Biocatalytic Synthesis of Polyesters Using Enzymes
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Biocatalytic Synthesis of Polyesters Using Enzymes Cary J. Morrow Introduction Plants and animals have been exploited as sources of materials for centuries but, as our ability to analyze and fractionate them has progressed, the extraordinary range of properties available from materials produced by living systems has continued to grow. Doi, in another article in this issue of the MRS Bulletin, presents a discussion of a group of naturally occurring polyesters related to poly(beta-hydroxybutyrate). These polyesters are formed in vivo by several microorganisms as part of an energy storage scheme. Research on these systems has allowed growth conditions to be found that can lead, in a controlled fashion, to a number of copolymers. Useful materials based on these bacterial polyesters appear to be at hand. The in-vivo formation of polyesters in microorganisms also illustrates several of the important reasons for examining biocatalytic polymer synthesis. First, unlike most industrial syntheses of polyesters, the poly(beta-hydroxybutyrate) biosynthesis occurs at a near-ambient temperature using a carbohydrate feedstock. Second, and perhaps most importantly, the stored polyesters are readily biodegraded by the bacteria that manufacture them, so materials based on these polyesters should also be biodegradable. Third, although there are side chains along the polymer backbone, they are introduced in a highly stereospecific fashion during in-vivo synthesis, leading to an entirely stereoregular polyester. However, along with these advantages, there are also significant limitations to bacterial polyester synthesis. First, there are some substrates that are not incorpoMRS BULLETIN/NOVEMBER 1992
rated into the polyester by the bacteria. Second, normal metabolism leads to the polyester, always incorporating a fraction of hydroxybutyrate monomers. Third, the backbone is always comprised of fouratom, A-B type 3-hydroxy acid repeat units with variations appearing in the side chain at carbon-3. The potential advantages of biocatalytic polymer preparation led several groups to explore alternative methods that will overcome the limitations. One approach is to use gene modification techniques to prepare genes having specific characteristics required for production of the desired material and then to introduce the modified gene or genes into a "factory" organism that synthesizes the target material. This complex technology is only in its infancy. Application of the method in the synthesis of some materials other than polyesters is described by Hilvert in this issue of the MRS Bulletin. Adapting the bacterial polyester synthesis apparatus to allow currently unattainable side chains to be introduced may be an achievable goal, but other modifications of the polymer product will probably require a much better understanding of the structure and function of the enzymes in the biosynthesis pathway than is available. A second approach to overcoming these limitations as well as the inherent selectivity of all enzymes is to carry out an invitro react
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