Tropoelastin cross-linking

After secretion from the cell, certain lysyl residues of tropoelastin are oxidatively deaminated to aldehydes by lysyl oxidase, the same enzyme involved in this process in collagen. However, the major cross-links formed in elastin are the desmosines, which result from the condensation of three of these lysine-derived aldehydes with an unmodified lysine to form a tetrafunctional cross-hnk unique to elastin. Once cross-linked in its mature, extracellular form, elastin is highly insoluble and extremely stable and has a very low turnover rate. Elastin exhibits a variety of random coil conformations that permit the protein to stretch and subsequently recoil during the performance of its physiologic functions. 

Sandberg, L. B., Weissman, N., and Gray, W. R. (1971). Structural features of tropoelastin related to the sites of cross-links in aortic elastin. Biochemistry 10, 52-56. 

The basic unit of elastin structure is tropoelastin, which has a molecular weight of about 72,000 and contains 800-850 amino acid residues. It has been proposed that tropoelastin units are present in the random coil conformation and are extensively cross-linked. This makes such a network kinetically free free to stretch and to recoil. It is the entropy effects that permit the stretched elastin... 

With respect to one of these precursors, the term, tropoelastin, has been used as a designation for a non-cross linked elastin precursor of approximately 70,000 daltons (1). Since it is currently the best characterized of the non-crosslinked elastins and is used extensively by those familiar with elastin, this term will be retained. Elastin will be used to designate the protein in its crosslinked form. This term, however, is at best operational, since elastin is only isolated from tissues or cell culture by procedures that would be offensive to most protein chemists. As a component of extracellular matrices, elastin is extremely insoluble and in close association with many other extracellular components (2). In order to remove these components, harsh treatments such as autoclaving, extraction with alkali or... 

The exact form in which non-crosslinked elastin is secreted from smooth muscle cells is yet to be clearly defined. Foster et al. (36) have suggested that a non-cross linked elastin (pro-elastin) is secreted from smooth muscle cells in a form that is approximately 120,000 to 140,000 daltons. They have suggested that proelastin is cleaved to smaller molecular weight forms of non-crosslinked elastin. It should be noted, however, that this view is not entirely supported by data from other laboratories. There are two reports on the use of isolated mRNA from chick aorta suggesting only a 70,000 dalton non-cross linked elastin is the major product of translation (37,38). There is also a recent report suggesting that aortic mRMA translates a 200,000 dalton putative elastin product (39). We have recently isolated a non-crosslinked elastin from the aortas of copper deficient chicks that appears to be 100,000 daltons (27). Its amino acid composition is similar to that for tropoelastin (Table III). A major problem in resolving these points is that the trypsin-like proteinase associated with elastin is not easily denatured or separated from the non-crosslinked forms of elastin. The proteinase is also not readily inhibited by commonly used inhibitors for trypsin-like proteinases (26). 

Figure 2. Synthesis of mature elastin fibers. Some evidence suggests the possibility for proforms to elastin that appear as the first products of translation. These products are cleaved to tropoelastin (27), which appears to combine with microfibrillar protein. Although post-translational events important to the synthesis of the microfibrillar protein have not been defined, it is clear that it is a major component on which is organized or assembled the profibrillar forms of elastin. Cross-linking is catalyzed by lysyl oxidase, a copper-requiring protein (30). Recent information on the elastin proteinase(s) involved in tropoelastolysis would suggest that proteolysis may also play a role in elastin fiber...

For purposes of this manuscript, we wish to concentrate only on the steps leading to the formation of desmosines, amino acids found predominantly in elastin. With respect to their formation, the following suggests their spontaneous formation from peptidyl lysine and the oxidation product, peptidyl allysine. Narayanan et al. (28,29) have shown that when purified lysyl oxidase and non-crosslined elastin, specifically tropoelastin, are incubated together, the desmosines are formed. Desmosine formation, however, only occurs at temperatures that favor fibrillar arrangements of tropoelastin. Subsequently, it is felt that the maturation of non-crosslinked elastin into cross-linked elastin appears to involve only two major steps, namely insolublization through the formation of fibrils and fixation of the fibrils by crosslinking. 

Cross-linking of the tropoelastin molecules within the tropoelastin-fibrillin aggregates is mediated by lysyl oxidase, the same enzyme responsible for cross-linking collagen fibers. In the KA domains, lysine residues are typically found in clusters of two or three amino acids, separated by two or three alanine residues. These regions are proposed to be a-helical with 3.6 residues per turn of helix, which has the effect of positioning two lysine... 

A feature of mature elastin is the presence of covalent cross-links that connect elastin polypeptide chains into a fiber network. The major cross-linkages involve desmo-sine and isodesmosine, both of which are derived from lysine residues. Several regions rich in lysine residues can provide cross-links. Two such regions that contain peptide sequences that are repeated several times in tropoelastin have the primary structure -Lys-Ala-Ala-Ala-Lys- and... 

On secretion from the cell, the tropoelastin is aligned with the microfibrils, and lysyl oxidase initiates the reactions that cross-link elastin molecules, using lysine residues within the hydrophilic alternating domains in the proteins. This cross-linking reaction is the same as that which occurs in collagen. In this reaction, 2, 3, or 4 lysine residues are cross- linked to form a stable stracture. The net result of the cross-linking is the generation of a fibrous mesh that encircles the cells. 

S.M. Mithieux, Y. Tu, E. Korkmaz, F. Braet, A.S. Weiss, In situ polymerization of tropoelastin in the absence of chemical cross-linking, Biomaterials 30 (2009) 431-435. 

Elastin is an elastic insoluble protein in connective tissue, with cross-linked tropoelastin as a major component. It is responsible for the contraction of skin, lung, and vascular tissues in our body [42]. The insolubility and enhanced immune response [43] of native elastin limits its biomedical application and demands new materials for tissues. The ability of elastin to maintain minimum platelet interaction makes it a suitable material for making biological coatings for synthetic vascular grafts [44,45]. Currently used synthetic vascular biomaterials like poly(ethylene tere-phthalate) (Dacron) and poly(tetrafluoroethylene) show... 

L. Nivison-Smith, J. Rnjak, A.S. Weiss, Synthetic human elastin microfibers stable cross-linked tropoelastin and cell interactive constructs for tissue engineering applications, Acta Biomater. 6 (2) (2010) 354-359. 

Covalent cross-linking of polypeptides (e.g., enzyme-based cross-linking via lysine-rich segments in the elastin precursor tropoelastin)... 

Another type of inportant structural ECM proteins, the elastin, is rich in elastic tissues and organs, such as the cardiovascular and pulmonary system and skin, and of extreme importance for their proper function. The precursors of native elastin, the tropoelastins, are composed of alternating hydrophobic domains, enabling hydrophobic interaction, and lysine-rich domains, allowing for covalent cross-linking through the mediation of the enzyme lysyl oxidase. 

D.W. Urry, K. Okamoto, R.D. Harris, CF. Hendrix, and M.M. Long, Synthetic, Cross-Linked Polypentapeptide of Tropoelastin An Anisotropic, Fibrillar Elastomer, Biochemistry 15,4083-4089,1976. 

Urry, D.W., Okamoto, K, Harris, R.D. and Hendrix, C.F. (1976) Synthetic, cross-linked polypentapeptide of tropoelastin an anisotropic fibrillar elastomer. Biochemistry, 15, 4083-4089. 

Narayanan, A. S., Page, R. C., Kuzan, F., and Cooper, C. G., 1978, Elastin cross-linking in vitro. Studies on factors influencing the formation of desmosines by lysyl oxidase action on tropoelastin, Biochem. J. 173 857-862. 

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