Genetic Production of Synthetic Protein Polymers
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Genetic Production of Synthetic Protein Polymers Joseph Cappello Introduction A goal of polymer chemistry is to control the final properties of a processed material by controlling the chemistry and physics of the polymer chain. Since the final formulated materials' properties are a complex function of chemical and physical interactions at both the micro and macro scales, it would be desirable to influence those interactions at the level of the molecular chemistry of the chain. However, in order to affect those macromolecular associations that occur via chemical functionalities separated over long distances, a chemistry is needed that can specify not only variability in chemical composition along the chain but can also specify the positions at which this chemical variability must occur over the entire length of the chain—several hundred nanometers. Protein-based materials in nature are formulated from protein chains which have evolved chemical compositions that through the specific sequence of their amino acid monomer constituents can result in diverse materials such as ultrahigh strength fibers,1 silks and collagens, and soft, ultradurable elastomers, elastin.2As described in the article by Kaplan in this issue, the fundamental six amino acid repeat of the crystalline fraction of Bombyx mori silk fibroin is composed of essentially three amino acids, glycine (G), alanine (A), and serine (S), in a 3:2:1 ratio. 3 The strength of the fiber is derived from these crystallized peptide blocks.45 They are positioned to a great extent as tandem repeats of the sequence GAGAGS. However, what properties would be obtained if the amino acids of silk fibroin were scrambled? What strength would GGGAAS give? What crystallinity would AGGSAG result in? What chain geometry would
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AAGGSG adopt? What properties would be obtained from an amino acid polymer consisting of a random statistical mixture of all 20 amino acids? What is the significance of sequence control on polymer chemistry, chain conformation, and materials properties? We can only infer indirectly from the comparison of systems that have it and those that do not. A good example is the degree of functional diversity of natural proteins versus that of storage and structural polysaccharides. Proteins can control their three-dimensional shape, allowing selective recognition and binding to occur between molecularly distinct entities such as an enzyme with its substrate, an antibody with its antigen, a cellular receptor with a protein ligand, or the monomeric chains of a homomeric or heteromeric protein assembly such as collagen tendon. The assembly and architecture of many proteinbased materials can be attributed to the specific nature of these interactions. Structural polysaccharides, on the other hand, are generally linear or branched homopolymers or simple copolymers whose conformations are far simpler and involve few, if any, interactions with other polysaccharides. At a high metabolic cost, nature has developed an elaborate cellular machinery system to carry out templated
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