Recombinant protein engineering can significantly increase protein yield over that which can be achieved by extraction of a native protein from animal tissues and offers the ability to use human amino acid sequences, so as to avoid adverse immunological responses. However, the most important impact of this technology lies in the potential to introduce precise changes in the amino acid sequence and/or to construct new proteins based upon the assembly of de novo peptide sequences or through the use of non-natural amino acids. One example is the generation of structural proteins, referred to as protein polymers that consist of sequentially repeated amino acid blocks. Typically, the incorporation of repetitive oligopeptide sequences, derived from a consideration of the primary amino acid structure of a native protein, imparts critical structural properties from the parent protein to the recombinant polypeptide. Moreover, opportunities to improve the biological, thermodynamic, and mechanical properties of the protein polymer exist through alteration of the peptide chain length, consensus repeat sequence, and the introduction of additional functional groups or oligopeptide units. For example, we have demonstrated that substituting different amino acids for those ordinarily occurring in the sequence can affect the susceptibility of the protein to proteolytic degradation or facilitate the placement of crosslinks at well-defined intervals along the polypeptide chain. It is also significant that the uniformity of macromolecular structure achieved by recombinant strategies provides exquisite control over macroscopic polymer properties, including material processability. In summary, the possibility now exists to generate synthetic polypeptides that mimic structural matrix proteins.
Recombinant elastin-like protein polymers (ELP) represent a promising new class of biomaterials with potential applications in drug delivery, tissue engineering, or as components of implanted medical devices. Through a multidisciplinary effort involving a diverse group of chemists and engineers, our group is designing and investigating a number of novel photochemically and virtually crosslinked elastomeric recombinant protein polymers. In particular, our studies have lead to the generation of ELP triblock copolymers, consisting of hydrophilic, elastomeric midblock sequences flanked by self-associating, hydrophobic endblocks in arranged in an ABA block format. Sequences with individual block sizes in excess of 35 kDa have resulted in protein-based biomaterials demonstrating structural polymorphism, allowing us to broadly tune material properties. By adjusting polymer sequence and processing conditions, resilient ELPs with a broad range of stiffness and strength have been formulated as films, gels, micelles, or nanofibers. Subcutaneous implants in mice have revealed that some ELPs largely resist biodegradation for at least a year following implant. Ex vivo baboon experiments have indicated that ELP coatings on the inner surface of small diameter vascular grafts dramatically reduce clotting (platelet and fibrin deposition) in the first hour of blood flow. Overall, our research has produced a series of ELPs in high yield, with molecular weights ranging from 100 to 250 kD, and with low endotoxin levels for a variety of applications in tissue engineering, drug delivery, and device design.
Nagapudi K, Brinkman WT, Thomas BS, Ok-Park J, Sreenivasarao M, Wright E, Conticello VP, Chaikof EL. Viscoelastic and mechanical behavior of recombinant protein elastomers. Biomaterials 2005; 26:4695-4706.
Wu X, Sallach R, Haller CA, Caves JA, Nagapudi K, Conticello VP, Levenston ME, Chaikof EL. Alterations in physical crosslinking modulate mechanical properties of two-phase protein polymer networks. Biomacromolecules 2005; 6:3037-304
Sallach RE, Cui W, Balderrama F, Martinez AW, Wen J, Haller CA, Taylor JV, Wright ER, Long RC, Chaikof EL. Long-term biostability of self-assembling protein polymers in the absence of covalent crosslinking. Biomaterials 2010; 31:779-91
Ravi S, Krishnamurthy VR, Caves JM, Haller CA, Chaikof EL. Maleimide-thiol coupling of a bioactive peptide to an elastin-like protein polymer. Acta Biomaterialia 2012; 8:627-635.