Natural Protein Fibers

Keywords

Hagfish • Slime • Silk thread • Tensile properties • Seawater

Hagfishes are marine craniates (animals that contain hard bone or cartilage skull) that produce large amounts of slime [84Dow]. The slime is composed of cells that are made up of threads similar to fibers seen in a silkworm cocoon. These elliptical­shaped cells are produced by highly specialized slime glands. When these gland cells are released into water, they release strands or threads that uncoil and increase the viscosity of the mucus [81Dow]. A typical cell in the hagfish slime is shown in Fig. 42.1. Each cell has threads that are 1-3 pm in diameter and may have lengths up to 60 cm [84Dow, 12Neg]. SDS-PAGE of the threads revealed that the proteins have a molecular weight of about 63,500 Da. Further analyses of the proteins have demonstrated the presence of three components, one major (a) and two minor (P, y) that have similar molecular weights but different isoelectric values of 7.56, 5.67, and 5.31 for the a, p, and y, respectively [84Spi]. The amino acid composition of the three components is shown in Table 42.1. The amino acid composition in the hagfish threads were similar to the keratin polypeptides found in humans and rats [84Spi]. Based on X-ray diffraction studies, it was suggested that the hagfish threads could undergo irreversible a-p transition, under large strains as observed in wool keratins [03Fud]. Using a glass microbeam force transducer apparatus, the tensile properties of the hagfish threads in seawater were determined. It was found that the threads had a low initial stiffness of 6.4 MPa (0.06 g/den) but considerably high strength (180 MPa) (1.6 g/den) and low elongation of 2.2 %.

At a molecular level, the slime threads were found to consist of 10 nm protein nanofibers that are made from non-repetitive genes. Therefore, it was envisaged that it was easier to replicate slime thread fiber properties through biotechnology compared to spider silk [10Fud]. More interestingly, the slime proteins self — assemble into the 10 nm fibers in aqueous buffers. Since it was observed that the proteins in slime undergo a~p transition under high strains, it was hypothesized that the stretching of the fibers could lead to improved mechanical properties. Hagfish slime threads were drawn in water and later dehydrated in ethanol and tested in dry air. Drawn threads showed considerable increase in strength that approached the strength of spider silks. Some of the properties of the slime threads before and after stretching, annealing, and cross-linking are shown in Table 42.2.

References

[81Dow] Downing, S. W., Spitzer, R. H., Salo, W. L., Downing, J. S., Saidel, L. J., Koch, E. A.: Science 212, 326 (1981)

[84Dow] Downing, S. W., Spitzer, R. H., Koch, E. A., Salo, W. L.: J. Cell Biol. 98(2), 653 (1984) [84Spi] Spitzer, R. H., Downing, S. W., Koch, E. A., Salo, W. L., Saidel, L. J.: J. Cell Biol. 98, 670 (1984)

[03Fud] Fudge, D. S., Gardner, K. H., Forsyth, T. V., Riekel, C., Gosline, J. M.: Biophys. J. 85, 2015 (2003)

[10Fud] Fudge, D. S., Hillis, S., Levy, N., Gosline, J. M.: Bioinspir. Biomim. 5, 1 (2010)