Enzyme polymer chemistry

Worldwide suppliers with bioengineering capabilities are displacing established polymers with cost-effective and higher performing plastics. An explosion of novel polymers has been made by enzymatic control. The use of enzymes for polymerization has drastically altered the landscape of polymer chemistry. Processors can request specific properties for each application as opposed to the usual making do with what is available. The supplier can deliver to the processor desired properties requested. 

Apart from the traditional organic and combinatorial/high-throughput synthesis protocols covered in this book, more recent applications of microwave chemistry include biochemical processes such as high-speed polymerase chain reaction (PCR) [2], rapid enzyme-mediated protein mapping [3], and general enzyme-mediated organic transformations (biocatalysis) [4], Furthermore, microwaves have been used in conjunction with electrochemical [5] and photochemical processes [6], and are also heavily employed in polymer chemistry [7] and material science applications [8], such as in the fabrication and modification of carbon nanotubes or nanowires [9]. 

Abstract Transferases are enzymes that catalyze reactions in which a group is transferred from one compound to another. This makes these enzymes ideal catalysts for polymerization reactions. In nature, transferases are responsible for the synthesis of many important natural macromolecules. In synthetic polymer chemistry, various transferases are used to synthesize polymers in vitro. This chapter reviews some of these approaches, such as the enzymatic polymerization of polyesters, polysaccharides, and polyisoprene. 

In this section, enzymes in the EC 2.4. class are presented that catalyze valuable and interesting reactions in the field of polymer chemistry. The Enzyme Commission (EC) classification scheme organizes enzymes according to their biochemical function in living systems. Enzymes can, however, also catalyze the reverse reaction, which is very often used in biocatalytic synthesis. Therefore, newer classification systems were developed based on the three-dimensional structure and function of the enzyme, the property of the enzyme, the biotransformation the enzyme catalyzes etc. [88-93]. The Carbohydrate-Active enZYmes Database (CAZy), which is currently the best database/classification system for carbohydrate-active enzymes uses an amino-acid-sequence-based classification and would classify some of the enzymes presented in the following as hydrolases rather than transferases (e.g. branching enzyme, sucrases, and amylomaltase) [91]. Nevertheless, we present these enzymes here because they are transferases according to the EC classification. 

With the increased use of enzymes in polymer chemistry, the enzymology terminology to describe the reaction kinetics and the enantioselectivity of a reaction has become more and more common in polymer literature. The parameter of choice to describe the enantioselectivity of an enzyme-catalyzed kinetic resolution is the enantiomeric ratio E. The enantiomeric ratio is defined as the ratio of the specificity constants for the two enantiomers, R) and S) (1) ... 

Chapter 2 summarizes the application of transferases in polymer chemistry. Transferases are enzymes transferring a group from one compound (donor) to another compound (acceptor). Of the three classes of enzymes used in polymer science, transferases are the least frequently applied, which is due to their sensitivity. Nonetheless, several transferases such as phosphorylases and synthases have been... 

Enzymes that belong to the class of hydrolases are by far the most frequently-applied enzymes in polymer chemistry and are discussed in Chaps. 3-6. Although hydrolases typically catalyse hydrolysis reactions, in synthetic conditions they have also been used as catalysts for the reverse reaction, i.e. the bond-forming reaction. In particular, lipases emerged as stable and versatile catalysts in water-poor media and have been applied to prepare polyesters, polyamides and polycarbonates, all polymers with great potential in a variety of biomedical applications. 

Macromolecules are very much like the crystalline powder just described. A few polymers, usually biologically-active natural products like enzymes or proteins, have very specific structure, mass, repeat-unit sequence, and conformational architecture. These biopolymers are the exceptions in polymer chemistry, however. Most synthetic polymers or storage biopolymers are collections of molecules with different numbers of repeat units in the molecule. The individual molecules of a polymer sample thus differ in chain length, mass, and size. The molecular weight of a polymer sample is thus a distributed quantity. This variation in molecular weight amongst molecules in a sample has important implications, since, just as in the crystal dimension example, physical and chemical properties of the polymer sample depend on different measures of the molecular weight distribution. 

