Purification and characterization of a thermostable keratinolytic serine alkaline proteinase from Streptomyces sp. strain AB1 with high stability in organic solvents

A keratinolytic alkaline proteinase (KERAB) was isolated from Streptomyces sp. strain AB1. Based on MALDI-TOF mass spectrometry analysis, the purified enzyme is a monomer with a molecular mass of 29850.17 Da. The NH2-terminal sequence of the enzyme was determined to be TQANPPSWGLDDIDQTAL. This keratinase was completely inhibited by phenylmethanesulfonyl fluoride (PMSF) and diiodopropyl fluorophosphates (DIFP), which suggests that it belongs to the serine protease family. Using keratin azure as a substrate, the optimum pH and temperature values for keratinase activity were pH 11.5 and 75 °C, respectively. This keratinase was stable between 30 and 60 °C and pH 4 and 11 for 4 and 96 h, respectively, and thermoactivity and thermostability were enhanced in the presence of 5 mM Mg2+. Its catalytic efficiency was higher than those of SAPB-L31I/T33S/N99Y, nattokinase and subtilisin Carlsberg. KERAB exhibited stability to detergents and high resistance against organic solvents and was able to degrade feathers completely. These properties make KERAB a potential candidate for future applications in detergent formulations, dehairing during leather processing, and non-aqueous peptide biocatalysis.