A serine protease from newly isolated Bacillus sp. for efficient silk degumming, sericin degrading and colour bleaching activities

- The bacteria-producing silk degumming protease was isolated and identified.
- The purified protease degraded only sericin (gum) without degrading the fibroin (silk yarn).
- The protease clearly bleached the yellow colour of raw silk-sheet.
- Degumming efficiency of the protease was explained in detail.
- This research emphasizes the protection and conservation of cultural heritage of Thailand.

An extracellular serine protease of Bacillus sp. C4 SS-2013 (C4), isolated in Thailand, exhibited a high specificity to sericin substrate. The crude enzyme showed the ability to completely remove sericin (with the specific activity of 25.0 U mg−1) without damaging the silk fibroin fiber even at long incubation periods. It was easily concentrated and suitable for longer storage at low temperatures. The purified enzyme exhibited a molecular weight of 28 kDa, 3.1-fold purification, 6.2% yield and specific activity of 78.0 U mg−1. Its optimal pH and temperature were at 8 and 50 °C, respectively and it remained stable at pH 5-11 and temperature at 30-40 °C. The enzyme activity was stimulated by Ca2+. This is the first report of kinetic values determined by sericin substrate. The Vmax, Km, Kcat and Kcat/Km values of this enzyme were 6.6 μmoL mL−1 min−1, 14.6 mg mL−1, 1222.2 s−1 and 83.7 mL (s mg)−1, respectively. Besides, its specificity to sericin and other glue proteins but not to fibrous proteins, the purified C4 enzyme displayed an excellent silk degumming, yellow colour bleaching and sericin degrading activities. It clearly degraded sericin substrate into peptides with 10-12 kDa in size. The C4 protease can thus be a good choice for silk, cosmetic and detergent industries etc.