Probing the Natural-Abundance 13C Populations of Insoluble Elastin Using 13C−1H Heteronuclear Correlation (HETCOR) NMR Spectroscopy

Protein-Based Biological Materials: Molecular Design and Artificial Production

Polymeric materials produced from fossil fuels have been intimately linked to the development of industrial activities in the 20th century and, consequently, to the transformation of our way of living. While this has brought many benefits, the fabrication and disposal of these materials is bringing enormous sustainable challenges. Thus, materials that are produced in a more sustainable fashion and whose degradation products are harmless to the environment are urgently needed. Natural biopolymers─which can compete with and sometimes surpass the performance of synthetic polymers─provide a great source of inspiration. They are made of natural chemicals, under benign environmental conditions, and their degradation products are harmless. Before these materials can be synthetically replicated, it is essential to elucidate their chemical design and biofabrication. For protein-based materials, this means obtaining the complete sequences of the proteinaceous building blocks, a task that historically took decades of research. Thus, we start this review with a historical perspective on early efforts to obtain the primary sequences of load-bearing proteins, followed by the latest developments in sequencing and proteomic technologies that have greatly accelerated sequencing of extracellular proteins. Next, four main classes of protein materials are presented, namely fibrous materials, bioelastomers exhibiting high reversible deformability, hard bulk materials, and biological adhesives. In each class, we focus on the design at the primary and secondary structure levels and discuss their interplays with the mechanical response. We finally discuss earlier and the latest research to artificially produce protein-based materials using biotechnology and synthetic biology, including current developments by start-up companies to scale-up the production of proteinaceous materials in an economically viable manner.

Development of Small-Diameter Elastin-Silk Fibroin Vascular Grafts

Globally, increasing mortality from cardiovascular disease has become a problem in recent years. Vascular replacement has been used as a treatment for these diseases, but with blood vessels <6 mm in diameter, existing vascular grafts made of synthetic polymers can be occluded by thrombus formation or intimal hyperplasia. Therefore, the development of new artificial vascular grafts is desirable. In this study, we developed an elastin (EL)-silk fibroin (SF) double-raschel knitted vascular graft 1.5 mm in diameter. Water-soluble EL was prepared from insoluble EL by hydrolysis with oxalic acid. Compared to SF, EL was less likely to adhere to platelets, while vascular endothelial cells were three times more likely to adhere. SF artificial blood vessels densely packed with porous EL were fabricated, and these prevented the leakage of blood from the graft during implantation, while the migration of cells after implantation was promoted. Several kinds of ¹³C solid-state NMR spectra were observed with the EL-SF grafts in dry and hydrated states. It was noted that the EL molecules in the graft had very high mobility in the hydrated state. The EL-SF grafts were implanted into the abdominal aorta of rats to evaluate their patency and remodeling ability. No adverse reactions, such as bleeding at the time of implantation or disconnection of the sutured ends, were observed in the implanted grafts, and all were patent at the time of extraction. In addition, vascular endothelial cells were present on the graft's luminal surface 2 weeks after implantation. Therefore, we conclude that EL-SF artificial vascular grafts may be useful where small-diameter grafts are required.

Alterations of elastin in female reproductive tissues arising from advancing parity

Female reproductive tissues undergo significant alterations during pregnancy, which may compromise the structural integrity of extracellular matrix proteins. Here, we report on modifications of elastic fibers, which are primarily composed of elastin and believed to provide a scaffold to the reproductive tissues, due to parity and parturition. Elastic fibers from the upper vaginal wall of virgin Sprague Dawley rats were investigated and compared to rats having undergone one, three, or more than five pregnancies. Optical microscopy was used to study fiber level changes. Mass spectrometry, ¹³C and ²H NMR, was applied to study alterations of elastin from the uterine horns. Spectrophotometry was used to measure matrix metalloproteinases-2,9 and tissue inhibitor of metalloproteinase-1 concentration changes in the uterine horns. Elastic fibers were found to exhibit increase in tortuosity and fragmentation with increased pregnancies. Surprisingly, secondary structure, dynamics, and crosslinking of elastin from multiparous cohorts appear similar to healthy mammalian tissues, despite fragmentation observed at the fiber level. In contrast, elastic fibers from virgin and single pregnancy cohorts are less fragmented and comprised of elastin exhibiting structure and dynamics distinguishable from multiparous groups, with reduced crosslinking. These alterations were correlated to matrix metalloproteinases-2,9 and tissue inhibitor of metalloproteinase-1 concentrations. This work indicates that fiber level alterations resulting from pregnancy and/or parturition, such as fragmentation, rather than secondary structure (e.g. elastin crosslinking density), appear to govern scaffolding characteristics in the female reproductive tissues.

