Research progress on the degradation mechanism and modification of keratinase

Recombinant keratin: Comprehensive review of synthesis, hierarchical assembly, properties, and applications

Keratin has gained attention for its remarkable mechanical properties, thermal stability, and beneficial biological properties, such as promoting hemostasis and wound healing. Traditionally, keratin has been extracted from natural sources, including human hair, wool, and feathers, and processed into biomaterials, including films, hydrogels, and nanoparticles, primarily for biomedical applications. However, extraction methods often result in heterogeneous keratin mixtures with residual impurities and structural degradation due to harsh purification conditions, complicating efforts to understand how specific keratins and their hierarchical assemblies contribute to desired material properties. Recombinant keratin technology addresses these challenges by enabling the synthesis of individual keratin types with high purity and batch-to-batch consistency. These advancements facilitate studies on how individual and combined keratins at various assembly stagesfrom molecular components and heterodimers to intermediate filaments (IFs) and IF networksimpact material properties. Moreover, this technology allows for precise genetic modifications, potentially leading to engineered keratin variants with tailored characteristics for targeted applications. Despite these advantages, translating recombinant keratin into practical applications requires overcoming key manufacturing challenges, such as optimizing large-scale production and improving purification efficiency. This review presents the current state of recombinant keratin research by highlighting its advancements and exploring current biomaterial applications. While its applications remain limited compared to extracted keratin at this early stage, its potential offers future opportunities for extending its use in advanced material design and beyond biomedical fields. STATEMENT OF SIGNIFICANCE: Keratin and keratinized structures provide essential protection to tissues against mechanical stress and environmental damage, serving as foundational elements across diverse biological systems. This review discusses advancements in recombinant keratin technology, enabling high-purity, reproducible synthesis with controlled composition modifications that effectively overcome the limitations of traditional extraction methods. The innovations deepen our understanding of hierarchical assembly in keratin structures across various length scales, along with their reinforcing mechanisms and mechanical and biofunctional properties. These insights lay the groundwork for biomaterials tailored to regenerative medicine, wound healing, and other biomedical applications. By focusing on the unique capabilities of recombinant keratin, this review offers a valuable resource for future advancements in high-performance biomaterials across biomedical and biotechnological fields.

Exploring the potential of spore surface-displayed keratinase for feather waste degradation using high-throughput screening

BACKGROUND

With the development of the global poultry industry, millions of tons of feathers are produced each year. A significant proportion is improperly discarded, leading to a substantial loss of keratin resources from feathers. Utilizing feathers in the feed industry could reduce costs substantially. However, the new fermented feed industry lacks acid-resistant keratinase, making it unable to convert feather keratin effectively in a lactic acid bacteria fermentation environment.

RESULTS

This study established a high-throughput screening method using spore surface-display to identify acid-resistant mutants generated by error-prone PCR. A mutant H4 (N43K/N143I) was obtained with a significant increase in activity under acidic conditions, and it was 1.34-fold more active than the wild type in a pH 5 environment. Bioinformatic analysis showed that the increase in stress resistance and activity of H4 was due to an increase in certain weak interaction forces and alterations in energy levels. The addition of H4 spore suspension as an enzyme preparation to feather meal fermentation feed with the participation of lactic acid bacteria improved the content of small peptides and free amino acids significantly. After being subjected to short-term high-temperature treatments, the H4 spore preparation was still able to degrade chicken feathers.

CONCLUSION

These results suggest that the spore surface-displayed keratinase mutant H4 has potential for improved effectiveness in feather degradation within the fermented feed industry. © 2025 Society of Chemical Industry.

