Optimizing Protein-Glutaminase Expression in Bacillus subtilis

Protein-glutaminase (PG) is a promising protein deaminase. It only hydrolyzes the side chain amido groups of protein-bound to generate ammonia and protein-L-glutamic acid and does not catalyze any other undesirable changes in protein structures. Deamidation of proteins via PG can influence the solubility, emulsification, foaming, and gelation properties of proteins, which are important properties for some food proteins. Therefore, there is great potential for the application of PG in the food industry. PG is derived from Chryseobacterium proteolyticum (C. proteolyticum); however, wild strains are difficult to industrialize because of their low levels of enzyme production. In this article, we studied different strategies for PG expression in B. subtilis. Results showed that PG produced from C. proteolyticum could be successfully secreted in B. subtilis WB800N, and actively secreted in B. subtilis 168(BS168) or DB403 containing a pro-peptide (pro-PG). The secreted PG from B. subtilis WB800N was inactive unless digested by exogenous proteases, such as trypsin, alkaline protease, and neutral protease. However, active PG was secreted by the self-processing of BS168 and DB403. The specific activity of purified PG reached 20.9 U/mg. PG showed maximum activity at pH 5.5, 55 °C and more than 80% of PG activity was retained within a range of pH 3.5-6.5. When Cbz-Gln-Gly was used as the substrate, PG activity was 31.1 ± 0.9 μM min^-1 mg^-1. Mg²⁺, Ca²⁺, and Zn²⁺ stabilized and even activated PG activity. These strategies concerning PG expression in B. subtilis and the enzymatic properties of PG provide efficient alternatives for PG research and contribute to the industrial-scale production of PG.