Direct observation of structure and dynamics during phase separation of an elastomeric protein

Despite its growing importance in biology and in biomaterials development, liquid-liquid phase separation of proteins remains poorly understood. In particular, the molecular mechanisms underlying simple coacervation of proteins, such as the extracellular matrix protein elastin, have not been reported. Coacervation of the elastin monomer, tropoelastin, in response to heat and salt is a critical step in the assembly of elastic fibers in vivo, preceding chemical cross-linking. Elastin-like polypeptides (ELPs) derived from the tropoelastin sequence have been shown to undergo a similar phase separation, allowing formation of biomaterials that closely mimic the material properties of native elastin. We have used NMR spectroscopy to obtain site-specific structure and dynamics of a self-assembling elastin-like polypeptide along its entire self-assembly pathway, from monomer through coacervation and into a cross-linked elastic material. Our data reveal that elastin-like hydrophobic domains are composed of transient β-turns in a highly dynamic and disordered chain, and that this disorder is retained both after phase separation and in elastic materials. Cross-linking domains are also highly disordered in monomeric and coacervated ELP3 and form stable helices only after chemical cross-linking. Detailed structural analysis combined with dynamic measurements from NMR relaxation and diffusion data provides direct evidence for an entropy-driven mechanism of simple coacervation of a protein in which transient and nonspecific intermolecular hydrophobic contacts are formed by disordered chains, whereas bulk water and salt are excluded.

Resolving nitrogen-15 and proton chemical shifts for mobile segments of elastin with two-dimensional NMR spectroscopy

In this study, one- and two-dimensional NMR experiments are applied to uniformly (15)N-enriched synthetic elastin, a recombinant human tropoelastin that has been cross-linked to form an elastic hydrogel. Hydrated elastin is characterized by large segments that undergo "liquid-like" motions that limit the efficiency of cross-polarization. The refocused insensitive nuclei enhanced by polarization transfer experiment is used to target these extensive, mobile regions of this protein. Numerous peaks are detected in the backbone amide region of the protein, and their chemical shifts indicate the completely unstructured, "random coil" model for elastin is unlikely. Instead, more evidence is gathered that supports a characteristic ensemble of conformations in this rubber-like protein.

A solid-state (17)O NMR study of L-tyrosine in different ionization states: implications for probing tyrosine side chains in proteins

We report experimental characterization of (17)O quadrupole coupling (QC) and chemical shift (CS) tensors for the phenolic oxygen in three l-tyrosine (l-Tyr) compounds: l-Tyr, l-Tyr.HCl, and Na(2)(l-Tyr). This is the first time that these fundamental (17)O NMR tensors are completely determined for phenolic oxygens in different ionization states. We find that, while the (17)O QC tensor changes very little upon phenol ionization, the (17)O CS tensor displays a remarkable sensitivity. In particular, the isotropic (17)O chemical shift increases by approximately 60 ppm upon phenol ionization, which is 6 times larger than the corresponding change in the isotropic (13)C chemical shift for the C(zeta) nucleus of the same phenol group. By examining the CS tensor orientation in the molecular frame of reference, we discover a "cross-over" effect between delta(11) and delta(22) components for both (17)O and (13)C CS tensors. We demonstrate that the knowledge of such "cross-over" effects is crucial for understanding the relationship between the observed CS tensor components and chemical bonding. Our results suggest that solid-state (17)O NMR can potentially be used to probe the ionization state of tyrosine side chains in proteins.