Combining Pro-peptide Engineering and Multisite Saturation Mutagenesis To Improve the Catalytic Potential of Keratinase

Development of a Universal One-Step Purification and Activation Method to Engineer Protein-Glutaminase through Rational Design

Cytotoxic enzymes often exist as zymogens containing prodomains to keep them in an inactive state. Protein-glutaminase (PG), which can enhance various functional characteristics of food proteins, is an enzyme containing pro-PG and mature-PG (mPG). However, poor activity and stability limit its application while tedious purification and activation steps limit its high-throughput engineering. Here, based on structural analysis, we replaced the linker sequence between pro-PG and mPG with the HRV3C protease recognition sequence and then coexpressed it with HRV3C protease in Escherichia coli to develop an efficient one-step purification and activation method for PG. We then used this method to obtain several mutants designed by a combination of computer-aided approach and beneficial point mutations. The specific activity (131.6 U/mg) of the best variant D1 was 4.14-fold that of the wild type, and t1/2 and T5010 increased by 13 min and 7 °C, respectively. D1 could effectively improve the solubility and emulsification of wheat proteins, more than twice the effect of the wild type. We also discussed the mechanism underlying the improved properties of D1. In summary, we not only provide a universal one-step purification and activation method to facilitate zymogen engineering but also obtain an excellent PG mutant.

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 scientific version of understanding "Why did the chickens cross the road"? - A guided journey through Bacillus spp. towards sustainable agriculture, circular economy and biofortification

The world, a famished planet with an overgrowing population, requires enormous food crops. This scenario compelled the farmers to use a high quantity of synthetic fertilizers for high food crop productivity. However, prolonged usage of chemical fertilizers results in severe adverse effects on soil and water quality. On the other hand, the growing population significantly consumes large quantities of poultry meats. Eventually, this produces a mammoth amount of poultry waste, chicken feathers. Owing to the protein value of the chicken feathers, these wastes are converted into protein hydrolysate and further extend their application as biostimulants for sustained agriculture. The protein profile of chicken feather protein hydrolysate (CFPH) produced through Bacillus spp. was the maximum compared to physical and chemical protein extraction methods. Several studies proved that the application of CFPH and active Bacillus spp. culture to soil and plants results in enhanced plant growth, phytochemical constituents, crop yield, soil nutrients, fertility, microbiome and resistance against diverse abiotic and biotic stresses. Overall, "CFPH - Jack of all trades" and "Bacillus spp. - an active camouflage to the surroundings where they applied showed profound and significant benefits to the plant growth under the most adverse conditions. In addition, Bacillus spp. coheres the biofortification process in plants through the breakdown of metals into metal ions that eventually increase the nutrient value of the food crops. However, detailed information on them is missing. This can be overcome by further real-world studies on rhizoengineering through a multi-omics approach and their interaction with plants. This review has explored the best possible and efficient strategy for managing chicken feather wastes into protein-rich CFPH through Bacillus spp. bioconversion and utilizing the CFPH and Bacillus spp. as biostimulants, biofertilizers, biopesticides and biofortificants. This paper is an excellent report on organic waste management, circular economy and sustainable agriculture research frontier.

Enhanced expression of a novel trypsin from Streptomyces fradiae in Komagataella phaffii GS115 through combinational strategies of propeptide engineering and self-degredation sites modification

This study aimed to enhance the expression level of a novel trypsin gene from Streptomyces fradiae ATCC14544 in Komagataella phaffii GS115 through the combinational use of propeptide engineering and self-degradation residues modification strategies. An artificial propeptide consisted of thioredoxin TrxA, the bovine propeptide DDDDK and the hydrophobic peptide FVEF was introduced to replace the original propeptide while the self-degradation residue sites were predicted and analyzed through alanine screening. The results showed that the quantity and enzymatic activity of asft with engineered propeptide reached 47.02 mg/mL and 33.9 U/mL, which were 9.6 % and 59.29 % higher than those of wild-type (42.9 mg/mL and 13.8 U/mL). Moreover, the introduction of R295A/R315A mutation further enhanced the enzymatic activity (58.86 U/mL) and obviously alleviated the phenomena of self-degradation. The tolerance of trypsin towards alkaline environment was also improved since the optimal pH was shifted from pH 9.0 to pH 9.5 and the half-life value at pH 10 was significantly extended. Finally, the fermentation media composition and condition were optimized and trypsin activity in optimal condition reached 160.58 U/mL, which was 2.73-fold and 11.64-fold of that before optimization or before engineering. The results obtained in this study indicated that the combinational use of propeptide engineering and self-degradation sites modification might have great potential application in production of active trypsins.

Structure prediction, docking studies and molecular cloning of novel Pichia kudriavzevii YK46 metalloprotease (MetPr) for improvement of feather waste biodegradation

This study addresses the environmental risks associated with the accumulation of keratin waste from poultry, which is resistant to conventional protein degradation methods. To tackle this issue, microbial keratinases have emerged as promising tools for transforming resilient keratin materials into valuable products. We focus on the Metalloprotease (MetPr) gene isolated from novel Pichia kudriavzevii YK46, sequenced, and deposited in the NCBI GenBank database with the accession number OQ511281. The MetPr gene encodes a protein consisting of 557 amino acids and demonstrates a keratinase activity of 164.04 U/ml. The 3D structure of the protein was validated using Ramachandran's plot, revealing that 93% and 97.26% of the 557 residues were situated within the most favoured region for the MetPr proteins of template Pichia kudriavzevii strain 129 and Pichia kudriavzevii YK46, respectively. Computational analyses were employed to determine the binding affinities between the deduced protein and beta keratin. Molecular docking studies elucidated the optimal binding affinities between the metalloprotease (MetPr) and beta-keratin, yielding values of - 260.75 kcal/mol and - 257.02 kcal/mol for the template strains Pichia kudriavzevii strain 129 and Pichia kudriavzevii YK46, respectively. Subsequent molecular cloning and expression of the MetPr gene in E. coli DH5α led to a significantly higher keratinase activity of 281 ± 12.34 U/ml. These findings provide valuable insights into the potential of the MetPr gene and its encoded protein for keratin waste biotransformation, with implications for addressing environmental concerns related to keratinous waste accumulation.

Modular Engineering to Enhance Keratinase Production for Biotransformation of Discarded Feathers

Biotransformation of wasted feathers via feather-degrading enzyme has gained immense popularity, low conversion efficiency hinders its scale application, and the main purpose of this study is to improve feather-degrading enzyme production in Bacillus licheniformis. Firstly, keratinase from Bacillus amyloliquefaciens K11 was attained with the best performance for feather hydrolysis, via screening several extracellular proteases from Bacillus; also, feather powder was proven as the most suitable substrate for determination of feather-degrading enzyme activity. Then, expression elements, including signal peptides and promoters, were optimized, and the combination of signal peptide SP_SacC with promoter Pdual3 owned the best performance, keratinase activity aggrandized by 6.21-fold. According to amino acid compositions of keratinase and feeding assays, Ala, