An aggregating elastin-like pentapeptide

Recombinant fibrous protein biomaterials meet skin tissue engineering

Natural biomaterials, particularly fibrous proteins, are extensively utilized in skin tissue engineering. However, their application is impeded by batch-to-batch variance, limited chemical or physical versatility, and environmental concerns. Recent advancements in gene editing and fermentation technology have catalyzed the emergence of recombinant fibrous protein biomaterials, which are gaining traction in skin tissue engineering. The modular and highly customizable nature of recombinant synthesis enables precise control over biomaterial design, facilitating the incorporation of multiple functional motifs. Additionally, recombinant synthesis allows for a transition from animal-derived sources to microbial sources, thereby reducing endotoxin content and rendering recombinant fibrous protein biomaterials more amenable to scalable production and clinical use. In this review, we provide an overview of prevalent recombinant fibrous protein biomaterials (collagens, elastin, silk proteins and their chimeric derivatives) used in skin tissue engineering (STE) and compare them with their animal-derived counterparts. Furthermore, we discuss their applications in STE, along with the associated challenges and future prospects.

Coarse-grained model of tropoelastin self-assembly into nascent fibrils

Elastin is the dominant building block of elastic fibers that impart structural integrity and elasticity to a range of important tissues, including the lungs, blood vessels, and skin. The elastic fiber assembly process begins with a coacervation stage where tropoelastin monomers reversibly self-assemble into coacervate aggregates that consist of multiple molecules. In this paper, an atomistically based coarse-grained model of tropoelastin assembly is developed. Using the previously determined atomistic structure of tropoelastin, the precursor molecule to elastic fibers, as the basis for coarse-graining, the atomistic model is mapped to a MARTINI-based coarse-grained framework to account for chemical details of protein-protein interactions, coupled to an elastic network model to stabilize the structure. We find that self-assembly of monomers generates up to ∼70 ​nm of dense aggregates that are distinct at different temperatures, displaying high temperature sensitivity. Resulting assembled structures exhibit a combination of fibrillar and globular substructures within the bulk aggregates. The results suggest that the coalescence of tropoelastin assemblies into higher order structures may be reinforced in the initial stages of coacervation by directed assembly, supporting the experimentally observed presence of heterogeneous cross-linking. Self-assembly of tropoelastin is driven by interactions of specific hydrophobic domains and the reordering of water molecules in the system. Domain pair orientation analysis throughout the self-assembly process at different temperatures suggests coacervation is a driving force to orient domains for heterogeneous downstream cross-linking. The model provides a framework to characterize macromolecular self-assembly for elastin, and the formulation could easily be adapted to similar assembly systems.

Self-assembly in elastin-like recombinamers: a mechanism to mimic natural complexity

The topic of self-assembled structures based on elastin-like recombinamers (ELRs, i.e., elastin-like polymers recombinantly bio-produced) has released a noticeable amount of references in the last few years. Most of them are intended for biomedical applications. In this review, a complete revision of the bibliography is carried out. Initially, the self-assembly (SA) concept is considered from a general point of view, and then ELRs are described and characterized based on their intrinsic disorder. A classification of the different self-assembled ELR-based structures is proposed based on their morphologies, paying special attention to their tentative modeling. The impact of the mechanism of SA on these biomaterials is analyzed. Finally, the implications of ELR SA in biological systems are considered.

Tuning self-assembly in elastin-derived peptides

Elastin-derived peptides are gaining increasing interest as potential biomaterials. Previous studies have demonstrated that short elastin-derived peptides are able to self-assemble into fibrils as the entire elastin protein. The motif responsible for that is the XGGZG motif at least three-fold repeated. In this work we have synthesized and studied, at molecular and supramolecular levels, four pentadecapeptides obtained by switching the X and Z residue with leucine and/or valine. We found that the four peptides formed different supramolecular structures corresponding to specific molecular conformations. Our results show that not only the residue type but also the exact position occupied by the residue in the motif is crucial in driving the self-aggregation. The aim of this work is to provide the basis for designing elastin-derived peptides with tunable supramolecular architecture.

