A novel method for silkworm cocoons self-degumming and its effect on silk fibers

Conventional hot-alkaline cocoon degumming techniques greatly weaken the physicochemical and mechanical properties of silk fibroin fiber, thus affecting the quality of silk fabric. Moreover, it causes massive energy waste and serious environmental pollution. This study aims to establish a novel cocoon self-degumming method by genetic modification of silkworm varieties and silk fibers. The self-degummed cocoon material was generated by specifically overexpressing trypsinogen protein in the sericin layer of silk thread; the effect of cocoon self-degumming method was evaluated by the degumming rate of sericin protein, the cleanliness and equivalent diameter of silk fibroin fiber; the basic characteristics of silk fibroin fiber degummed by cocoon self-degumming method and conventional hot-alkaline degumming technique were determined by electron microscopy, Fourier infrared spectroscopy, X-ray diffraction and tensile tests; the composition and biological activity of degummed sericin protein was respectively analyzed by liquid chromatograph-mass spectrometry and cytological experiments. The genetically engineered self-degumming cocoon containing trypsinogen protein was successfully created, and the content of trypsinogen protein in silk was 47.14 ± 0.90 mg/g. The sericin protein in the self-degumming cocoon was removed out in water or 1 mM Tris-HCl buffer (pH=8.0). Compared to alkaline-degummed silk fibroin, self-degummed silk fibroin had better cleanliness, thicker equivalent diameter, more complete silk structure and better mechanical property. In addition, sericin protein degummed from self-degumming cocoons significantly promoted cell proliferation and caused no obvious cytotoxicity. Compared to conventional hot-alkaline degumming technique, the cocoon self-degumming method by genetically overexpressing trypsinogen protein in sericin layer of silk thread can self-degummed in a mild degumming condition, and gain silk fiber with better quality and more biologically active sericin protein products. This strategy can not only reduce the environmental impact, but also generate greater economic value, which will accelerate its application in the silk and pharmaceutical industries.