chondroitin were prepared via hyaluronidase-catalyzed polymerizations. A slightly different approach is to use glycosidases to prepare oligosaccharides (17,37). A recent development is silicon bioscience (38). An example in this book is the paper by Bassindale et al (15) where several lipases and proteases were engaged to catalyze the condensation of alkoxysilanes. It may be noted that lipases are also commonly used for the synthesis of organic compounds, including monomers and reactive oligomers. For example, Kalra et al (32) prepared vinylethylglucoside, and Gu et al (33) prepared substituted acrylic monomers, both with lipases. These monomers were subsequently polymerized by conventional methods.


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Hydrolase-Catalyzed Polymer Modifications Instead of polymer synthesis, hydrolases can be used to add, change, or remove functional groups on existing polymers. Several examples are given in this book, including protease-catalyzed acylation of polysaccharides (21), papain-catalyzed amidation of pectin (21), hydrolase-catalyzed amidation of carboxymethylcellulose (33), and lipase-catalyzed syntheses of fatty acid diester of poly(ethylene glycol) (33), fatty acid ester of cationic guar (33), modified starch (26), and glycosilicone conjugates (16). Hydrolysis with Hydrolases As their name implies, hydrolysis is the preferred reaction for hydrolases in water under optimal conditions. Some examples in the polysaccharide area include cellulolytic enzymes for biomass conversion (5), proteases for the degradation of guar gum (21), cellulase for viscosity reduction of xanthan gum (21), and β-D-galactosidase for pectin hydrolysis (21). Two examples in the non-polysaccharide area are the use of Alcalase® protease for the hydrolysis of end-terminated esters in polyamide (24) and nitrilase for the bioconversion of nitriles to carboxylic acids (7).


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Oxidoreductases (EC 1) Many enzymes in this family require cofactors, which are inconvenient and often expensive in commercial applications. Not surprisingly, most of the reactions reported thus far in polymer science deal with enzymes that require no cofactors.


Oxidoreductases in Polymer Synthesis Thus far, oxidoreductases are mostly engaged in polymer synthesis in two major areas. First, they are employed for the polymerization of phenols and anilines. In the former case, the products include polyphenols (39) and poly(phenylene oxide) (40). In the latter case, water-soluble polyaniline polymers can be made (41). Secondly, oxidoreductases are employed for the free-radical polymerization of vinyl monomers (42). Oxidoreductases in Polymer Modifications Oxidoreductases tend to be specific with respect to their substrates. Whereas this feature limits the scope of the reactions, these enzymes can be used in suitable cases for specific functionalization. An example is galactose oxidase which can specifically oxidize the C6 alcohol on galactose in guar to an aldehyde (43). Another oxidoreductase is tyrosinase, which has been shown to functionalize chitosan (44). In this book, Payne et al (11) grafted two proteins onto chitosan with this enzyme. Although strictly speaking a hydrolase, a suitable lipase when combined with H 0 and a carboxylic acid can carry out oxidation reaction through the formation of a peracid (45). This reaction has been used for polymer modification, e.g., epoxidation of polybutadiene (46a) and oxidation of hydroxyethylcellulose (46b). 2

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Oxidoreductases in Polymer Hydrolysis One type of reactions that has attracted a lot of attention is the enzymatic removal of lignin in wood pulp (also known as "biobleaching") (47). The enzymes involved include laccase (in combination with oxygen and a mediator), lignin peroxidase (in conjunction with H 0 ), and manganese peroxidase (also with H 0 ). These enzymatic reactions modify the lignin molecules, which can then be removed in a subsequent alkaline washing step. 2

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Transferases (EC 2) A lot of creative work has been done with transferases, primarily for polymer synthesis and modification reactions.


Transferases in Polymer Synthesis A major application is the use of glycosyltransferase for the synthesis of oligosaccharides and polysaccharides (48). Elegant methods have been developed to recycle the cofactors and render the processes economical. In this book, Wang et al (17) reviewed several approaches relating to glycosyltransferases. DeAngelis (19) described two approaches using recombinant Pasteurella multocida synthase to generate glycosaminoglycans. Another transferase reaction involves dextransucrase, which catalyzes the formation of dextran and some oligosaccharides (49). Yet another transferase reaction entails the use of potato starch phosphorylase in the synthesis of lowmolecular-weight amylose (50). Transferases in Polymer Modifications Glycosyltransferases can also carry out glycan chain modifications, especially at outer or terminal positions (17). Transglutaminases are acyl transfer enzymes that catalyzed the condensation of glutamine and lysine residues of proteins (51). They have been utilized for food processing. In this book a calcium-independent microbial transglutaminase has been used by Payne et al (11) to crosslink the protein in gelatin-chitosan blends Lyases (EC 4) Lyases are effective enzymes for the degradation of polysaccharides. Examples are pectin lyase, pectate lyase, xanthan lyase, alginate lyase, hyaluronate lyase, and heparin lyase. BioPreparation™, which was developed by NOVOZYMES A/S for the removal of polymeric materials from the cotton surface, contains the pectate lyase (5). Isomerases (EC 5) Isomerases are involved in polymer science only in specific occasions. An example is the recent use of epimerase to convert mannuronate to guluronate in alginates (52), oxidized konjac glucomannan (53a), and oxidized galactomannan (53b).


