Physiology of keratin waste biodegradation and keratinase production by mycelial fungi: a review

Abstract

The ability to degrade keratin is widespread among all groups of microorganisms. From an environmental and economic perspective, the biodegradation of keratinous wastes by microorganisms and their keratinolytic enzymes is considered the most effective alternative for their processing and valorization. Moreover, the use of keratinases to deconstruct keratin waste is a promising approach for manufacturing animal feed, plant fertilizers, cosmetics, pharmaceuticals, tannery and leather, detergents, and bioplastics. As a result, the demand for these enzymes has increased significantly in recent years, requiring large quantities of low-cost enzymes to meet industrial needs. Ascomycota and Basidiomycota fungi are considered potential producers of keratinases in submerged and solid-state fermentations of keratinous waste materials. To increase keratinase yields, various approaches and strategies, such as the use of cheap materials as growth substrates, optimization of fermentation media and cultivation conditions, and development of improved bioprocess technologies, have been widely exploited. However, compared to bacteria, understanding the regulatory mechanisms of keratin-degrading enzyme production in filamentous fungi is still in its infancy. This review provides the first systematic analysis of the cultural and physicochemical factors modulating the keratinolytic activity of Ascomycota and Basidiomycota fungi. The need for a thorough study of the microbial diversity of keratinolytic fungi to identify fungi that overproduce keratinase is emphasized. Understanding key aspects of the physiological regulation of target enzyme synthesis by each specific fungal strain is crucial for increasing keratinase yield.