Elasticity, elastin

Elastin. Elastic, load-bearing protein fibers of ani-... 

The first elastomeric protein is elastin, this structural protein is one of the main components of the extracellular matrix, which provides stmctural integrity to the tissues and organs of the body. This highly crosslinked and therefore insoluble protein is the essential element of elastic fibers, which induce elasticity to tissue of lung, skin, and arteries. In these fibers, elastin forms the internal core, which is interspersed with microfibrils [1,2]. Not only this biopolymer but also its precursor material, tropoelastin, have inspired materials scientists for many years. The most interesting characteristic of the precursor is its ability to self-assemble under physiological conditions, thereby demonstrating a lower critical solution temperature (LCST) behavior. This specific property has led to the development of a new class of synthetic polypeptides that mimic elastin in its composition and are therefore also known as elastin-like polypeptides (ELPs). 

This coacervation process forms the basis for the self-assembly, which takes place prior to the crosslinking. The assembly of tropoelastin is based on an ordering process, in which the polypeptides are converted from a state with little order to a more structured conformation [8]. The insoluble elastic fiber is formed via the enzymatic crosslinking of tropoelastin (described in Sect. 2.1). Various models have been proposed to explain the mechanism of elasticity of the elastin fibers. 

Resilin and elastin, unlike other structural proteins, fulfill both definitions of an elastic material. Colloquially speaking, resilin and elastin are stretchy or flexible. They also fulfill the strict definition of an elastic material, i.e., the ability to deform in proportion to the magnitude of an applied stress without a loss of energy, and the recovery of the material to its original state when that stress is removed. Resilin and elastin are alone in the category of structural proteins (e.g., collagen, silk, etc.) in that they have the correct blend of physical properties that allow the proteins to fulfill both definitions of elasticity. Both proteins have high extensibility and combine that property with remarkable resilience [208]. 

Resilin and elastin have relatively high extensibility and resilience, but as compared to the collagen and the silks, the proteins sacrifice stiffness (elastic modulus) and strength (see Table 2). Collagen and dragUne sflk are much stiffer materials, but lack the extensibility that is characteristic of the rubber-like proteins. On the other hand, the mussel byssus fibers and the viscid silk have the extensibility of resilin and elastin, but lack the resilience [208]. 

Elastin confers extensibihty and elastic recoil on tissues. Elastin lacks hydroxylysine, Gly-X-Y sequences, triple hehcal stmcture, and sugars but contains desmosine and isodesmosine cross-links not found in collagen. 

Elastin-mimetic protein polymers have been fabricated into elastic networks primarily via y-radiation-induced, radical crosslinking of the material in the coacervate state [10]. Although effective, this method cannot produce polymers gels of defined molecular architecture, i.e., specific crosslink position and density, due to the lack of chemoselectivity in radical reactions. In addition, the ionizing radiation employed in this technique can cause material damage, and the reproducibility of specimen preparations may vary between different batches of material. In contrast, the e-amino groups of the lysine residues in polymers based on Lys-25 can be chemically crosslinked under controllable conditions into synthetic protein networks (vide infra). Elastic networks based on Lys-25 should contain crosslinks at well-defined position and density, determined by the sequence of the repeat, in the limit of complete substitution of the amino groups. 

The pleural tissue is a typical connective tissue that consists mostly of matrix the fibrous proteins (collagen, elastin), and mucopolysaccharides, and a few scattered mesothelial cells, capillaries, venules, and ducts. Anatomists have defined several layers (Fig. 3.4) for each of the pleura. Layers 3 and 5 in Fig. 3.4 contain an abundance of fibrous protein, especially elastin. Both the interstitial (Layer 4) and mesothelial (1 and 2) layers contain capillaries of the vascular system and lymphatic channels. The matrix (ground substance) gives the pleura structural integrity and is responsible for its mechanical properties such as elasticity and distensibility. 

Collagens (see p. 344), of which there are at least 19 different varieties, form fibers, fibrils, networks, and ligaments. Their characteristic properties are tensile strength and flexibility. Elastin is a fiber protein with a high degree of elasticity. 

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