Natural Protein Fibers

Keywords

Spider silk • Extraordinary tensile strength • Tensile strength • Silk gland • Fiber production • Major ampullate gland • Spidroin

Spider silks are recognized for their extraordinary properties due to their composi­tion and structure. Enormous literature is available on the structure and properties of spider silks, on the mechanisms of silk production, and on reproducing the properties of the spider silks through biotechnology. Considerable variation in tensile properties is seen among the fibers produced from different spiders as seen in Table 43.1 and also between the fibers produced from different glands in the spiders. Figure 43.1 depicts the major components in spider and the four most common silk-producing glands. Figure 43.2 shows a schematic of the process of production of spider silk [12Eis]. Dragline silk produced by the major ampullate gland is the most common type of silk fiber studied. Recently, the structure and composition of this gland have been studied. As seen in Fig. 43.3, the gland consists of a tail, a sac, and an elongated duct. The sac can be divided into three distinct epithelial regions (A, B, and C), and it was found that sections A and B produce spidroins, but spidroins were lacking in the C region. Spidroins are proteins that have about 3,500 amino acid residues and consist of N-terminal (NT) and C-terminal (CT) domains which are considered to be responsible for fiber formation [99Hay, 13And]. A two-layered silk fibers consisting of a core and skin produced by zones A and B, respectively, were proposed. It was found that the nonterminal spidroin was homogenously distributed and was also discovered in the skin region.

Silks produced by the dragline glands are typically composed of two major proteins called spidroins. However, the composition and structure of the spidroins in the fibers vary from species to species. For example, in Nephila clavipes, the proteins are classified as major ampullate spidroins MaSp1 and major ampullate spidroin MaSp2 [10Hei], whereas fibers from Araneus diadematus contain fibroins 3 and 4 that are referred to as ADF-3 and ADF-4. MaSp1 and MaSp2 show similar

Source	Strength (g/den)	Elongation (%)	Modulus (g/den)	Toughness (MJ/m3)
L. hesperus	12.1 ± 0.9	28 ± 1	148 ± 16.5	243 ± 29
A. diadematus	14.8 ± 1.8	23 ± 1	87.0 ± 13.0	225 ± 29
N. edulis	10.4 ± 1.7	39 ± 8	69.6 ± 16.5	215 ± 36
B. mori	5.2	28	130	150

Table 43.1 Comparison of the properties of natural silk fibers produced by various spiders in comparison to B. mori silk [09Fu1, 09Fu2]

cylindrical:

outer egqsac

Flagelliforme Aggregate tylmdncale

Minor Ampullate

acini form,
trapping *i
and

packing silk

Fig. 43.1 Major components of spider and the four most common silk-producing glands (Major ampullate, minor ampullate, piriforme, aciniforme). From Vollrath [00Vol], reproduced with permission from Elsevier primary structure with highly repetitive core domain containing iterated repeats of alanine — and glycine-rich domains and non-repetitive terminal regions. In addition, MaSp1 is said to be homogeneously distributed throughout the core, whereas the MaSP2 is present as clusters in the core of the fibers [10Hei]. A schematic of the proposed structure of spider silks is shown in Fig. 43.4. The MaSp2 component is composed of about 15 % of proline residues and shown to form the matrix and is mainly responsible for the elongation of the fibers, whereas the MaSp1 is proline free and forms the crystalline regions. The crystalline regions are composed of crystalline p-sheets and provide strength to the fibers. In addition, glycine-rich motifs such as GGX or GPGXX that have flexible helical structures connect crystalline regions and also provide elasticity to the fibers.

Several theories have been proposed on the arrangement, structure, properties, and role of the different components in spider silks. Although recent evidence

Fig. 43.2 Schematic depiction of the process of producing spider silk. Reproduced from Eisoldt et al. [12Eis] with permission from Wiley

Fig. 43.3 Schematic of the different zones in the major ampullate gland. From Andersson et al. [13And] reproduced with permission from the American Chemical Society

Fig. 43.4 Model of MAS primary structure. The protein core comprises iterated repeats of characteristic consensus motifs (X, Y). A consensus motif is typically built of 10-50 amino acid residues and is repeated up to 100 times. The repetitive core is flanked by N — and C-terminal domains with unique non-repetitive primary structures, each harboring a cysteine residue involved in intermolecular disulfide bridge formation [10Hei]

provides a clearer understanding of the structure, there are contradictory results presented by different studies. One such study suggests that there is a bimodal distribution of crystalline regions in the fibers with two distinct sizes of crystallites.

Crystallites that are about 2-3 nm long and contain highly ordered and tightly packed p-sheets of polyalanine are found to be inter-dispersed with less ordered crystallites measuring 70-500 nm and consist of different silk motifs. However, using atomic force microscopy (AFM), it was found that fibers produced from the black widow spider Latrodectus hesperus had both unordered and highly ordered region composed of two fibers with diameters of 300 nm that were oriented parallel to the fiber axis and fibers that measured 10-100 nm that were oriented across the fiber axis [99Gou].

Yet another study has reported that spider silks contain highly crystalline regions composed of pleated p-sheets of polyalanine that provide strength and amorphous regions that are rich in glycine and are responsible for the elasticity of the fibers [04Hue1]. Spiders store freshly secreted silk as liquid crystalline spinning dope in concentrations up to 50 %. The silk solution in the spider’s gland is water soluble but becomes water insoluble after extrusion into fibers, a phenomenon also observed in B. mori silks.

At a molecular level, the assembly of the proteins differs considerably even though the amino acid sequences are similar [04Hue2]. Two major proteins ADF3 and ADF4 from the garden spider A. diadematus were used to study the assembly of proteins and their influence of fiber properties. ADF3 and ADF4 have similar amino acid sequences but have remarkably different properties. For example, ADF3 is soluble at high concentrations, but ADF4 is insoluble and self-assembles into filaments under specific conditions. To investigate the structure further, different repetitive and non-repetitive units in ADF3 and ADF4 were constructed by cloning. It was found that acidification and increase in phosphate concentration promoted self assembly but decreased solubility, and this effect was more pronounced in the non-repetitive regions [04Hue2]. Such an effect was attributed to the hydrophobicity of the two regions.

References

[99Gou] Gould, S. A.C., Tran, K. T., Spagna, J. C., Moore, A. M.F., Shulman, J. B.: Int. J. Biol. Macromol. 24, 151 (1999)

[99Hay] Hayashi, C. Y., Shipley, N. H., Lewis, R. V.: Int. J. Biol. Macromol. 24, 271 (1999) [00Vol] Vollrath, F.: J. Biotechnol. 74, 67 (2000)

[04Hue1] Huemmerich, D., Helsen, C. W., Quedzuweit, S., Oschmann, J., Rudolph, R., Scheibel, T.: Biochemistry 43, 13604 (2004)

[04Hue2] Huemmerich, D., Scheibel, T., Vollrath, F., Cohen, S., Gat, U., Ittah, S.: Curr. Biol. 14, 2070 (2004)

[09Fu1] Fu, C., Porter, D., Shao, Z.: Macromolecules 42, 7877 (2009)

[09Fu2] Fu, C., Shao, Z., Fritz, V.: Chem. Commun. 42, 6515 (2009)

[10Hei] Heim, M., Romer, L., Scheibel, T.: Chem. Soc. Rev. 39, 156 (2010)

[12Eis] Eisoldt, L., Thamm, C., Scheibel, T.: Biopolymers 97(6), 355 (2012)

[13And] Andersson, M., Holm, L., Ridderstrale, Y., Johansson, J., Rising, A.: Biomacro­molecules 14, 2945 (2013)