Structure of elastin

From the foregoing sections of this review we may conclude that the elastic fibers of yellow connective tissue are composed in the main of a protein which is characteristic of this type of tissue and which is quite distinct from collagen or other fibrous proteins. Elastic fibers have characteristic staining properties, but they vary in thickness and morphological 

It seems therefore that the protein of the microfibrils is probably homogeneous, and closely similar in amino acid composition for all the tissues of a single mammalian species. No general survey of the elastic tissues of the manunals has yet been undertaken but there is enough evidence to 

Partridge et al. (W ib) observed that when elastin from ligamentum nuchae of cattle was repeatedly extracted with 0.2. 5 M oxalic acid at 100°C the fibers completely dissolved after about 5 hr total extraction. On dialysis in cellophane only about. 5 % of the total nitrogen of the reaction mixture diffused through the membrane. The bulk of the product was a protein which was soluble in distilled water or buffer solutions at temperatures below 2f °C, but on raising the temperature of the solution in dilute buffer at pH 4-6 a coacervate phase consisting of liquid droplets separated. The soluble material was thus similar in properties to the hemi-elastin of Horbaczewski (1882). 

The soluble protein showed a single boundary peak in the Tiselius apparatus in buffers of ionic strength 0.02 at all pH values in the range pH 2-9, but its isoelectric point was markedly dependent upon the salt concentration. At ionic strength 0.2, in the presence of sodium chloride, the isoelectric point both from electrophoretic mobility measurements and membrane potential determinations was pH 3.9-4,0. At lower ionic strength (0.02) the protein was isoelectric at pH 4.8 in the electrophoresis experiments and pH 4.7 in membrane potential determinations. 

The amino acid analysis of the a- and d-proteins was compared with that of the purified elastin fibers from which they were prepared. It was found that the composition of the three proteins was closely similar thus showing that the original fibers must be homogeneous as regards amino acid composition even though they show a fibrillar structure. There were 

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Native elastin is an insoluble, highly cross-linked protein. The chemistry, properties and structure of elastin have been frequently reviewed in the past330-333. In this context, aspects of calcification processes will be discussed. 

The tight y turn215 and the proline-containing P turn shown in Fig. 2-24 are thought to be major components of the secondary structure of elastin.216-218 This stretchable polymer, which consists largely of nonpolar amino acids, is the most abundant protein of the elastic fibers of skin, lungs, and arteries. The... 

Numerous studies have been undertaken to elucidate the secondary structure of soluble elastin. These studies have been performed on elastin, elastin solubilized by oxalic acid (a-elastin) or potassium hydroxide (/, -elastin). synthetic polypeptide models of elastin, and tropoelastin. Techniques used include circular dichroism, FT-Raman, and electron microscopy. No consensus has been reached on the overall structure of elastin. 

The four protein conformations that provide mechanical stability to cells, tissues, and organs include the random coil or amorphous structure that characterizes a part of the structure of elastin, the a helix, which is represented by the keratin molecule, the collagen triple helix, and the p structure of silk. In humans the P structure is found only in short sequences connecting parts of other structures such as the a helix, but serves as an example of the relationship between protein structure and properties. The ultimate tensile strength and modulus of each structure differs as discussed below. 

Fig. 5. Structure of elastin around the ohromophoric nucleus (indicated by the circle). The nucleus links two short peptide chains together. The structure is represented as the DNP-derivative of the peptide isolated, which contains glycine (1 mole), proline (1 mole), and alanine (2 moles). One mole of alanine is A -terminal but the position of the other three amino acid residues is not known (Thomas and Partridge, 1962).

Several models of the macromolecular structure of elastin have been suggested to account for its elasticity cross-linked globular elastin subunits, cross-linked... 

Hydrophilic cross-linking exons Hydrophobic exons Fig. 49.6. The cDNA structure of elastin, indicating the repeating cross-linking and hydrophobic domains. 

Debelle L, Alix AJP. The structures of elastins and their function. Biochimie. 1999 81 981-94. 

The amino acid sequence (Val-Pro-Gly-Val-Gly) occurs frequently in the primary structure and is responsible for the (3-tums in the secondary structure of elastin. 

Keeley, F.W., Bellingham, C.M., and Woodhouse, K.A., Elastin as a self-organising biomaterial Use of recombinantly expressed human elastin polypeptides as a model system for investigations of structure and self-assembly of elastin, Philos. Trans. R. Soc. Lond. B Biol. Sci., 357, 185-189, 2002. 

Miao M, Cirulis JT, Lee S et al (2005) Structural determinants of cross- linking and hydrophobic domains for self-assembly of elastin-like polypeptides. Biochemistry 44 14367-14375... 

Nakamura F, Yamazaki K and Suyama K (1992) Isolation and structural characterization of a new crosslinking amino acid, cyclopentenosine, from the acid hydrolysate of elastin. Biochem Biophys Res Comm 186, 1533-1538. 

Secondary structures for proteins are generally fibrous and globular. Proteins such as keratins, collagen, and elastin are largely fibrous and have secondary structures of sheets and helices. Many of the globular proteins are composed of protein chains present in secondary structures approximating helices and sheets. 

Structure of tropoelastin Elastin is synthesized from a precursor, tropoelastin, that is rich in proline and lysine, but contains only a little hydroxyproline and no hydroxylysine. In the extracellular matrix, tropoelastin is converted to elastin. 

Figure 2-24 Tight turns found in polypeptide chains. Two types of 3 turn are shown. A third variant, the type III or 310 turn resembles the type I turn but has the cp, y angles of a 310 helix. Type II P turns containing proline and tighter y turns are thought to be major structural components of elastin. Another P turn, lacking the hydrogen bond has a cz s-proline residue at position 3.

One of the key arguments for neutral site binding is the presence of (3-turns and associated conformations. This puts certain restrains on the structure of the fibrous protein. For elastin, conformations with bound calcium are likely to be inside-out with respect to hydrophobicity. Such structures are acceptable only for molecules functioning in a non-polar environment (cell-membranes) but not for a hydrated elastin fibre. Binding of calcium would stabilize a rigid inside-out conformation437. ... 

Tropoelastin molecules are crosslinked in the extracellular space through the action of the copper-dependent amine oxidase, lysyl oxidase. Specific members of the lysyl oxidase-like family of enzymes are implicated in this process (Liu etal, 2004 Noblesse etal, 2004), although their direct roles are yet to be demonstrated enzymatically. Lysyl oxidase catalyzes the oxidative deamination of e-amino groups on lysine residues (Kagan and Sullivan, 1982) within tropoelastin to form the o-aminoadipic-6-semialdehyde, allysine (Kagan and Cai, 1995). The oxidation of lysine residues by lysyl oxidase is the only known posttranslational modification of tropoelastin. Allysine is the reactive precursor to a variety of inter- and intramolecular crosslinks found in elastin. These crosslinks are formed by nonenzymatic, spontaneous condensation of allysine with another allysine or unmodified lysyl residues. Crosslinking is essential for the structural integrity and function of elastin. Various crosslink types include the bifunctional crosslinks allysine-aldol and lysinonorleucine, the trifunctional crosslink merodes-mosine, and the tetrafunctional crosslinks desmosine and isodesmosine (Umeda etal, 2001). 

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