Coacervation of tropoelastin

The coacervation of tropoelastin represents the first major stage of elastic fiber assembly. The process has been modeled in vitro by numerous studies, initially with mixtures of solubilized elastin, and subsequently with synthetic elastin peptides that represent hydrophobic repeat units, isolated hydrophobic domains, segments of alternating hydrophobic and cross-linking domains, or the full-length monomer. Tropoelastin coacervation in vitro is characterized by two stages: an initial phase separation, which involves a reversible inverse temperature transition of monomer to n-mer; and maturation, which is defined by the irreversible coalescence of coacervates into large species with fibrillar structures. Coacervation is an intrinsic ability of tropoelastin. It is primarily influenced by the number, sequence, and contextual arrangement of hydrophobic domains, although hydrophilic sequences can also affect the behavior of the hydrophobic domains and thus affect coacervation. External conditions including ionic strength, pH, and temperature also directly influence the propensity of tropoelastin to self-associate. Coacervation is an endothermic, entropically-driven process driven by the cooperative interactions of hydrophobic domains following destabilization of the clathrate-like water shielding these regions. The formation of such assemblies is believed to follow a helical nucleation model of polymerization. Coacervation is closely associated with conformational transitions of the monomer, such as increased β-structures in hydrophobic domains and α-helices in cross-linking domains. Tropoelastin coacervation in vivo is thought to mainly involve the central hydrophobic domains. In addition, cell-surface glycosaminoglycans and microfibrillar proteins may regulate the process. Coacervation is essential for progression to downstream elastogenic stages, and impairment of the process can result in elastin haploinsufficiency disorders such as supravalvular aortic stenosis.

Alzheimer's disease: a pathogenetic autoimmune disorder caused by herpes simplex in a gene-dependent manner

Herpes simplex is implicated in Alzheimer's disease and viral infection produces Alzheimer's disease like pathology in mice. The virus expresses proteins containing short contiguous amino acid stretches (5–9aa “vatches” = viralmatches) homologous to APOE4, clusterin, PICALM, and complement receptor 1, and to over 100 other gene products relevant to Alzheimer's disease, which are also homologous to proteins expressed by other pathogens implicated in Alzheimer's disease. Such homology, reiterated at the DNA level, suggests that gene association studies have been tracking infection, as well as identifying key genes, demonstrating a role for pathogens as causative agents. Vatches may interfere with the function of their human counterparts, acting as dummy ligands, decoy receptors, or via interactome interference. They are often immunogenic, and antibodies generated in response to infection may target their human counterparts, producing protein knockdown, or generating autoimmune responses that may kill the neurones in which the human homologue resides, a scenario supported by immune activation in Alzheimer's disease. These data may classify Alzheimer's disease as an autoimmune disorder created by pathogen mimicry of key Alzheimer's disease-related proteins. It may well be prevented by vaccination and regular pathogen detection and elimination, and perhaps stemmed by immunosuppression or antibody adsorption-related therapies.

Supramolecular organization of elastin and elastin-related nanostructured biopolymers

The ultrastructure of elastin has been extensively analyzed by different methodologies. Starting from the first descriptions, where elastin was depicted as an amorphous structure, more complex and, in some cases, varied morphologies were revealed. The supramolecular structures found for elastin have been compared with those found for other elastin-related polypeptides, such as alpha-elastin and tropoelastin, and very similar features emerged. This review will deal with the supramolecular organization exhibited by many elastin-related compounds, starting from elastin, going through polypeptides constituted by different domains of tropoelastin, up to polymers containing repetitive sequences of elastin. In particular, recent developments on biopolymers of general type poly(Val-Pro-Gly-Xaa-Gly) and poly(Xaa-Gly-Gly-Zaa-Gly) (Xaa, Zaa = Val, Leu, Lys, Glu, Orn) obtained either by chemical synthesis or recombinant DNA techniques will be discussed in detail. The general aim is to describe the supramolecular features useful for the identification of elastin-inspired nanostructured biopolymers for developing highly functional and biocompatible vascular grafts as well as scaffolds for tissue regeneration.