Biochemical characterization and molecular docking of a novel alkaline-stable keratinase from Amycolatopsis sp. BJA-103

Amycolatopsis sp. BJA-103 was isolated for its exceptional feather-degradation capability, leading to the purification, cloning, and heterologous expression of the keratinase enzyme, KER0199. Sequence analysis places KER0199 within the S8 protease family, revealing <60 % sequence similarity to known proteases. The recombinant KER0199-His6 demonstrates a broad substrate range, along with remarkable thermostability and alkaline stability, exhibiting optimal activity at pH 11.0 and 60 °C, despite the absence of cysteine residues essential for disulfide bonding. Structural modeling reveals a predominantly negatively charged surface and a flat, low-electrostatic-potential substrate-binding pocket. Substrate-binding models, predicted using AlphaFold3 and molecular dynamics simulations, indicate that substrates such as casein, chicken feather β-keratin P2450, and hemoglobin bind to this pocket, forming anti-parallel β-sheets with residues G97 to G99 and establishing extensive hydrogen bonds with key residues near the enzyme's active site. These findings suggest that AlphaFold-based substrate binding predictions, combined with an analysis of intermolecular forces, provide a valuable tool for assisting in the elucidation of enzyme specificity and substrate recognition. KER0199, the first characterized S8 family keratinase from the Amycolatopsis genus, shows great potential for industrial applications.

Enzymatic processes for animal hide/skin collagen fiber purification processing: Recent progress, challenges and recommendations

Collagen fiber purification is the most important pretreatment process in the recycling of animal hide/skin, by-products of meat production, and can be utilized to produce value-added materials. Traditional animal hide/skin resource utilization technologies face serious challenges in the aspect of production efficiency and environmental sustainability. Enzymatic collagen fiber purification processing is thought to be one of the most promising technologies that can minimize the use of chemicals and energy, reduce CO2-eq emissions, and achieve sustainable development of animal hide/skin reutilization. However, enzymatic processes have not been well accepted for industrial-scale applications in factories so far. In this review, recent progress and challenges of enzymatic collagen fiber purification processing were comprehensively overviewed in the aspect of the key mechanisms and technologies of enzyme application. Recommendations for the direction of enzyme selection and development were put forward, which is expected to pave the way for the industrial-scale application of enzymes in animal hide/skin collagen fiber purification processing.

Three-step surface design of spore-displayed keratinase improved acid tolerance and feather degradation

The global chicken business has grown rapidly, producing millions of tons of feather waste annually. Keratinase is a special enzyme that catalyzes the degradation of keratin and can be applied to the feed industry. In this study, we initially set the tone for the acid-resistant mutation of spore surface-display keratinase cotG-KERQ7 by replacing base-catalytic residues in the active center. We then performed molecular dynamics simulations of the KERQ7-AAPF, the enzyme-substrate complex, to enhance both acid stability and activity by substituting unstable positively charged amino acids on the surface. Finally, based on the change of protein rigidity-flexibility, a flexible tentacle outside the catalytic pocket was introduced to enhance the feather meal degradation activity of KERQ7 in an acidic environment. The Ding-F was finally obtained through the three-step design of keratinase surface. The mutant Ding-F was able to break down feather meal better in both lactobacillus-fermented feed environments and simulated gastric fluid digestion environments. This study not only establishes a theoretical foundation for the development of keratinases used in lactobacillus-fermented feed, but it also generates a three-step surface design method that can serve as a guide for future general strategies aimed at modifying the acid resistance of serine keratinases.

Keratinous and corneous-based products towards circular bioeconomy: A research review

Keratins and corneous proteins are key components of biomaterials used in a wide range of applications and are potential substitutes for petrochemical-based products. Horns, hooves, feathers, claws, and similar animal tissues are abundant sources of α-keratin and corneous β-proteins, which are by-products of the food industry. Their close association with the meat industry raises environmental and ethical concerns regarding their disposal. To promote an eco-friendly and circular use of these materials in novel applications, efforts have focused on recovering these residues to develop sustainable, non-animal-related, affordable, and scalable procedures. Here, we review and examine biotechnological methods for extracting and expressing α-keratins and corneous β-proteins in microorganisms. This review highlights consolidated research trends in biomaterials, medical devices, food supplements, and packaging, demonstrating the keratin industry's potential to create innovative value-added products. Additionally, it analyzes the state of the art of related intellectual property and market size to underscore the potential within a circular bioeconomic model.

