Keratinases: microbial sources, mechanisms, and industrial applications in waste valorization

Keratinous waste, a major by-product of agriculture and animal husbandry, is produced in massive quantities and is notoriously recalcitrant to degradation. With the expansion of the poultry and livestock industries, keratinous waste accumulation (e.g., feathers, hooves, and horns) has become a pressing environmental concern. Keratin's highly cross-linked disulfide bond structure is resistant to breakdown by common proteases. Keratinase, a specialized protease capable of specifically degrading keratin, has emerges as a pivotal tool for the valorization of keratinous waste, demonstrating significant potential in waste management and resource recovery. This review systematically summarizes the enzymatic properties, mechanisms of action, and microbial sources of keratinases. It elaborates on innovative keratinase applications in waste valorization (including biogas production, the generation of bioactive peptides and amino acid feedstocks, and bioplastic manufacturing) and green industries (including leather and textile processing), as well as in the pharmaceutical, cosmetic, and detergent sectors. This review provides an in-depth discussion of the major challenges hindering industrial-scale keratinase application, including low heterologous expression efficiency and insufficient stability under industrial conditions. Finally, it outlines future research directions, encompassing protein engineering, artificial intelligence (AI)-assisted design, and multi-enzyme synergistic catalysis systems, aiming to offer forward-looking theoretical insights for advanced keratinase development and industrial application.