Challenges and Opportunities in Identifying and Characterising Keratinases for Value-Added Peptide Production

Abstract

Keratins are important structural proteins produced by mammals, birds and reptiles. Keratins usually act as a protective barrier or a mechanical support. Millions of tonnes of keratin wastes and low value co-products are generated every year in the poultry, meat processing, leather and wool industries. Keratinases are proteases able to breakdown keratin providing a unique opportunity of hydrolysing keratin materials like mammalian hair, wool and feathers under mild conditions. These mild conditions ameliorate the problem of unwanted amino acid modification that usually occurs with thermochemical alternatives. Keratinase hydrolysis addresses the waste problem by producing valuable peptide mixes. Identifying keratinases is an inherent problem associated with the search for new enzymes due to the challenge of predicting protease substrate specificity. Here, we present a comprehensive review of twenty sequenced peptidases with keratinolytic activity from the serine protease and metalloprotease families. The review compares their biochemical activities and highlights the difficulties associated with the interpretation of these data. Potential applications of keratinases and keratin hydrolysates generated with these enzymes are also discussed. The review concludes with a critical discussion of the need for standardized assays and increased number of sequenced keratinases, which would allow a meaningful comparison of the biochemical traits, phylogeny and keratinase sequences. This deeper understanding would facilitate the search of the vast peptidase family sequence space for novel keratinases with industrial potential.Catalysts 2020, 10, 184 2 of 23 with keratinolytic activity for keratin hydrolysis protects the integrity of the keratin amino acids in most cases and allows control over the peptide size in the hydrolysate that is not readily achievable with other methods [5]. This degree of control allows the production of bespoke medical biomaterials, smart biocomposites, protein feed supplements with enhanced nutritional and bioactive properties as well as personal care products with enhanced functional and bioactive properties.Identifying peptidases with keratinolytic activity is an inherent problem associated with the search for new enzymes. Keratinase activity however appears to be dependent on the accessibility of the keratin substrate to the enzyme [6,7]. Thermochemical or biochemical treatment of the keratin, with emphasis on the reduction of the disulphide bond and disruption of other important bonds involved in the structural stability of keratin like isopeptide, hydrogen and glycolytic bonds [6,[8][9][10], appears to be the prerequisite for enzymatic hydrolysis. Sulphitolysis, which involves reduction of the disulphide bond in keratin, often acts synergistically with keratinases in nature [6,7]. Although destabilization of the keratin structure is a prerequisite for keratin hydrolysis, not all peptidases can hydrolyse keratin. Peptidases like trypsin, papain and pepsin cannot hydrolyse keratin as efficiently ...

Microbial Keratinase: Next Generation Green Catalyst and Prospective Applications

“…The bio-decomposition process significantly promotes soil fertility via the supply, slow but regular release of organic nitrogenous compounds (Nafady et al, 2018). The foreseeable implication of the bioconversion of keratinous biomass into utilizable units include food chain nourishment, bio-geo-ecological balance, and safety to public and animal health as pathogens would have lost habitation consequent to organic matter decomposition (Kowalczyk et al, 2018;De Oliveira Martinez et al, 2020). Another significant contribution of the keratinase producing microbes is the recycling of refractory materials with keratin composition, thus, reducing the pollution levels of the planet (Callegaro et al, 2018).…”

“…Microbial keratinase-assisted solubilization of keratin is important in the bio-economy innovation value chain. It enables a sustainable waste management, while promoting the valorization of intractable waste bio-resources into valueadded products for the green economy (De Oliveira Martinez et al, 2020). However, the mechanisms of action leading to the complete breakdown of keratinous substrates by keratinolytic proteases are not yet fully elucidated.…”

Bacillus sp. CSK2 produced thermostable alkaline keratinase using agro-wastes: keratinolytic enzyme characterization

“…The bio-based approach for the valorization of poultry feathers (keratin-rich polymer) represents an efficient route for novel products. Therefore, it portends an effective and sustainable reintegration of the agro residues into a value chain for the bio-economy [ 13 ]. Consequently, some microbes including bacteria, actinobacteria and fungi have expressed keratinolytic potentials [ 14 , 15 ].…”

Biochemical and Molecular Characterization of a Thermostable Alkaline Metallo-Keratinase from Bacillus sp. Nnolim-K1

“…The biodegradation of feathers by keratinolytic microbes is mediated through the expression and extracellular production of keratinase. The degree of feather degradation is contingent on the enzyme yield and catalytic efficiency of the produced keratinolytic enzyme [ 2 ]; however, not all the peptidases could efficiently liberate soluble proteins from keratin even after structural alteration by sulfitolytic system [ 11 ].…”