A Thermostable Aluminum-Tolerant Protease Produced by Feather-Degrading Bacillus thuringiensis Isolated from Tea Plantation

Background: Proteases with keratinolytic activity are widely used in biotechnologies. The feather-degrading Bacillus thuringensis isolated from soil sample of a tea plantation produced high level of extracellular keratinase. Objective: This study aimed to analyze the properties by biochemical and enzymological methods to gain information for better utilization of the enzyme. Methods: The enzyme was purified with ion exchange and size exclusion chromatography. The substrate preference, optimal pH and temperature, and the effects of organic solvents and ions were checked. Circular dichroism was performed to compare the secondary structures of the native and apo-enzyme. Results: The enzyme worked best at 50°C, and it was an acidic serine protease with an optimal pH of 6.2. Ions Ca²⁺ and Mg²⁺ were essential for its activity. Organic solvents and other metal ions generally deactivated the enzyme in a concentration-dependent manner. However, Mn²⁺ and DMSO, which were frequently reported as inhibitors of protease, could activate the enzyme at low concentration (0.01 to 2 mmol/L of Mn²⁺; DMSO <2%, v/v). The enzyme exhibited high resistance to Al³⁺, which might be explained by the soil properties of its host’s residence. Circular dichroism confirmed the contribution of ions to the structure and activity. Conclusion: The enzyme was a thermostable aluminum-tolerant serine protease with unique biochemical properties.