Many other topics also can be considered for inclusion in the first-year physical chemistry course. Enzyme kinetics, the study of the solid state and crystallography (42), and polymer chemistry would be especially interesting to the biochemistry students in our classes. The choices are limited only by the creativity of faculty teaching the physical chemistry courses. 

At a glance, the rapprochement between biochemistry and polymer chemistry seems to have played an important role in the methodological development of preparations for immobilized biocatalysts. A number of articles on the preparation and characterization of immobilized biocatalysts, together with their applications in a variety of fields besides synthetic chemical reactions - chemical and clinical analysis, medicine, and food processing, for example - have already been published. These results have been reviewed by many of the pioneers in this and related fields [1-20]. The technology for immobilizing enzymes and cells is believed to be relatively mature at this point. In addition, the nature of immobilized biocatalysts has become somewhat more transparent to us. The key now is to come up with new uses and new systems which can fulfill specific needs [21]. 

Mabrouk, P.A., The use of poly(ethylene glycol) enzymes in nonaqueous enzymology in poly(ethylene glycol), In Chemistry and Biological Applications (ACS Symposium Series, No 680), J. Milton Harris, Samuel Zalipsky, eds., American Chemical Society Division of Polymer Chemistry, Calif. American Chemical Society Meeting 1997, San Francisco, pp 118. 

Scheme 4.87 Formation of enzyme—polymer membranes in a microchannel. Reprinted with permission from [344]. Copyright 2005 The Royal Society of Chemistry.

A range of excellent and recent reviews can be found, in which the use of enzymes within specific branches or disciplines of organic chemistry is highlighted. These include biocatalysis in carbohydrate chemistry [39], polymer chemistry [40] and for protecting group manipulations [41]. The present chapter is focused on immobilized enzymes. Hence, as an appetizer, a few selected applications with Novozym 435 are presented below, followed by a short subsection discussing industrial-scale applications of immobilized enzymes. 

To date, the limited use of the enantioselectivity of biocatalysts in polymerization conditions and the lengthy synthetic procedures required to prepare optically pure monomers have hampered full exploitation of chemo-enzymatic approaches in polymer chemistry. However, a combined multidisciplinary effort at the interface of biocatalysis, polymer chemistry and organic catalysis, will allow to convert methods well-established in organic chemistry such as tandem catalysis, to the field of polymer chemistry. Undoubtedly, in the near future the exploitation of the selectivity of enzymes and the advantages of chemo-enzymatic approaches in a wide variety of polymerization chemistries will be recognized. This may lead to a paradigm shift in polymer chemistry and allow a higher level of structural complexity in macromolecules, reminiscent to those found in Nature. 

Natural rubber (pages 5 and 6) is a member of this group of polymers although it is synthesised in the rubber tree by a quite different process. NR is a high member of the terpene family and is made by the same kind of enzyme-controlled chemistry as the terpene squalene, which is the precursor of cholesterol in animals. Squalene is often used as a low Mj model to study the reactions of cis-poly(isoprene) (PI). 

The CAD-Kit enzyme polymers are a good example of how immobilization can lead to a robust assay platform for use by military or homeland security personnel. Figure 6 illustrates the minimum shelf life of all CAD-Kit pens under elevated temperatures. The shelf life exceeds 10 days during storage at 60°C, and more than 2 years during storage at room temperature. The multipoint immobilization of the pol30irethane polymer chemistry described in the previous... 

But the term biological model has simpler and cleaner connotation, which actually corresponds more closely to the philological meaning of the word model . The paradigm is now to look at the biological system in order to obtain a hint for a new synthetic system. Thus, the construction of synthetic macromolecules as synthetic catalysts based on model enzymes is very interesting and useful from the point of view of polymer chemistry, but one should not try to use them the other way round, i. e. to obtain information about the catalytic and biological activity of enzymes. 

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