Molecular and supramolecular structural studies on human tropoelastin sequences

One of the unusual properties of elastin is its ability to coacervate, which has been proposed to play an important role in the alignment of monomeric elastin for cross-linking into the polymeric elastin matrix. The temperature at which this transition takes place depends on several factors including protein concentration, ionic strength, and pH. Previously, polypeptide sequences encoded by different exons of the human tropoelastin gene have been analyzed for their ability to coacervate and to self-assemble. Few of them were indeed able to coacervate and only one, that encoded by exon 30 (EX30), gave amyloid fibers. In this article, we report on two chemically synthesized peptides-a decapeptide and an octadecapeptide-whose sequences are contained in the longer EX30 peptide and on a polypeptide (EX1-7) of 125 amino-acid residues corresponding to the sequence coded by the exons 1-7 and on a polypeptide (EX2-7) of 99 amino-acid residues encoded by exons 2-7 of human tropoelastin obtained by recombinant DNA techniques. Molecular and supramolecular structural characterization of these peptides showed that a minimum sequence of approximately 20 amino acids is needed to form amyloid fibers in the exon 30-derived peptides. The N-terminal region of mature tropoelastin (EX2-7) gives rise to a coacervate and forms elastinlike fibers, whereas the polypeptide sequence containing the signal peptide (EX1-7) forms mainly amyloid fibers. Circular dichroism spectra show that beta-structure is ubiquitous in all the sequences studied, suggesting that the presence of a beta-structure is a necessary, although not sufficient, requirement for the appearance of amyloid fibers.

Transformation of Amyloid-like Fibers, Formed from an Elastin-Based Biopolymer, into a Hydrogel: An X-ray Photoelectron Spectroscopy and Atomic Force Microscopy Study

Previous studies have revealed the propensity of elastin-based biopolymers to form amyloid-like fibers when dissolved in water. These are of interest when considered as "ancestral units" of elastin in which they represent the simplest sequences in the hydrophobic regions of the general type XxxGlyGlyZzzGly (Xxx, Zzz = Val, Leu). We normally refer to these biopolymers based on elastin or related to elastin units as "elastin-like polypeptides". The requirement of water for the formation of amyloids seems quite interesting and deserves investigation, the water representing the natural transport medium in human cells. As a matter of fact, the "natural" supramolecular organization of elastin is in the form of beaded-string-like filaments and not in the form of amyloids whose "in vivo" deposition is associated with some important human diseases. Our work is directed, therefore, to understanding the mechanism by which such hydrophobic sequences form amyloids and any conditions by which they might regress to a non-amyloid filament. The elastin-like sequence here under investigation is the ValGlyGlyValGly pentapeptide that has been previously analyzed both in its monomer and polymer form. In particular, we have focused our investigation on the apparent stability of amyloids formed from poly(ValGlyGlyValGly), and we have observed these fibers evolving to a hydrogel after prolonged aging in water. We will show how atomic force microscopy can be combined with X-ray photoelectron spectroscopy to gain an insight into the spontaneous organization of an elastin-like polypeptide driven by interfacial interactions. The results are discussed also in light of fractal-like assembly and their implications from a biomedical point of view.

Dissection of human tropoelastin: supramolecular organization of polypeptide sequences coded by particular exons

Polypeptide sequences encoded by some exons of the human tropoelastin gene (EDP, elastin-derived peptide) have been analysed for their ability to coacervate and to self-assembly. The great majority of them were shown to form organized structures, but only a few were indeed able to coacervate. Negative staining and rotary shadowing transmission electron microscopy showed the polypeptides to adopt a variety of supramolecular organization, from filaments, as those typical of tropoelastin, to amyloid-like fibers. The results obtained gave significant insight to the possible roles played by specific polypeptide sequences of tropoelastin.