Unraveling dynamics and interactions of core microorganisms in the biodegradation of keratin-based feather wastes

Waste feathers, abundant byproducts of the poultry industry, pose significant environmental challenges. Although microbial degradation has been investigated, the core microorganisms and their interactions remain underexplored. This study examined microbial community dynamics during feather degradation, using diverse feather sources and under varying temperatures. Significant divergences were observed in bacterial communities, with Firmicutes, Actinobacteria, and Acidobacteriota (56.65%, 18.13%, and 11.14%) as dominant phyla. A core microbial consortium of 51 taxa was identified, with 8 core genera from the Bacilli class, significantly enriched during degradation. Higher temperature (50 °C) accelerated degradation. Dynamics patterns showed the enrichment of and depletion of some strains. Functional prediction highlighted the mechanisms for keratin biodegradation. This study identified core microorganisms and enzymes during keratin degradation, providing evidence to microbial treatment of keratin-based waste to reduce agricultural pollution.

High keratinase and other types of hydrolase activity of the new strain of Bacillus paralicheniformis

Keratinases, a subclass of proteases, are used to degrade keratin thereby forming peptones and free amino acids. Bacillus paralicheniformis strain T7 was isolated from soil and exhibited high keratinase, protease, collagenase, amylase, xylanase, lipase, and phosphatase activities. Keratinases of the strain showed maximum activity at 70°C and pH 9.0 as well as high thermal stability. A mass-spectrometric analysis identified seven peptidases with molecular masses of 26.8-154.8 kDa in the secretory proteome. These peptidases are members of S8 and S41 serine peptidase families and of M14, M42, and M55 metallopeptidase families. Additionally, α-amylase (55.2 kDa), alkaline phosphatase (59.8 kDa), and esterase (26.8 kDa) were detected. The strong keratinolytic properties of the strain were confirmed by degradation of chicken and goose feathers, which got completely hydrolyzed within 4 days. Submerged fermentation by strain B. paralicheniformis T7 was carried out in a pilot bioreactor, where the highest keratinase production was noted after 19 h of cultivation. After the fermentation, in the culture fluid, the keratinase activity toward keratin azure was 63.6 ± 5.8 U/mL. The protease activity against azocasein was 715.7 ± 40.2 U/mL. The possibility of obtaining enzyme preparations in liquid and powder form was demonstrated, and their comparative characteristics are given. In the concentrate, the keratinase, protease, α-amylase, phosphatase, and esterase/lipase activities were 2,656.7 ± 170.4, 29,886.7 ± 642.9, 176.1 ± 16.3, 23.9 ± 1.8, and 510.9 ± 12.2 U/mL, respectively. In the lyophilizate, these activities were 57,733.3 ± 8,911.4, 567,066.7 ± 4,822.2, 2,823.0 ± 266.8, 364.2 ± 74.8, and 17,618.0 ± 610.3 U/g, respectively. In the preparation obtained by air flow drying at 55°C, these activities were 53,466.7 ± 757.2, 585,333.3 ± 4,277.1, 2,395.8 ± 893.7, 416.7 ± 52.4, and 15,328.1 ± 528.6 U/g, respectively. The results show high potential of B. paralicheniformis strain T7 as a producer of keratinases and other enzymes for applications in agricultural raw materials and technologies for processing of keratin-containing animal waste.