6 Ligases (EC 6) This class of enzymes has not found much application in conventional polymeric materials thus far.


Whole-Cell Biocatalysis Whole-cell biotransformations typically utilize the metabolic pathways of microorganisms (or plant or animal cells) to produce desirable products (1). These are among the oldest technologies used for food products. Some common items include alcohol, cheese, soy sauce and tofii, among others (54). Microbial biocatalysis is often employed for polymer synthesis. A successful application area is the poly(hydroxyalkanoate) (PHA). Thus, Noda et al (22) reported the development of Nodax™ family of copolymers, which were shown to have versatile physical properties. Srienc et al (23) engineered yeast to produce PHAs comprising 6-13 carbon monomers. Henderson (5) included whole-cell approaches and metabolic engineering in the biomass conversion program. The synthesis of oligosaccharides and polysaccharides can also be achieved via microbial biocatalysis. For example, Wang et al (17) devised the "Superbug" method that could produce oligosaccharides of less than 4 sugar units efficiently. A successful industrial process was described by Riehle (24), using a microbial process to convert the waste materials to innocuous compounds

New Enzyme Methodologies One of the reasons for the increasing popularity of biocatalysis is the rapid pace of progress in biotechnology, which has a synergistic effect on biocatalysis. Bioprocess engineering has also helped to improve the product yields and the process economics. Given below is a sampling of the methodologies: Enzyme discovery and improvement: high-throughput screening, biodiversity, directed evolution, gene shuffling, combinatorial chemistry, computational chemistry, bioinformatics, DNA microarray technology Enzyme applications: cofactor recycling techniques, enzyme immobilization, enzyme solubilization, enzyme recycling, multi-enzyme clusters, mini- and micro-scale application tests, new enzyme analytical and separation techniques.


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Bioengineering: enzyme production (including gene expression, enzyme purification, and formulation), fermentation, membrane technology, process systems engineering. Microbial methodologies: culture enrichment and microbial strain improvement, metabolic engineering, integration of multi-pathways in cells, cell entrapment, functional genomics These advances are reflected in a number of articles in this book. For example, Henderson (5) mentioned several programs at Novozymes A/S where some of these techniques were involved. Conboy et al (7) used H NMR as a tool for high throughput screening to identify new enzymatic activity. An ingenious alternative to high throughput assays was proposed by Minshull (6). Genetically modified enzymes were involved in the articles by DeAngelis (19), Wang et al (17), and Henderson (5). A genetically modified protein was prepared by Patwardhan et al (14). Wang et al (17) devised multi-enzyme clusters (Superbeads) and cells containing multiple pathways (Superbugs). Immobilization is a useful tool for biocatalysis. Several methods are often used: 1) Binding to a carrier, e.g., through covalent bonding, physical adsorption, electrostatic interaction, or biospecific binding. 2) Crosslinking with bifunctional or multifunctional reagents, or derivatization with a reactive group and subsequent polymerization. 3) Entrapment in a gel, microcapsules, liposomes, hollow fibers, or ultrafiltration membranes. 4) Combination of the above methods (3a). Nevertheless, new and improved methods are desirable. In this book, Hsieh et al (8) described the successful binding of lipase onto cellulose. Gitsov et al (9) trapped laccase in an amphophilic linear-dendritic block copolymer. DeAngelis (19) used an immobilized enzyme reactor in one of his approaches. A popular and effective lipase is Novozym® 435, an immobilized Candida antartica Lipase Β (CALB), available commercially from Novozymes A/S. This enzyme was invoked in at least 11 of the articles in this book ( 16,20,25-33).


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Novel Bio-Related Materials Biomaterials are synthetic or natural materials that are in contact with biological tissues or fluids and may enhance or replace tissues, bones, organs, or body functions (4). They include metals, alloys, glasses, ceramics, natural or synthetic polymers, biomimetics, and composites. Typical biomaterials may be used in artificial skin, tissues, and bones, dental fillings, wire plates and pins for bone repair, artificial hips and joints, implantable drug delivery systems, and other dental, surgical, and medical devices. New and improved biomaterials continue to be sought.