Supramolecular Amyloid-like Assembly of the Polypeptide Sequence Coded by Exon 30 of Human Tropoelastin

Elastin is known to self-aggregate in twisted-rope filaments. However, an ultrastructural organization different from the fibrils typical of elastin, but rather similar to those shown by amyloid networks, is shown by the polypeptide sequence encoded by exon 30 of human tropoelastin. To better understand the molecular properties of this sequence to give amyloid fibers, we used CD, NMR, and FTIR (Fourier transform infrared spectroscopy) to identify the structural characteristics of the peptide. In this study, we have demonstrated, by FTIR, that antiparallel beta-sheet conformation is predominant in the exon 30 fibers. These physical-chemical studies were combined with transmission electron microscopy and atomic force microscopy to analyze the supramolecular structure of the self-assembled aggregate. These studies show the presence of fibrils that interact side-by-side probably originating from an extensive self-interaction of elemental cross beta-structures. Similar sequences, of the general type XGGZG(X, Z = V, L, A, I), are widely found in many proteins such as collagens IV and XVII, major prion protein precursor, amyloid beta A4 precursor protein-binding family, etc., thus suggesting that this sequence could be involved in contributing to the self-assembly of amyloid fibers even in other proteins.

AFM Study of the Elastin-like Biopolymer Poly(ValGlyGlyValGly)

In this paper, we report an AFM study on the supramolecular structures adopted by the synthetic polypentapeptide poly(ValGlyGlyValGly), whose monomeric sequence is an abundant, simple building block of elastin. The polypeptide was analyzed by deposition from both methanolic and aqueous suspensions, showing different behaviors. In methanol, the polypeptide is able to evolve, in a time-dependent way, from layers to ribbons to beaded filaments. When the equilibrium is reached, the formation of well-defined dendritic structures is also observed. This restructuring of the polypentapeptide seems to be reminiscent of a sort of Rayleigh instability. When deposited from aqueous suspensions, the polypeptide self-assembles either in fibrillar networks or in amyloid-like patterns, both of them being found in elastin or elastin-related polypeptides. As a general finding, poly(ValGlyGlyValGly) seems to constitute an excellent mimetic of the supramolecular properties of native elastin.

Recombinant human elastin polypeptides self-assemble into biomaterials with elastin-like properties

Processes involving self-assembly of monomeric units into organized polymeric arrays are currently the subject of much attention, particularly in the areas of nanotechnology and biomaterials. One biological example of a protein polymer with potential for self-organization is elastin. Elastin is the extracellular matrix protein that imparts the properties of extensibility and elastic recoil to large arteries, lung parenchyma, and other tissues. Tropoelastin, the approximately 70 kDa soluble monomeric form of elastin, is highly nonpolar in character, consisting essentially of 34 alternating hydrophobic and crosslinking domains. Crosslinking domains contain the lysine residues destined to form the covalent intermolecular crosslinks that stabilize the polymer. We and others have suggested that the hydrophobic domains are sites of interactions that contribute to juxtaposition of lysine residues in preparation for crosslink formation. Here, using recombinant polypeptides based on sequences in human elastin, we demonstrate that as few as three hydrophobic domains flanking two crosslinking domains are sufficient to support a self-assembly process that aligns lysines for zero-length crosslinking, resulting in formation of the crosslinks of native elastin. This process allows fabrication of a polymeric matrix with solubility and mechanical properties similar to those of native elastin.

13C CPMAS NMR studies of the elastin-like polypeptide (LGGVG)n

The elucidation of structure-function relationships in insoluble elastin is often approached using elastin-like polypeptides. In this manner, the characterization of the different regions in this extensive biopolymer may be facilitated in a "piece-wise" manner. Our solid-state NMR experiments indicate that (LGGVG)n has structural similarities to elastin and some elastin peptides, providing support for the utility of the mimetic peptides. Furthermore, previous NMR and CD studies indicated that the structure of the elastin-like polypeptide (LGGVG)n in solution is best described as a "conformational ensemble" with a mixture of type I and II beta-turns, in addition to unfolded regions. Our data indicate that the peptide does not adopt a single conformation in the solid state, lending further support to models for elastin that involve significant conformational heterogeneity.

