Purification and Characterization of a Novel Extracellular Haloprotease Vpr from Bacillus licheniformis Strain KB111.

Research background: Haloalkaline proteases are one of the most interesting types of commercial enzymes in various industries due to their high specific activity and stability under extreme conditions. Biochemical characterization of enzymes is an important requirement for determining their potential for application in industrial fields. Most of microbial proteases have been isolated from Bacillus spp. In this study, the purification and characterization of an extracellular haloprotease produced from Bacillus sp. KB111 strain, which was previously isolated from mangrove forest sediments, are investigated for industrial applications. Experimental approach: The whole genome of KB111 strain was identified by DNA sequencing. Its produced protease was purified by salting out and anion-exchange chromatography, characterized based on protease activity and stability using a peptide substrate, and identified by LC-MS/MS. Results and conclusions: The strain KB111 was identified as Bacillus licheniformis. The molecular mass of its extracellular protease, termed KB-SP, was estimated to be 70 kDa. The optimal pH and temperature for the activity of this protease were 7 and 50 °C, respectively, while the enzyme exhibited maximal activity in the broad salinity range of 2-4 M NaCl. It was fully stable at an alkaline pH range of 7-11 at 50 °C with a half-life of 90 min. Metal ions such as K+, Ca2+ and Mg2+ could enhance the enzyme activity. Therefore, this protease indicates a high potential for the applications in the food and feed industry, as well as the waste management since it can hydrolyse protein at high alkaline pH and salt concentrations. The amino acid profiles of the purified KB-SP determined by LC-MS/MS analysis showed high score matching with the peptidase S8 of B. licheniformis LMG 17339, corresponding to the mature domain of a minor extracellular protease (Vpr). Amino acid sequence alignment and 3D structure modelling of KB-SP showed a conserved catalytic domain, a protease-associated (PA) domain and a C-terminal domain. Novelty and scientific contribution: A novel extracellular haloprotease from B. licheniformis was purified, characterized and identified. The purified protease was identified as being a minor extracellular protease (Vpr) and this is the first report on the halotolerance of Vpr. This protease has the ability to work in harsh conditions, with a broad alkaline pH and salinity range. Therefore, it can be useful in various applications in industrial fields.