Harnessing the potential of microbial keratinases for bioconversion of keratin waste

The increasing global consumption of poultry meat has led to the generation of a vast quantity of feather keratin waste daily, posing significant environmental challenges due to improper disposal methods. A growing focus is on utilizing keratinous polymeric waste, amounting to millions of tons annually. Keratins are biochemically rigid, fibrous, recalcitrant, physiologically insoluble, and resistant to most common proteolytic enzymes. Microbial biodegradation of feather keratin provides a viable solution for augmenting feather waste's nutritional value while mitigating environmental contamination. This approach offers an alternative to traditional physical and chemical treatments. This review focuses on the recent findings and work trends in the field of keratin degradation by microorganisms (bacteria, actinomycetes, and fungi) via keratinolytic and proteolytic enzymes, as well as the limitations and challenges encountered due to the low thermal stability of keratinase, and degradation in the complex environmental conditions. Therefore, recent biotechnological interventions such as designing novel keratinase with high keratinolytic activity, thermostability, and binding affinity have been elaborated here. Enhancing protein structural rigidity through critical engineering approaches, such as rational design, has shown promise in improving the thermal stability of proteins. Concurrently, metagenomic annotation offers insights into the genetic foundations of keratin breakdown, primarily predicting metabolic potential and identifying probable keratinases. This may extend the understanding of microbial keratinolytic mechanisms in a complex community, recognizing the significance of synergistic interactions, which could be further utilized in optimizing industrial keratin degradation processes.

Design, development and characterization of a chimeric protein with disulfide reductase and protease domain showing keratinase activity

Keratin is one of the major components of solid waste, and the degradation products have extensive applications in various commercial industries. Due to the complexity of the structure of keratin, especially the disulfide bonds between keratin polypeptides, keratinolytic activity is efficient with a mixture of proteins with proteases, peptidases, and oxidoreductase activity. The present work aimed to create an engineered chimeric protein with a disulfide reductase domain and a protease domain connected with a flexible linker. The structure, stability, and substrate interaction were analyzed using the protein modeling tools and codon-optimized synthetic gene cloned, expressed, and purified using Ni2+-NTA chromatography. The keratinolytic activity of the protein was at its maximum at 70 °C. The suitable pH for the enzyme activity was pH 8. While Ni2+, Mg2+, and Na+ inhibited the keratinolytic activity, Cu2+, Ca2+, and Mn2+ enhanced it significantly. Biochemical characterization of the protease domain indicated significant keratinolytic activity at 70 °C at pH 10.0 but was less efficient than the chimeric protein. Experiments using feathers as the substrate showed a clear degradation pattern in the SEM analysis. The samples collected from the degradation experiments indicated the release of proteins (2-fold) and amino acids (8.4-fold) in a time-dependent manner. Thus, the protease with an added disulfide reductase domain showed excellent keratin degradation activity and has the potential to be utilized in the commercial industries.

Molecular Modification Strategies of Nitrilase for Its Potential Application in Agriculture

Some feed source plants will produce secondary metabolites such as cyanogenic glycosides during metabolism, which will produce some poisonous nitrile compounds after hydrolysis and remain in plant tissues. The consumption of feed-source plants without proper treatment affect the health of the animals' bodies. Nitrilases can convert nitriles and have been used in industry as green biocatalysts. However, due to their bottleneck problems, their application in agriculture is still facing challenges. Acid-resistant nitrilase preparations, high-temperature resistance, antiprotease activity, strong activity, and strict reaction specificity urgently need to be developed. In this paper, the application potential of nitrilase in agriculture, especially in feed processing industry was explored, the source properties and catalytic mechanism of nitrilase were reviewed, and modification strategies for nitrilase application in agriculture were proposed to provide references for future research and application of nitrilase in agricultural and especially in the biological feed scene.

Transcriptomics Reveals the Mechanism of Purpureocillium lilacinum GZAC18-2JMP in Degrading Keratin Material