Sequence and structure determinants for the self-aggregation of recombinant polypeptides modeled after human elastin

Elastin is a polymeric structural protein that imparts the physical properties of extensibility and elastic recoil to tissues. The mechanism of assembly of the tropoelastin monomer into the elastin polymer probably involves extrinsic protein factors but is also related to an intrinsic capacity of elastin for ordered assembly through a process of hydrophobic self-aggregation or coacervation. Using a series of simple recombinant polypeptides based on elastin sequences and mimicking the unusual alternating domain structure of native elastin, we have investigated the influence of sequence motifs and domain structures on the propensity of these polypeptides for coacervation. The number of hydrophobic domains, their context in the alternating domain structure of elastin, and the specific nature of the hydrophobic domains included in the polypeptides all had major effects on self-aggregation. Surprisingly, in polypeptides with the same number of domains, propensity for coacervation was inversely related to the mean Kyte-Doolittle hydropathy of the polypeptide. Point mutations designed to increase the conformational flexibility of hydrophobic domains had the unexpected effect of suppressing coacervation and promoting formation of amyloid-like fibers. Such simple polypeptides provide a useful model system for understanding the relationship between sequence, structure, and mechanism of assembly of polymeric elastin.

Model peptide studies of sequence regions in the elastomeric biomineralization protein, Lustrin A. I. The C-domain consensus-PG-, -NVNCT-motif

The lustrin superfamily represents a unique group of biomineralization proteins localized between layered aragonite mineral plates (i.e., nacre layer) in mollusk shell. Recent atomic force microscopy (AFM) pulling studies have demonstrated that the lustrin-containing organic nacre layer in the abalone, Haliotis rufescens, exhibits a typical sawtooth force-extension curve with hysteretic recovery. This force extension behavior is reminiscent of reversible unfolding and refolding in elastomeric proteins such as titin and tenascin. Since secondary structure plays an important role in force-induced protein unfolding and refolding, the question is, What secondary structure(s) exist within the major domains of Lustrin A? Using a model peptide (FPGKNVNCTSGE) representing the 12-residue consensus sequence found near the N-termini of the first eight cysteine-rich domains (C-domains) within the Lustrin A protein, we employed CD, NMR spectroscopy, and simulated annealing/minimization to determine the secondary structure preferences for this sequence. At pH 7.4, we find that the 12-mer sequence adopts a loop conformation, consisting of a "bend" or "turn" involving residues G3-K4 and N7-C8-T9, with extended conformations arising at F1-G3; K4-V6; T9-S10-G11 in the sequence. Minor pH-dependent conformational effects were noted for this peptide; however, there is no evidence for a salt-bridge interaction between the K4 and E12 side chains. The presence of a loop conformation within the highly conserved -PG-, -NVNCT- sequence of C1-C8 domains may have important structural and mechanistic implications for the Lustrin A protein with regard to elastic behavior.

Elastin-based biopolymers: chemical synthesis and structural characterization of linear and cross-linked poly(OrnGlyGlyOrnGly)

Poly(OrnGlyGlyOrnGly) was synthesized by classical procedures in solution. The monomeric sequence -OrnGlyGlyOrnGly- was chosen as a modification of -ValGlyGlyValGly-, typical of elastin, to impart primary amine functionality, susceptible to cross-linking with appropriate bifunctional reagents. Herein we focus on the cross-linking of poly(OrnGlyGlyOrnGly) with glutaraldehyde. The polymers, both linear and cross-linked, were characterized and investigated for their molecular and supramolecular properties. Circular dichroism studies performed on linear poly(OrnGlyGlyOrnGly) revealed a variety of conformations similar to elastin. At a supramolecular level, different kinds of aggregates were found such as the elastin-like twisted-rope pattern of filaments and fibrils, together with other specific morphologies, similar to those recently identified in some elastin-mimetic polypeptides.

