Secondary structure, elastin

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. 

Collagen and elastin are examples of common, well-characterized fibrous proteins that serve structural functions in the body. For example, collagen and elastin are found as components of skin, connective tissue, blood vessel walls, and sclera and cornea of the eye. Each fibrous protein exhibits special mechanical properties, resulting from its unique structure, which are obtained by combining specific amino acids into reg ular, secondary structural elements. This is in contrast to globular proteins, whose shapes are the result of complex interactions between secondary, tertiary, and, sometimes, quaternary structural elements. 

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. 

CD analysis of recombinant human tropoelastin shows that the molecule is composed of 3% a-helix, 41% /3-sheet, 21% /3-turn, and 33% other structure (Vrhovski et al., 1997). FT-Raman studies on human elastin demonstrate derived secondary structures containing 8% a-helix, 36% /3-strand, and 56% unordered conformation (Debelle et al., 1998). 

Debelle, L., Alix, A. J., Jacob, M. P., Huvenne, J. P., Berjot, M., Sombret, B., and Legrand, P. (1995). Bovine elastin and kappa-elastin secondary structure determination by optical spectroscopies. / Biol. Chem. 270, 26099-26103. 

Despite different sequences and repetitive motives, all gliadins have the same secondary structure of loose spirals which are a balanced compromise between the p-spiral and poly-L-proline structure (polyproline helix II) (Parrot et al., 2002), the balance is dependent on temperature, type of solvent, and hydration level (Miles et al., 1991). Similar sequences can be found in other proteins, mainly animal proteins such as elastin and collagen, and they are responsible for particular biomechanical properties connected to reverse P-spirals or p-sheet structures (Tatham and Shewry, 2000). 

Protein molecules are synthesized rapidly (3-5 minutes) in vivo with a high degree of precision. The error level in the incorporation of specific amino acids into a growing polypeptidyl chain to give the primary sequence of a specific protein is estimated at about one error in every 104 to 105 amino acids incorporated. Unlike carbohydrates, every molecule of a given protein is identical in molecular weight, amino acid sequence, and secondary structure. When the proteins are released from the ribosomes they immediately are confronted with a hostile environment. Some proteins survive in the environment for only a few minutes while others last several years. For example, the half life of ornithine decarboxylase normally is about 11 minutes, while that of elastin is not readily measurable. 

Elastin - Elastin is a highly elastic fiber present in ligaments and arterial blood vessels. The polypeptide is rich in glycine, alanine, and valine. Its secondary structure is the most random of the fibrous proteins described here. Like collagen, elastin contains lysine groups involved in cross-links between the chains. In elastin, however, four lysine chains can be combined to form a desmosine cross-link (see here). Thus, fewer cross-links are needed to provide strength for the chains and a more elastic network is created. 

The polypeptide chain of elastin is rich in glycine, alanine, and valine and is very flexible and easily extended. In fact, its conformation probably approximates that of a random coil, with little secondary structure at all. However, the sequence also contains frequent lysine side chains, which can be involved in cross-links. These cross-links prevent the elastin fibers from being extended indefinitely, causing the fibers to snap back when tension is removed. The cross-links in elastin are rather different from those in collagen, for they are designed to hold several chains together. Four lysine side chains can be combined to yield a desmosine cross-link (see here)... 

Urry, D. W., Mitchell, L. W., and Ohnishi, T. (1974). Biochem. Biophys. Res. Comm. 59, 62. Solvent Dependence of Peptide Carbonyl Carbon Chemical Shifts and Polypeptide Secondary Structure The Repeat Tetrapeptide of Elastin. 

Pioneering work by the Alix laboratory on the secondary structure of human elastin and the solubilized K-elastin, estimated the molecule to be composed of 10% a-helices, 35% P-strands and 55% undefined conformation. These estimations were based on Fourier transform infrared (FTIR), near infrared Fourier transform Raman spectroscopy and circular dichroism (CD) (15). To further investigate the nature of the elasticity, polypeptides of hydrophobic sequences containing exons 3, 7, and 30 of human elastin were analyzed by CD and Classic Raman spectroscopy, revealing polyproline II (PPII) helix secondary structures in both the aqueous and solid phase. Further analysis of exon 30 by FTIR spectroscopy determined that this sequence was characterized by both PPII as well as p-sheets structures (15). The presence of these structures were dependent on temperature, concentration and / or time, where lower temperatures and concentrations favored the PPII structure and higher temperatures and concentrations favored p-sheets (16). 

D.W. Urry and M.M. Long, Conformations of the Repeat Peptides of Elastin in Solution An Application of Proton and Carbon-13 Magnetic Resonance to the Determination of Polypeptide Secondary Structure. CRC Crit Rev. Biochemistry, 4,1-45,1976. 

The right-handed helices, which seem to be the preferred secondary structure, have 3.6 amino acids per tnm and are stabilised by hydrogen bonding between the NH and the CO groups further along the chain. Helices of this kind are associated in different ways to form tertiary structures of super helices in other fibrous proteins such as collagen, elastin, wool and so forth (Figure 10.16). 

H.R. Kricheldorf, D. Muller, Secondary structure ofpeptides.15. C-13 NMR Cp Mas study of solid elastin and proline-containing copolyesters, Int. J. Biol. Macromol. 6 (1984) 145-151. 

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. 

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