Microbial degradation of keratin is characterized by its inherent safety, remarkable efficiency, and the production of copious degradation products. All these attributes contribute to the effective management of waste materials at high value-added and in a sustainable manner. Microbial degradation of keratin materials remains unclear, however, with variations observed in the degradation genes and pathways among different microorganisms. In this study, we sequenced the transcriptome of Purpureocillium lilacinum GZAC18-2JMP mycelia on control medium and the medium containing 1% feather powder, analyzed the differentially expressed genes, and revealed the degradation mechanism of chicken feathers by P. lilacinum GZAC18-2JMP. The results showed that the chicken feather degradation rate of P. lilacinum GZAC18-2JMP reached 64% after 216 h of incubation in the fermentation medium, reaching a peak value of 148.9 μg·mL-1 at 192 h, and the keratinase enzyme activity reached a peak value of 211 U·mL-1 at 168 h, which revealed that P. lilacinum GZAC18-2JMP had a better keratin degradation effect. A total of 1001 differentially expressed genes (DEGs) were identified from the transcriptome database, including 475 upregulated genes and 577 downregulated genes. Kyoto encyclopedia of genes and genomes (KEGG) enrichment analysis of the DEGs revealed that the metabolic pathways related to keratin degradation were mainly sulfur metabolism, ABC transporters, and amino acid metabolism. Therefore, the results of this study provide an opportunity to gain further insight into keratin degradation and promote the biotransformation of feather wastes.

Perspective on Agricultural Industrialization: Modification Strategies for Enhancing the Catalytic Capacity of Keratinase

Keratinases is a special hydrolytic enzyme produced by microorganisms, which has the ability to catalyze the degradation of keratin. Currently, keratinases show great potential for application in many agricultural and industrial fields, such as biofermented feed, leather tanning, hair removal, and fertilizer production. However, these potentials have not yet been fully unleashed on an industrial scale. This paper reviews the sources, properties, and catalytic mechanisms of keratinases. Strategies for the molecular modification of keratinases are summarized and discussed in terms of improving the substrate specificity, thermostability, and pH tolerance of keratinases. The modification strategies are also enriched by the introduction of immobilized enzymes and directed evolution. In addition, the selection of modification strategies when facing specific industrial applications is discussed and prospects are provided. We believe that this review serves as a reference for the future quest to extend the application of keratinases from the laboratory to industry.

Molecular strategies to enhance the keratinase gene expression and its potential implications in poultry feed industry

The tons of keratin waste are produced by the poultry and meat industry which is an insoluble and protein-rich material found in hair, feathers, wool, and some epidermal wastes. These waste products could be degraded and recycled to recover protein, which can save our environment. One of the potential strategy to achieve this target is use of microbial biotreatment which is more convenient, cost-effective, and environment-friendly by formulating hydrolysate complexes that could be administered as protein supplements, bioactive peptides, or animal feed ingredients. Keratin degradation shows great promise for long-term protein and amino acid recycling. According to the MEROPS database, known keratinolytic enzymes currently belong to at least 14 different protease families, including S1, S8, S9, S10, S16, M3, M4, M14, M16, M28, M32, M36, M38, and M55. In addition to exogenous attack (proteases from families S9, S10, M14, M28, M38, and M55), the various keratinolytic enzymes also function via endo-attack (proteases from families S1, S8, S16, M4, M16, and M36). Biotechnological methods have shown great promise for enhancing keratinase expression in different strains of microbes and different protein engineering techniques in genetically modified microbes such as bacteria and some fungi to enhance keratinase production and activity. Some microbes produce specific keratinolytic enzymes that can effectively degrade keratin substrates. Keratinases have been successfully used in the leather, textile, and pharmaceutical industries. However, the production and efficiency of existing enzymes need to be optimized before they can be used more widely in other processes, such as the cost-effective pretreatment of chicken waste. These can be improved more effectively by using various biotechnological applications which could serve as the best and novel approach for recycling and degrading biomass. This paper provides practical insights about molecular strategies to enhance keratinase expression to effectively utilize various poultry wastes like feathers and feed ingredients like soybean pulp. Furthermore, it describes the future implications of engineered keratinases for environment friendly utilization of wastes and crop byproducts for their better use in the poultry feed industry.

