A Review: Biodegradation and Applications of Keratin Degrading Microorganisms and Keratinolytic Enzymes, Focusing on Thermophiles and Thermostable Serine Proteases

Keratins are hard-degrading fibrous proteins, insoluble in water and organic solvents, often accumulated in nature and major components in feathers, skins, hair, horn, nail, hoof etc., Keratin-degrading microorganisms such as bacteria, archaea, actinomycetes and fungi employ keratinases to attack keratin. Keratinases belonging to subtilisin-like serine proteases were classified based on similarity of amino acid sequences. Keratinolytic thermophilic or hyperthermophilic bacteria and archaea have been known to degrade keratin at ≥70°C. General properties of thermozymes such as stability to heat and resistance to denaturing conditions; e.g., solvents and detergents have drawn attention to various biotechnological industries. Some bacterial and archaeal keratinases degrade not only fibrous keratin but also digest recalcitrant prion proteins, an etiologic agent of spongiform encephalopathies of brain and nervous system. Keratin and keratinase have a number of applications in various sectors, i.e., biotechnology, cosmetic and pharmaceutical industries, medical therapy and waste management. On the other hand, accumulation of excess amount of keratin is recognized as solid waste and troublesome environmental pollutant. Biodegradation involving either keratinolytic thermophiles or thermostable keratinases not only improve digestibility and nutritive values of keratinous meals (for mixers in animal feed stuffs), but also minimize risk from infection and microbial toxin transmitted to livestock.