Elastin as a self-organizing biomaterial: use of recombinantly expressed human elastin polypeptides as a model for investigations of structure and self-assembly of elastin

Elastin is the major extracellular matrix protein of large arteries such as the aorta, imparting characteristics of extensibility and elastic recoil. Once laid down in tissues, polymeric elastin is not subject to turnover, but is able to sustain its mechanical resilience through thousands of millions of cycles of extension and recoil. Elastin consists of ca. 36 domains with alternating hydrophobic and cross-linking characteristics. It has been suggested that these hydrophobic domains, predominantly containing glycine, proline, leucine and valine, often occurring in tandemly repeated sequences, are responsible for the ability of elastin to align monomeric chains for covalent cross-linking. We have shown that small, recombinantly expressed polypeptides based on sequences of human elastin contain sufficient information to self-organize into fibrillar structures and promote the formation of lysine-derived cross-links. These cross-linked polypeptides can also be fabricated into membrane structures that have solubility and mechanical properties reminiscent of native insoluble elastin. Understanding the basis of the self-organizational ability of elastin-based polypeptides may provide important clues for the general design of self-assembling biomaterials.

On (GGLGY) synthetic repeating sequences of lamprin and analogous sequences

The repetitive sequence GGLGY was found in lamprin, the most important matrix protein of lamprey annular cartilage by Keeley and co-workers. Similar sequences appear also in other proteins, i.e. elastin, spidroin, spider minor ampullate silk proteins, in matrix proteins of the chorion or egg shell membrane of insects and others. We synthesized (GGLGY)n, n=1, 2, 6, because the sequence is repeated six times in the aggregated protein. The peptides were studied both in solution and in the solid state. Because the CD spectra were dominated by aromatic contribution, we synthesized GGLGF and GGLGA in order to carefully interpret the CD spectra. The conformational analysis suggests that all synthetic peptides do adopt the same secondary structure. In solution the peptides present a flexible conformation with a significant amount of PPII structure. In the solid state PPII, beta-pleated-sheets and beta-turns possibly co-exist.

Chemical synthesis of cross-linked poly(KGGVG), an elastin-like biopolymer

Previous studies afforded on peptides and polypeptides containing repetitive sequences of elastin have largely demonstrated that their molecular and supramolecular properties are fully representative of those of tropoelastin, the soluble, linear precursor of elastin itself. In the attempt to synthesize cross-linked elastin-mimetic polypeptides, the repeating sequence VGGVG (V: valine; G: glycine), typical of elastin, was modified to incorporate lysine residues, yielding the polymer poly(KGGVG) (K: lysine). This imparts primary amine functionality susceptible to cross-linking reaction with appropriate bifunctional cross-linking reagents. We report herein the chemical synthesis and cross-linking of poly(KGGVG) with glutaraldehyde (GTA) and with disuccinimidyl glutarate (DSG). In both cases, the characterization of the polymers, both linear and cross-linked, has been carried out by CD spectroscopy and transmission electron microscopy measurements. The obtained results, although not conclusive, demonstrate that poly(KGGVG), both linear and cross-linked, may be considered very similar to tropoelastin and mature elastin, as concerns its molecular and supramolecular properties.