A Comparative Transcriptome Analysis Unveils the Mechanisms of Response in Feather Degradation by Pseudomonas aeruginosa Gxun-7

Microbial degradation of feathers offers potential for bioremediation, yet the microbial response mechanisms warrant additional investigation. In prior work, Pseudomonas aeruginosa Gxun-7, which demonstrated robust degradation of feathers at elevated concentrations, was isolated. However, the molecular mechanism of this degradation remains only partially understood. To investigate this, we used RNA sequencing (RNA-seq) to examine the genes that were expressed differentially in P. aeruginosa Gxun-7 when exposed to 25 g/L of feather substrate. The RNA-seq analysis identified 5571 differentially expressed genes; of these, 795 were upregulated and 603 were downregulated. Upregulated genes primarily participated in proteolysis, amino acid, and pyruvate metabolism. Genes encoding proteases, as well as those involved in sulfur metabolism, phenazine synthesis, and type VI secretion systems, were notably elevated, highlighting their crucial function in feather decomposition. Integration of Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) taxonomies, combined with a review of the literature, led us to propose that metabolic feather degradation involves environmental activation, reducing agent secretion, protease release, peptide/amino acid uptake, and metabolic processes. Sulfite has emerged as a critical activator of keratinase catalysis, while cysteine serves as a regulatory mediator. qRT-PCR assay results for 11 selected gene subset corroborated the RNA-seq findings. This study enhances our understanding of the transcriptomic responses of P. aeruginosa Gxun-7 to feather degradation and offers insights into potential degradation mechanisms, thereby aiding in the formulation of effective feather waste management strategies in poultry farming.

Structural and enzymatic characterization of a novel metallo-serine keratinase KerJY-23

KerJY-23 was a novel keratinase from feather-degrading Ectobacillus sp. JY-23, but its enzymatic characterization and structure are still unclear. In this study, the KerJY-23 was obtained by heterologous expression in Escherichia coli BL21(DE3), and enzymatic properties indicated that KerJY-23 was optimal at 60 °C and pH 9.0 and could be promoted by divalent metal ions or reducing agents. Furthermore, KerJY-23 had a broad substrate specificity towards casein, soluble keratin, and expanded feather powder, but its in vitro degradation against chicken feathers required an additional reducing agent. Homology modeling indicated that KerJY-23 contained a highly conserved zinc-binding HELTH motif and a His-Asp-Ser catalytic triad that belonged to the typical characteristics of M4-family metallo-keratinase and serine-keratinase, respectively. Molecular docking revealed that KerJY-23 achieved a reinforced binding on feather keratin via abundant hydrogen bonding interactions. This work not only deepened understanding of the novel and interesting metallo-serine keratinase KerJY-23, but also provided a theoretical basis for realizing the efficient use of waste feather keratin.

Effective biodegradation on chicken feather by the recombinant KerJY-23 Bacillus subtilis WB600: A synergistic process coupled by disulfide reductase and keratinase

Keratin wastes are abundantly available but rich in hard-degrading fibrous proteins, and the keratinase-producing microorganisms have gained significant attention due to their biodegradation ability against keratinous materials. In order to improve the degradation efficiency of feather keratins, the keratinase gene (kerJY-23) from our previously isolated feather-degrading Ectobacillus sp. JY-23 was overexpressed in Bacillus subtilis WB600 strain. The recombinant KerJY-23 strain degraded chicken feathers rapidly within 48 h, during which the activities of disulfide reductase and keratinase KerJY-23 were sharply increased, and the free amino acids especially the essential phenylalanine and tyrosine were significantly accumulated in feather hydrolysate. The results of structural characterizations including scanning electron microscopy, Fourier transform infrared spectrum, X-ray diffraction, and X-ray photoelectron spectroscopy, demonstrated that the feather microstructure together with the polypeptide bonds and SS bonds in feather keratins were attacked and destroyed by the recombinant KerJY-23 strain. Therefore, the recombinant KerJY-23 strain contributed to feather degradation through the synergistic action of the secreted disulfide reductase to break the SS bonds and keratinase (KerJY-23) to hydrolyze the polypeptide bonds in keratins. This study offers a new insight into the underlying mechanism of keratin degradation, and provides a potential recombinant strain for the valorization of keratin wastes.

