Mature elastin

Lysyl oxidase (also called protein-lysine 6-oxidase) Required for cross-linking of collagen and elastin maturation of collagen. 

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. 

Bailey AJ, Ranta MH, Nicholls AC, Partridge SM and Elsden DP (1977) Isolation of a -amino adipic acid from mature dermal collagen and elastin. Evidence for an oxidative pathway in the maturation of collagen and elastin. Biochem Biophys Res Comm 78, 1403-1410. 

Fibrillin microfibrils are widely distributed extracellular matrix assemblies that endow elastic and non elastic connective tissues with long-range elasticity. They direct tropoelastin deposition during elastic fibrillogenesis and form an outer mantle for mature elastic fibers. Microfibril arrays are also abundant in dynamic tissues that do not express elastin, such as the ciliary zonules of the eye. Mutations in fibrillin-1—the principal structural component of microfibrils—cause Marfan syndrome, a heritable disease with severe aortic, ocular, and skeletal defects. Isolated fibrillin-rich microfibrils have a complex 56 nm beads-on-a-string appearance the molecular basis of their assembly and... 

Elastin is a macromolecule synthesized as a 70,000 single peptide chain, termed tropoelastin and secreted into the extracellular matrix where it is rapidly crosslinked to form mature elastin. The carboxy-terminal end of elastin is highly conserved with the sequence Gly-Gly-Ala-Cys-Leu-Gly-Leu-Ala-Cys-Gly-Arg-Lys-Arg-Lys. The two Cys residues that form disulfide crosslinks are found in this region as well as a positively charged pocket of residues that is believed to be the site of interaction with microfibrillar protein residues. Hydrophobic alanine-rich sequences are known to form a helices in elastin these sequences are found near lysine residues that form crosslinks between two or more chains. Alanine residues not adjacent to lysine residues found near proline and other bulky hydrophobic amino acids inhibit a helix formation. Additional evidence exists for (3 structures and 3 turns within elastin thereby giving an overall model of the molecule that contains helical stiff segments connected by flexible segments. 

This review focuses upon the post-translational modification and chemical changes that occur in elastin. Outlined are the steps currently recognized as important in the assembly of pro-fibrillar elastin subunits into mature fibers. Descriptions of some of the proposed mechanisms that appear important to the process are also presented. It will be emphasized that from the standpoint of protein deterioration, elastin is a very novel protein. Under normal circumstances, the final product of elastin metabolism, the elastin fiber does not undergo degradation that is easily measured. Unlike the metabolism of many other proteins, deterioration or degradation is most evident biochemically in the initial stages of synthesis rather than as a consequence of maturation. Since the presence of crosslinks is an essential component of mature elastin, a section of this review also addresses important features of crosslink formation. 

For purposes of definition, we will use the following terms to designate the various forms of elastin. The term, non-cross-linked elastin, will be used as a general description for proposed precursors to mature elastin that appear to be rapidly modified during the initial stages of elastic fiber formation. 

The ratio of microfibrillar protein to elastin, however, appears to decrease upon maturation (2). Other proteins are also secreted with microfibrillar protein and elastin. It is now clear that bound to elastin in its non-crosslink form(s) is a trypsin-like neutral proteinase (26). This proteinase effects... 

AMINO ACID COMPOSITION (EXPRESSED AS RESIDUES PER 1000 TOTAL RESIDUES) OF TYPICAL MATURE ELASTIN, MATRIX COLLAGEN AND MICROFIBRILLAR PROTEIN PREPARATIONS. 

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. 

When the metabolic turnover of elastin in arterial tissue or in lung is examined, it is extremely difficult to demonstrate active turnover. Once an elastin fiber is formed it appears to be fixed. The turnover of rat aorta elastin is best measured in years (8). Data shown in Figure 5 also suggests negliable turnover. The animal used for this study, the Japanese quail, was chosen because it fully matures at 5-6 weeks of age. Similar to the rat its elastin appears to turn over in amounts best estimated in years. 

The work on metabolic turnover underlines the biological and chemical unreactivity of elastin which is perhaps its most characteristic property. Apart from the destructive effects of recognizable disease it has seemed to many authors that the mature elastic fiber, once laid down, is retained for life. In this situation it may be expected that changes may occur in elastic fibers which could be ascribed to the results of aging alone. Such changes have been looked for in a number of laboratories. 

Blomfiel J, Farrar JF (1969) Fluorescent properties of maturing arterial elastin. Cardiovasc Res 3 161-170... 

Elastin is a major structural protein element found in many connective tissues that is known to impart resiliency and elasticity to mechanically loaded tissues. It is therefore no suiprise that the material has become an important tool in tissue engineering research [98]. Mature elastic fibers found in tissues contain structural subunits called tropoelastin, an amorphous 64 kDa monomer composed of 830 amino acids [14]. In the extiacellular space, the cell-synthesized tropoelastin is exposed to lysyl oxidase-mediated oxidation at available lysine residues and then... 

Mature elastin is a linear polypeptide, tropoelastin, which has a molecular weight of about 72,000 and contains about 850 amino acid residues. Although glycine accounts for one third of the residues, the repeat sequence Gly-X-Y characteristic of collagen is not present in elastin. Instead, glycine residues are present in the repeat units Gly-Gly-Val-Pro, Pro-Gly-Val-Gly-Val, and Pro-Gly-Val-Gly-Val-Ala. Elastin is relatively rich in the nonpolar amino acids alanine, valine, and proline. In contrast to collagen, only a few hydroxyproline residues are present in elastin. Elastin contains no hydroxylysine or sugar residues. 

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... 

The turnover rate of mature elastin in healthy persons is relatively low. Insoluble elastin in healthy elastic tissue is usually stable and subjected to minimal proteolytic degradation. In several clinical conditions (e.g., emphysema, advanced atherosclerosis, pancreatitis), increased degradation of fragmentation of elastic fibers may play a significant role. The interaction between insoluble elastin and soluble elastolytic enzymes, and the regulation of these enzymes, may shed light on certain cardiovascular diseases, in view of the role of elastin in arterial dynamics. 

Elastase is the name given to proteinases that possess the ability to hydrolyze mature cross-linked elastin [18]. Elastin is an insoluble structural protein responsible for the elastic properties of the lung, skin, and arteries and is quite resistant to most proteinases. Elastin is high in hydrophobic amino acid residues such as valine, alanine, glycine, and proline [19]. Insoluble elastin fibers contain cross-links usually between four lysine residues, which form a unique cyclic product, desmosine. The presence of soluble desmosine cross-links in plasma can be used as a measure of elastin breakdown. Of all the elastases in humans, neutrophil elastase has received the most attention over the years due to its broad substrate specificity and abundance within the cell. However, neutrophils and macrophages contain several proteinases (Table 1), which are capable of degrading elastin. 

It is well known that repair of tissues damaged as a result of inflammation, a consequence of many varied insults, requires cross-linking and extracellular repair and maturation of collagen and elastin. Since enzymes responsible for this, lysyl oxidases, are copper-dependent [568-572], this aspect of wound or tissue repair assumes particular significance. Lysyl oxidase activity is induced in copper-deficient chickens with copper(II) sulphate [571], and (+ )-catechin... 

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