Microbial proteases for sustainable protein-waste valorization and circular bioeconomy.

Microbial proteases are among the most commercially important enzyme groups, contributing approximately 20% of the global industrial enzyme market because of their broad substrate specificity, operational robustness, and cost-effectiveness. These enzymes catalyze peptide bond hydrolysis under mild and environmentally benign conditions, supporting cleaner manufacturing and circular bioeconomy strategies. This review focuses on the production, engineering, and industrial application of microbial proteases, with particular emphasis on the valorization of protein-rich agro-industrial, food, and animal-processing residues generated in multi-million-ton quantities annually. The review adopts a circular bioprocessing perspective that links protease performance with waste-stream conversion, process integration, and resource-efficiency outcomes. Applications in food and feed processing, detergent formulations, leather processing, and protein hydrolysate production are critically discussed in the context of sustainable industrial processing. Advances in enzyme engineering, secretion and production optimization, and integrated bioprocess design are examined alongside key technical and economic constraints. Future progress will depend on the development of robust proteases, intensified and continuous processing platforms, and decentralized waste-to-value systems supported by improved process monitoring and standardized product characterization.