The flexible linker and CotG were more effective for the spore surface display of keratinase KERQ7

Feather, horn, hoof, and other keratin waste are protein-rich but limited by natural keratinase synthesis, activity, pH, and temperature stability. It is challenging to realize its large-scale application in industries. Bacillus subtilis spores are a safe, efficient, and highly resistant immobilized carrier, which can improve target proteins' resistance. In this research, KERQ7, the keratinase gene of Bacillus tequilensis strain Q7, was fused to the Bacillus subtilis genes coding for the coat proteins CotG and CotB, respectively, and displayed on the surface of B. subtilis spores. Compared with the free KERQ7, the immobilized KERQ7 showed a greater pH tolerance and heat resistance on the spore surface. The activity of CotG-KERQ7 is 1.25 times that of CotB-KERQ7, and CotG-KERQ7 is more stable. When the flexible linker peptide L3 was used to connect CotG and KERQ7, the activity was increased to 131.2 ± 3.4%, and the residual enzyme activity was still 62.5 ± 2.2% after being kept at 60 ℃ for 4 h. These findings indicate that the flexible linker and CotG were more effective for the spore surface display of keratinase to improve stress resistance and promote its wide application in feed, tanning, washing, and other industries.

Identification and characterization of a versatile keratinase, KerZJ, from Stenotrophomonas sp. LMY

Keratinases have drawn increasing attention in recent decades owing to their catalytic versatility and broad applications from agriculture to medicine. In the present study, we isolated a highly keratinolytic and fibrinolytic bacterium from the campus soil and named it Stenotrophomonas sp. LMY based on genetic information. To identify the potential keratinase genes, the genome sequence of the strain was obtained and analyzed. Sequence alignment and comparison revealed that the protein 1_737 (KerZJ) had the highest sequence homology to a reported keratinase KerBL. We recombinantly expressed KerZJ in Escherichia coli Origami™ (DE) pLysS and purified it to homogeneity. KerZJ showed the highest activity at 40 °C and pH 9.0, and metal ions exhibited no significant effects on its activity. Although reducing agents would break the disulfide bonds in KerZJ and reduce its activity, KerZJ still exhibited the ability to hydrolyze feather keratin in the presence of β-ME. KerZJ could efficiently digest human prion proteins. In addition, KerZJ showed fibrinolytic activity on fibrin plates and effectively eliminated blood clots in a thrombosis mouse model without side effects. Our results suggest that KerZJ is a versatile keratinase with significant potential for keratin treatment, decontamination of prions, and fibrinolytic therapy.

Comprehensive insights into the mechanism of keratin degradation and exploitation of keratinase to enhance the bioaccessibility of soybean protein

Keratin is a recalcitrant protein and can be decomposed in nature. However, the mechanism of keratin degradation is still not well understood. In this study, Bacillus sp. 8A6 can completely degrade the feather in 20 h, which is an efficient keratin degrader reported so far. Comprehensive transcriptome analysis continuously tracks the metabolism of Bacillus sp. 8A6 throughout its growth in feather medium. It reveals for the first time how the strain can acquire nutrients and energy in an oligotrophic feather medium for proliferation in the early stage. Then, the degradation of the outer lipid layer of feather can expose the internal keratin structure for disulfide bonds reduction by sulfite from the newly identified sulfite metabolic pathway, disulfide reductases and iron uptake. The resulting weakened keratin has been further proposedly de-assembled by the S9 protease and hydrolyzed by synergistic effects of the endo, exo and oligo-proteases from S1, S8, M3, M14, M20, M24, M42, M84 and T3 families. Finally, bioaccessible peptides and amino acids are generated and transported for strain growth. The keratinase has been applied for soybean hydrolysis, which generates 2234 peptides and 559.93 mg/L17 amino acids. Therefore, the keratinases, inducing from the poultry waste, have great potential to be further applied for producing bioaccessible peptides and amino acids for feed industry.

