Protein silk fibroin

Compare structural features and properties of the following proteins silk fibroin, a-keratin, collagen, and bovine serum albumin. 

Table 11.6 gives a list of the main material proteins, along with the production techniques used for each of them. Other proteins have also been used for film-forming applications rye, pea, barley, sorghum and rice proteins, silk fibroins, fish flesh proteins and serum albumin [26, 83, 137, 138]. 

Silk fibroin and collagen illustrate the close linkage of protein stmcture and biologic function. Diseases of collagen mamration include Ehlers-Danlos syndrome and the vitamin C deficiency disease scurvy. 

Naturally, very long T,c values are expected for solids as viewed from the expected correlation time at the low temperature side of the T c minimum (0.1-0.2 s) as shown in Figure 1. Indeed, their values turns out to be the order of 10-30 s for carbon sites in the absence of internal fluctuations as in polysaccharides such as (1 — 3)-p-D-glucan and (1 —> 3)-p-D-xylan,46 8 fibrous proteins such as collagen49 and silk fibroin,50 free and metal-complexed ionophores,51 or in some instances up to 1000 s as in crystalline polyethylene.52... 

Y.H. Wu, Q.C. Shen, and S.S. Hu, Direct electrochemistry and electrocatalysis of heme-proteins in regenerated silk fibroin film. Anal. Chim. Acta 558, 179-186 (2006). 

Raw silk was dissolved in hexafluoro-iso-propanol (HFIP) [17, 33]. A typical working concentration for spinning was 2.5% (w/v) silk fibroin in HFIP. The spinning solution was pressed through a small needle (0 80-250 pm) into a precipitation bath (methanol for Bombyx mori silk proteins and acetone for Nephila clavipes silk proteins) and the silk solution immediately precipitated as a fiber. The best performing fibers approached the maximum strength measured for native fibers of Bombyx mori, but did not achieve the mechanical properties of natural spider silk. 

Asakura, T., Sugino, R., Okumura, T., and Nakazawa, Y. (2002a). The role of irregular unit, GAAS, on the secondary structure of Bombyx mod silk fibroin studied with C-13 CP/MAS NMR and wide- angle X-ray scattering. Protein Sci. 11, 1873-1877. 

Inoue, S., Tsuda, H., Tanaka, T., Kobayashi, M., Magoshi, Y., and Magoshi, J. (2003). Nanostructure of natural fibrous protein In vitro nanofabric formation of Sarnia cynthia ricini wild silk fibroin by self-assembling. Nano Lett. 3, 1329-1332. 

Rossle, M., Panine, P., Urban, V. S., and Riekel, C. (2004). Structural evolution of regenerated silk fibroin under shear Combined wide- and small-angle x-ray scattering experiments using synchrotron radiation. Biopolymers 74, 316-327. Rousseau, M. E., Lefevre, T., Beaulieu, L., Asakura, T., and Pezolet, M. (2004). Study of protein conformation and orientation in silkworm and spider silk fibres using Raman microspectroscopy. Biomacromolecules 5, 2247-2257. 

Silk is produced from the spun threads from silkworms (the larvae of the moth Bombyx mori and related species). The main protein in silk, fibroin, consists of antiparallel pleated sheet structures arranged one on top of the other in numerous layers (1). Since the amino acid side chains in pleated sheets point either straight up or straight down (see p. 68), only compact side chains fit between the layers. In fact, more than 80% of fibroin consists of glycine, alanine, and serine, the three amino acids with the shortest side chains. A typical repetitive amino acid sequence is (Gly-Ala-Gly-Ala-Gly-Ser). The individual pleated sheet layers in fibroin are found to lie alternately 0.35 nm and 0.57 nm apart. In the first case, only glycine residues (R = H) are opposed to one another. The slightly greater distance of 0.57 nm results from repulsion forces between the side chains of alanine and serine residues (2). 

Silk-fibroin is composed of practically only three amino acids, glycine, alanine and tyrosine, and is probably the simplest protein known. It resembles the fibroin of spider s silk very closely, but here the presence of so much glutamic acid is one of the characteristics. 

The results of numerous investigations were published in i860 by Stadeler, who found tyrosine in silk-fibroin, mucin and various other proteins, and who also noted its occurrence in the free state in various organs, generally in conjunction with leucine. Since then, tyrosine has been constantly obtained from proteins by hydrolysis with acids and by the action of trypsin, and has long been regarded as a constituent of the protein molecule. 

Some protein structures limit the kinds of amino acids that can occur in the J3 sheet. When two or more /3 sheets are layered close together within a protein, the R groups of the amino acid residues on the touching surfaces must be relatively small. J3-Keratins such as silk fibroin and the fibroin of spider webs have a very high content of Gly and Ala residues, the two amino acids with the smallest R groups. Indeed, in silk fibroin Gly and Ala alternate over large parts of the sequence. 

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