Amino acid-based polymers polypeptide synthesi

Protein polymers based on Lys-25 were prepared by recombinant DNA (rDNA) technology and bacterial protein expression. The main advantage of this approach is the ability to directly produce high molecular weight polypeptides of exact amino acid sequence with high fidelity as required for this investigation. In contrast to conventional polymer synthesis, protein biosynthesis proceeds with near-absolute control of macromolecular architecture, i.e., size, composition, sequence, topology, and stereochemistry. Biosynthetic polyfa-amino acids) can be considered as model uniform polymers and may possess unique structures and, hence, materials properties, as a consequence of their sequence specificity [11]. Protein biosynthesis affords an opportunity to completely specify the primary structure of the polypeptide repeat and analyze the effect of sequence and structural uniformity on the properties of the protein network. 

Another important technique was based on the observation that synthetic trinucleotides induced the binding to ribosomes of tRNA molecules that were "charged" with their specific amino acids 38/39 For example, the trinucleotides UpUpU and ApApA stimulated the binding to ribosomes of 14C-labeled phenylalanyl-tRNA and lysyl-tRNA, respectively. The corresponding dinucleotides had no effect, an observation that not only verified the two codons but also provided direct evidence for the triplet nature of the genetic code. Another powerful approach was the use of artificial RNA polymers, synthesized by combined chemical and enzymatic approaches.40 For example, the polynucleotide CUCUCUCUCU led to the synthesis by ribosomes of a regular alternating polypeptide of leucine and serine. 

Proteins are informational macromolecules, the ultimate heirs of the genetic information encoded in the sequence of nucleotide bases within the chromosomes. Each protein is composed of one or more polypeptide chains, and each peptide chain is a linear polymer of amino acids. The order of the amino acids commonly found in the polypeptide chain is determined by the order of nucleotides in the corresponding messenger RNA template. In this chapter we examine four aspects of protein metabolism (fig. 29.1) (1) The process whereby amino acids are ordered and polymerized into polypeptide chains (2) posttranslational alterations in polypeptides, which occur after they are assembled on the ribosome (3) the targeting process whereby proteins move from their site of synthesis to their sites of function and (4) the proteolytic reactions that result in the return of proteins to their starting material, amino acids. 

Non-natural amino acids can be incorporated into peptides and polypeptides via several different methodologies. Solid-phase peptide synthesis (SPPS) is a straightforward method for incorporation of non-natural amino acids and allows the incorporation of essentially any amino acid but is limited by the size of the peptides produced 18). Suppression-based strategies, both in vitro and in vivo, have been developed for site specific incorporation of diverse set non-natural amino acids into natural and synthetic polypeptides 19). Alternatively, auxotrophic expression hosts have been used for multisite incorporation of nonnatural amino acid in protein polymers, where multiple natural amino acids of one type can be replaced with non-natural analogues during protein biosynthesis (20, 21). Multisite incorporation of non-natural amino acids in the synthesis of protein polymeric materials facilitates chemical modification at multiple sites and can modulate the physical properties of the protein polymers (22). 

Synthesis.—A number of interesting developments in synthetic techniques have taken place recently. The use of template polymerization has up to now been confined to vinyl polymerization and the preparation of polypeptides from the A-carboxyanhydrides of amino-acids. The template polycondensation of active esters containing nucleic acid bases with diamines has been reported. The enantioselective ester synthesis in the presence of optically active polymers, a not dissimilar process to template polymerization, has also been reported. ... 

Synthetic peptide-based polymers are not new materials homopolymers of polypeptides have been available for many decades, yet have only seen limited use in materials applications. However, new methods in chemical synthesis have made possible the preparation of increasingly complex polypeptide sequences of controlled molecular weight that display properties far superior to ill-defined homopolypeptides. Examination of the different methods for polypeptide synthesis reveals the limitations of these techniques for preparation of well-defined copolymers. Conventional solid-phase peptide synthesis is neither useful nor practical for direct preparation of large polypeptides (> 100 residues) due to unavoidable deletions and truncations that result from incomplete deprotection and coupling steps. The most economical and expedient process for synthesis of long polypeptide chains is the polymerization of a-amino acid N-carboxyanhydrides (NCAs)... 

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