Biology and Materials? Part II
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ch enzyme binds its particular substrate, chemically converts it to product, and releases it. Doi describes a family of polyesters produced by bacteria. These are environmentally degradable thermoplastics with a wide range of potential applications. The bacteria carry three genes for the process, each gene coding for an enzyme involved in the three-step pathway converting a common metabolic intermediate (itself produced from sugar through a ten-step pathway) to the polymer. The genes have been cloned and studied in great detail. Morrow discusses his work in using purified enzymes in reactions outside the organism to produce novel polymers. By virtue of the substrate and reaction specificity of the enzymes, their use allows the synthesis of optically active and therefore highly stereoregular polymers. Enzyme use outside the organism provides an increased level of control and flexibility of the reaction, but requires considerable work in designing appropriate reaction conditions. Hilvert takes the system one step further. The specificity of enzymes is an advantage in that it rigorously controls the structure of the reaction product. It is, however, a disadvantage in limiting the flexibility of the system. One would like to use variants of the natural substrate to allow the enzyme to produce variants of the product. These would have novel structures and therefore novel properties. Enzymes that are highly specific for their substrate cannot recognize altered substrates and catalyze reactions converting them to altered products. Hilvert describes the genetic engineering technique of "site-directed mutagenesis," which allows the rational redesign of an enzyme to allow it to catalyze a reaction leading to a new product with new and, with luck, interesting properties. He goes on to discuss the newer techniques that allow the
substitution of non-natural amino acids in the enzyme, providing greater flexibility in modifying its structure. Finally, he discusses the production of catalytic antibodies, proteins with enzyme activity that can be generated for catalysis of targeted reactions, whether they occur in nature or not. Callstrom and Bednarski's article in the October issue is also relevant here. Once enzymes have been selected for use in materials synthesis, their inherent instability must be addressed. One application of the new carbohydrate-based polymer described in that article is its unique ability to stabilize enzymes at high temperature, in organic solvents, and in distilled water without buffer. Membranes surround and contain virtually all biological structures. They selfassemble from lipid and protein substituents and form barriers between the surrounded structure and its environment. The lipid components form the thin film structure of the membrane in which the proteins are embedded. These proteins perform a variety of biological functions. After briefly discussing the features of naturally occurring membrane systems, Hampp et al. describe their efforts to use these materials in optoelectronic applications. They
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