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In this book Song et al (10) described a novel nucleation and mineral growth process to produce a bone-like biomineral composite. The crosslinked gelatinchitosan blend made by Payne et al (11) may perhaps be used as biomimetic soft tissue or for bioencapsulation. The sorbitol-based polyesters synthesized by Mei at al (27) and Kulshrestha et al (26) may possibly find applications in tissue engineering. Biswas et al (13) described the preparation and the mechanical properties of modified zein. Fishman et al (12) made pectin-starch and pectinpoly(vinyl alcohol) blends and found them to be strong, flexible films. Several papers in this book dealt with silicon-containing materials. Patwardhan et al (14) carried out bioinspired mineralization of silica in the presence of a genetically engineered protein. Bassindale et al (15) screened several enzymes to look at the condensation of silanes. Sahoo et al (16) prepared glycosilicone conjugates.

References 1.

General books on biocatalysis include: (a) Biocatalysis and Biodegradation; Wackett, L. P.; Hershberger, C. D.; A S M Press, Washington, DC, 2001. (b) Biotransformations in Organic Chemistry, 3 Ed.; Faber, K.; Springer, Berlin, Germany, 1997. (c) Introduction to Biocatalysis Using Enzymes and Micro-organisms; Roberts, S. M., Turner, N. J.; Willetts, A. J.; Turner, M. Cambridge Univ. Press, Cambridge, UK, 1995. (d) Applied Biocatalysis; Cabral, J. M. S.; Best, D.; Boross, L.; Tramper, J., Eds.; Harwood, Char, Switzerland, 1994. (e) Enzymes in Synthetic Organic Chemistry; Wong, C. H.; Whiteside, G. M.; Elsevier, Oxford, UK, 1994. 2. Some recent books on polymer biocatalysis include: (a) Biocatalysis in Polymer Science; Gross, R. Α.; Cheng, Η. N., Eds.; Amer. Chem. Soc., Washington, DC, 2003. (b) Enzymes in Polymer Synthesis; Gross, R. A; Kaplan, D. L.; Swift, G., Eds.; Amer. Chem. Soc., Washington, DC, 1998. 3 Some reviews on polymer biocatalysis include: (a) Cheng, H. N.; Gross, R. A. ACS Symp. Ser. 2002, 840, 1. (b) Gross, R. Α.; Kumar, Α.; Kalra, B. Chem. Rev. 2001, 101, 2097. (c) Kobayashi, S.; Uyama, H . ; Kimura, S. Chem. Rev. 2001, 101, 3793. 4. Some books on biomaterials include: (a) Biomaterials: Principles and Applications; Park, J. B.; Bronzino, J. D.; CRC Press, 2002. (b) Biomaterials Science; Ratner, B. D.; Hoffman, A. S.; Schoen, F. J.; Lemons, J. E. Eds.; Academic Press, Orlando, 1996. (c) Biomaterials Science and Biocompatibility; Silver, F. H.; Christiansen, D. L.; Springer-Verlag, New York, 1999. 5. Henderson, L. A. ACS Symp. Ser. (this volume), Chapter 2. 6. Ness, J. E.; Cox, T.; Govindarajan, S.; Gustafsson, C.; Gross, R.A.; Minshull, J. ACS Symp. Ser. (this volume), Chapter 3. rd


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51. For example: (a) Kamiya, N . ; Takazawa, T.; Tanaka, T.; Ueda, H.; Nagamune, T. Enzyme Microb. Technol. 2003, 33, 492. (b) Dickinson, E. Trends Food Sci. Technol. 1997, 8, 334. 52. For example, (a) Ertesvag, H.; Doseth, B.; Larsen, B.; Skjak-Braek, G.; Valla, S. J. Bacteriol. 1994, 176, 2846. (b) Franklin, M . J.; Chitnis, C. E.; Gacesa, P.; Sonesson, Α.; White, D. C.; Ohman, D. E. J. Bacteriol. 1994, 176, 1821. (c) Harmann, M . ; Holm, Ο. B.; Johansen, G.-A. B.; Skjak-Braek, G.; Stokke, Β. T. Biopolymers 2002, 63, 77. (d) Harmann, M . ; Dunn, A. S.; Markussen, S.; Grasdalen, H.; Valla, S.; Skjak-Braek, G. Biochim. Biophys. Acta 2002, 1570, 104. 53. (a) Crescenzi, V.; Skjak-Braek, G.; Dentini, M . ; Maci, G.; Bernalda, M . S.; Risica, D.; Capitani, D.; Mannina, L.; Segre, A. L. Biomacromolecules 2002, 3, 1343. (b) Crescenzi, V.; Dentini, M . ; Risica, D.; Spadoni, S.; Skjak-Braek, G.; Capitani, D.; Mannina, L.; Viele, S. Biomacromolecules

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