Hydrolysis of glycine-containing elastin pentapeptides by LasA, a metalloelastase from Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen that causes severe infections in vulnerable hosts. It may produce various virulence factors including proteases. Among them, LasA possesses both elastolytic and staphylolytic (hydrolysis of pentaglycine cross-links in the cell wall peptidoglycan) activities. To understand if its elastolytic activity results from a preference for glycine-rich substrates, we studied its ability to hydrolyse the 65 pentapeptides of human tropoelastin containing at least three glycines. As demonstrated by capillary electrophoresis (CE), 22 of these peptides were hydrolysed by LasA, generally at a single peptide bond and the catalytic ratio kcat/KM was determined for most of them. The highest value was obtained for LGGGA, 59 +/- 9 min(-1) x mmol(-1) x L. The specificity of hydrolysis was elucidated by CE, liquid secondary ion mass spectrometry and, in some cases, collision activated dissociation-mass analysis of ion kinetic energy. The preferred cleavage sites are GG and GA peptide bonds, the sequence GG(cleavage site)A being especially sensitive to hydrolysis. Both positions P2 and P'2 must be occupied for hydrolysis and the presence of an amino acid in P3 (but not in P'3) significantly increases the catalytic ratio. Considering these results, about 30 GGX sequences (X: G, A or Y) of human tropoelastin could be susceptible to LasA elastolysis.

Deficient coacervation of two forms of human tropoelastin associated with supravalvular aortic stenosis

Human tropoelastin associates by coacervation and is subsequently cross-linked to make elastin. In Williams syndrome, defective elastin deposition is associated with hemizygous deletion of the tropoelastin gene in supravalvular aortic stenosis (SVAS). Remarkably, point-mutation forms of SVAS correspond to incomplete forms of tropoelastin which include in-frame termination by nonsense mutations, yet the resulting phenotype of these disorders is not explained because expression variably occurs from both normal and mutant alleles. Proteins corresponding to two truncated tropoelastin mutants were expressed and purified to homogeneity. Coacervation of these proteins occurred as expected with increasing temperature, but substantially contrasted with that of the performance of a normal tropoelastin. Significantly, association by coacervation of the truncated SVAS tropoelastin molecules was negligible at 37 degrees C, which contrasted with the substantial coacervation seen for normal tropoelastin. Furthermore their midpoints of coacervation increased and correlated with the extent of deletion, in accord with the loss of hydrophobic regions required for tropoelastin association. Their secondary structures are similar, as evidenced by CD studies. We propose a model for point-mutation SVAS in which aberrant tropoelastin molecules are incompetent and are mainly excluded from participation in coacervation and consequently in elastogenesis. These forms of SVAS may consequently be considered functionally similar to a hemizygous deletion, and mark point-mutation SVAS as a disorder of defective coacervation.

Model peptide studies of sequence repeats derived from the intracrystalline biomineralization protein, SM50. I. GVGGR and GMGGQ repeats

We report solution-state pulsed field gradient nmr studies of a native sequence-derived 23-residue peptidomimetic, N alpha-acetyl-QPGVGGRQPGMGGQPGVGGRQPG-C alpha-amid, that incorporates the prevalent GVGGR and GMGGQ repeats found in the sea urchin embryo intracrystalline spicule matrix protein, SM50 (Strongylocentrotus purpuratus). These repeats are sequence homologues of elastin protein repeats (VPGVG, VGGVG, and APGVGV) and spider dragline silk protein repeats (GPGG, GQGG, and QPGYG). Using rotating frame nuclear Overhauser effect (ROE) connectivities, CH alpha proton conformational shifts, 3JNH-CH alpha coupling constants, amide temperature shift coefficients, and pulsed field gradient ROE spectroscopy solvent exchange measurements, we find that the 23-mer peptidomimetic possesses a multiple beta-turn structure in aqueous solution, in equilibria with an extended or coil structure (60% beta-turn: 40% random coil). The GVGGR sequence adopts a double beta-turn conformation that is stabilized by two hydrogen bonds (R7-->V4, R20-->V17; G6-->G3, G19-->G16). The GMGGQ region adopts a single beta-turn conformation that is stabilized by a hydrogen bond involving residues Q14 and M11. Repeating beta-turn structures, or beta-spirals, may play an important role with regard to matrix assembly, protein stability, molecular elasticity, and/or protein-crystal recognition within the spicule mineralized matrix.

