Efficient Production of a Novel Subtilisin-like Serine Protease in <i>Aspergillus niger</i> for the Preparation of DPP-IV Inhibitory Activity Peptides from Protein Byproducts

Proteases play a crucial role in the bioconversion of proteins into bioactive peptides. <i>Aspergillus niger</i> is an important cell factory for enzyme production due to its strong post-translational modification capabilities and excellent protein secretion system. In this study, a novel subtilisin-like serine protease (<i>Mo</i>S8) from <i>Monascus purpureus</i> was efficiently expressed extracellularly in <i>A. niger</i> FBL-B for the first time using a polycistronic system and the coexpression strategy of the <i>vgb</i> gene (hemoglobin from <i>Vitreocilla</i>). The recombinant <i>A. niger</i> FBL-B produced a high protease activity of up to 5250.5 U/mL with a protein concentration of 6.5 g/L through fed-batch fermentation in a 5 L fermenter. The purified <i>Mo</i>S8 showed optimal activity at pH 10.0 and 50 °C. It displayed broad substrate specificity and a high specific activity of 1578.5 U/mg toward casein. Furthermore, <i>Mo</i>S8 efficiently hydrolyzed nine industrial protein byproducts to produce valuable dipeptidyl peptidase IV (DPP-IV) inhibitory activity peptides. Among them, the walnut meal and whey protein hydrolysates exhibited higher DPP-IV inhibitory activity, with IC₅₀ values of 3.24 and 3.53 mg/mL, respectively. This study provides valuable strategies for the efficient production of foreign enzymes in <i>A. niger</i> and the high-value utilization of protein byproducts.