Reducing Cellular Autolysis of Bacillus subtilis to Improve Keratinase Production

Bacillus subtilis has been shown to be an excellent expression host for keratinases due to its powerful secretion system. However, cellular autolysis limits its production capacity. Here, we selected seven genes with significantly upregulated transcript levels from 15 genes associated with cellular autolysis as knockout targets by qRT-PCR and constructed a total of 127 strains to reduce cellular autolysis. Among them, the biomass of B. subtilis BSΔXLPC-ker deficient in xpf, lytC, pcfA, and cwlC increased by 57%. This was confirmed by cell staining, green fluorescent protein imaging, and extracellular nucleic acid leakage assay. Keratinase activity was increased by 1.46-fold in the 5 L fermenter. In addition, the activities of nattokinase and subtilisin E were also increased by 1.50-fold and 1.43-fold, respectively, in the modified chassis cells, which further confirms the generalizability of the strategy. Thus, reducing cellular autolysis to increase the ability of B. subtilis to produce subtilisins is promising.

Fusion of Substrate-Binding Domains Enhances the Catalytic Capacity of Keratinases and Promotes Enzymatic Conversion of Feather Waste

The unique role of keratinases in keratin hydrolysis has garnered huge interest in the recovery of feather waste. However, owing to the high hydrophobicity of feather keratins, the catalytic capacity of keratinases for hydrolyzing feathers is typically low. In this study, we aimed to improve the keratinase feather hydrolysis efficiency by fusing a substrate-binding domain into the enzyme. We screened several carbohydrate-binding modules (CBMs) and linking peptides. We selected the most promising candidates to construct, clone, and express a fusion keratinase enzyme KerZ1/CBM-L8 with a feather hydrolysis efficiency of 7.8 × 10^-8 g/U. Compared with those of KerZ1, KerZ1/CBM-L8 has a feather hydrolysis efficiency that is 2.71 times higher, a k_cat value that is 179% higher, which translates to higher catalytic efficiency, and K_m and binding constant (K) values that are lower, which indicate a higher KerZ1/CBM-L8-keratin binding affinity. Moreover, the number of binding sites to the substrate (N), determined using isothermal titration calorimetry, was 24.1 times higher than that of KerZ1. Thus, the fusion of the substrate-binding domain improved the binding ability of the keratinase enzyme to the hydrophobic substrate, which improved its feather hydrolysis efficiency. Therefore, using the fusion keratinase would significantly improve the recovery of feather waste.

Isolation of a novel feather-degrading Ectobacillus sp. JY-23 strain and characterization of a new keratinase in the M4 metalloprotease family

Microbial keratinases have prominent potential in biotransformation of recalcitrant keratin substrates to value-added products which has made keratinases a research focus in the past decades. In this study, an efficient feather-degrading bacterium was isolated and identified as a novel species in Ectobacillus genus and designated as Ectobacillus sp. JY-23. The degradation characteristics analysis revealed that Ectobacillus sp. JY-23 could utilize chicken feathers (0.4% w/v) as the sole nutrient source and degraded 92.95% of feathers in 72 h. A significant increase in sulfite and free sulfydryl group content detected in the feather hydrolysate (culture supernatant) indicated efficient reduction of disulfide bonds, which inferred that the degradation mechanism of isolated strain was a synergetic action of sulfitolysis and proteolysis. Moreover, abundant amino acids were also detected, among which proline and glycine were the predominant free amino acids. Then, the keratinase of Ectobacillus sp. JY-23 was mined and Y1_15990 was identified as the keratinase encoding gene of Ectobacillus sp. JY-23 and designated as kerJY-23. Escherichia coli strain overexpressing kerJY-23 degraded chicken feathers in 48 h. Finally, bioinformatics prediction of KerJY-23 demonstrated that it belonged to the M4 metalloprotease family, which was a third keratinase member in this family. KerJY-23 showed low sequence identity to the other two keratinase members, indicating the novelty of KerJY-23. Overall, this study presents a novel feather-degrading bacterium and a new keratinase in the M4 metalloprotease family with remarkable potential in feather keratin valorization.