Elastin: molecular description and function

Elastin, the protein responsible for the elastic properties of vertebrate tissues, has been thought to be solely restricted to that role. As a consequence, elastin was conventionally described as an amorphous polymer. Recent results in the biomedical, biochemical and biophysical fields have lead to the conclusion that the presence of elastin in the extracellular space has very complex implications involving many other molecules. The present review describes the current state of knowledge concerning elastin as an elastic macromolecule. First, the genetic, biological, biochemical and biophysical processes leading to a functional polymer are described. Second, the elastic function of elastin is discussed. The controversy on elastin structure and elasticity is discussed and a novel dynamic mechanism of elasticity proposed. Finally, pathologies where the elastin molecule is involved are considered. This updated description of functional elastin provides the required background for the understanding of its pathologies and defines clearly the properties a substance should possess to be qualified as a good elastic biomaterial.

Quantum molecular modeling of the elastinic tetrapeptide Val-Pro-Gly-Gly

The free Val-Pro-Gly-Gly tetrapeptide belonging to the Proline-rich sequences of elastin has been studied both theoretically and experimentally. The molecular modelisation was carried out using AM1 and ab initio quantum computations while the conformation in solution was ascertained by circular dichroism spectroscopy performed on the synthesized tetrapeptide. Experimental and theoretical investigations lead to the conclusion that the most probable structure is constituted by a type II beta-turn.

Structure and dynamics of elastin building blocks. Boc-LG-OEt, Boc-VGG-OH

Short di- and tripeptides such as Boc-LG-OEt, Boc-VG-OEt and Boc-VGG-OH, corresponding to abundant repetitive sequences in elastin, have been extensively studied both in solid state, by X-ray diffraction, and in solution by circular dicroism and nuclear magnetic resonance. Furthermore, theoretical procedures such as simulated annealing and molecular dynamics were also performed on these peptides. In general, the results indicate that no one single structure (be folded or extended) could be representative for these sequences in the protein, but rather that a multiplicity of interconverting conformers, ranging from folded to extended structures, should be considered. In any case, these structures, e.g. beta-turns, polyglycine II and beta-conformations, are those previously suggested to participate to conformational equilibria of elastin.

Coacervation characteristics of recombinant human tropoelastin

Coacervation of soluble tropoelastin molecules is characterized by thermodynamically reversible association as temperature is increased under appropriately juxtaposed ionic conditions, protein concentration and pH. Coacervation plays a critical role in the assembly of these elastin precursors in elastic fiber formation. To examine the effect of physiological parameters on the ability of tropoelastin molecules to associate, solutions of recombinant human tropoelastin were monitored spectrophotometrically by light scattering over a broad range of temperatures. Coacervation of recombinant human tropoelastin is strongly influenced by the concentration of protein and NaCl and to a lesser extent on pH. Trends towards maximal association are apparent when each of these parameters is varied. Remarkably, optimal coacervation is found at 37 degrees C, 150 mM NaCl and pH 7-8. Using the data generated by time courses, estimates of thermodynamic parameters were made. These estimates confirm that coacervation is endothermic and is marked by a strong entropic contribution. Circular dichroism of recombinant human tropoelastin revealed that, rather than being random, the structure is compatible with being largely that, of an all-beta protein (with secondary structure estimated to be 3% alpha-helix, 41% beta-sheet, 21% beta-turn and 33% other), exhibiting a spectrum as previously seen for tropoelastin populations and soluble elastin from naturally-derived sources.

Conformational and electrostatic properties of V-G-G-V-G, a typical sequence of the glycine-rich regions of elastin. An ab initio quantum molecular study

The conformational analysis and electrostatic properties of the monomeric sequence V-G-G-V-G of the glycine-rich regions of elastin is presented with the aim of explaining NMR and CD experimental results. On the basis of the molecular model NH+3-V-G-G-V-G-COO, Gaussian 92 quantum-molecular computations were performed by using principally an ab initio method at the 3-21G level and AM1. The occurrence of local secondary structures and of beta I, beta II, beta II' and VIa turns is discussed. Our results clearly demonstrate that the transconformations beta I-->half turn (which was invoked to explain experimental results) and beta I-->beta II' are theoretically allowed.