Silkworm structure

Secondary Structure. The silkworm cocoon and spider dragline silks are characterized as an antiparaHel P-pleated sheet wherein the polymer chain axis is parallel to the fiber axis. Other silks are known to form a-hehcal (bees, wasps, ants) or cross- P-sheet (many insects) stmctures. The cross-P-sheets are characterized by a polymer chain axis perpendicular to the fiber axis and a higher serine content. Most silks assume a range of different secondary stmctures during processing from soluble protein in the glands to insoluble spun fibers. 

The structures of some natural protein-based materials, such as silk and wool, result in strong, tough fibers. Spiders and silkworms use proteins as a structural material of remarkable strength (Fig. 19.22). Chemists are duplicating nature by making artificial spider silk (Fig. 19.23), which is one of the strongest fibers known. 

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. 

Sirichaisit, J., Brookes, V. L., Young, R.J., and Vollrath, F. (2003). Analysis of structure/ property relationships in silkworm (Bombyx mori) and spider dragline (Nephila edulis) silks using Raman spectroscopy. Biomacromolecules 4, 387-394. 

This signal chemical accomplishment is the work of a German chemist Adolf Butenandt and his coworkers and was completed in the late 1950s. Butenandt later won the Nobel Prize in Chemistry for his achievements, including the identification of the silkworm moth sex pheromone. This was not easy work it required about 20 years of effort. Nearly half a million female silkworm moths had to be processed to yield a mere 6 milligrams (mg), about three ten-thousandths of an ounce, of the sex pheromone. The structure of the substance was deduced from work on this very small amount of material. Given today s powerful tools of chemistry, the work would prove far less troublesome than it did in the time of Butenandt s work. 

Figure 5.1 The relationship between geometry at carbon-carbon double bonds and biological activity for the female sex attractant of the silkworm moth Bombyx mori. The figures associated with each structure are the relative effective concentrations required to elicit a response in 50% of male silkworm moths.

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). 

The structure of the sex attractant of the silkworm, bombykol, is given in Section 5-6 as structure 30. The compound has been synthesized by the route given below. Write the structures of each of the synthetic intermediates A-F. 

Second-generation juvenoids incorporate more substantial structural departures from neotenin and are more resistant to metabolic and environmental degradation. Epiphenonane, 2-ethyl-3-[3-ethyl-5-(4-ethylphenoxy)-pent-3-en-yl] 2-methyloxirane (131), has a rat oral LD50 of 4000 mg/kg. It and similar juvenoids are used in China and Japan to prolong the last larval instar of the silkworm so that silk production is increased 10—15%. Fenoxycarb, ethyl [2-(4-phenoxyphenoxy)ethyl] carbamate (132) (mp 53°C, vp 0.0078 mPa at 20°C), is soluble in water to 6 mg/L. The rat oral LD50 is >16,800 mg/kg. Fenoxycarb has a wide spectrum of activity, interfering with the developmental processes of fleas, cockroaches, and ants. 

A good example of a pheromone is bombykol, (10E,12Z)-hexadeca-10,12 dien 1 ol (2.10), a pheromone which is used by the female silkworm moth to attract a mate (Figure 2.26a). Its predominantly hydrocarbon structure makes it water insoluble and thus it remains localised near the source. Bombykol was one of the first pheromones to be isolated (in 1959) and is used in minute quantities to confuse male moths as to the location of females and thus control numbers. 

Fig. 13.4. This figure reinforces the picture of nondiscontiguity between relaxin structures and the purported age of the owners of these relaxins. Skate is no closer to humans than to a shark or to a silkworm in terms of the molecular genealogy record whereas the skate, of course, lived 300,000,000 years before humans and other mammals appeared on earth.

Sandler B. H., Nikonova L., Leal W. S. and Clardy J. (2000) Sexual attraction in the silkworm moth structure of the pheromone-binding-protein-bombykol complex. Chem. Biol. 7, 143-151. 

Ichikawa T., Hasegawa K., Shimizu I., Katsuno K., Kataoka H. and Suzuki A. (1995) Structure of neurosecretory cells with immunoreactive diapause hormone and pheromone biosynthesis activating neuropeptide in the silkworm, Bombyx mori. Zool. Sci. 12, 703-712. 

Imai K., Konno T., Nakazawa Y., Komiya T., Isobe M., Koga K., Goto T., Yaginuma T., Sakakibara K. and Hasegawa K., el al. (1991) Isolation and structure of diapause hormone of the silkworm, Bombyx mori. Proc. Japan Acad. 67(B), 98-101. 

Suwan S., Isobe M., Yamashita O., Minakata H. and Imai K. (1994) Silkworm diapause hormone, structure-activity relationships indispensable role of C-terminal amide. Insect Biochem. Mol. Biol. 24, 1001-1007. 

Leal W. S., Nikonova L. and Peng G. (1999) Disulfide structure of the pheromone binding protein from the silkworm moth, Bombyx mori. FEBS Letters. 468, 85-90. 

From the viewpoint of zootaxa, the silkworm and the spider belong to insect and arachnid of arthropod, respectively. Their silk proteins (fibroin for silkworm silk and spidroin for spider major ampullate silk) do not have any genetic heritage in common and their amino acids sequence compositions are different too. However, the silkworm and spider employ a similar spinning process to produce silk. Furthermore, the silkworm silk and the major ampullate silk have a number of similar structural characteristics, both at the level of the secondary protein structure and the condensed silk morphology. Therefore, for the sake of convenience, they are discussed together in some parts of this text. 

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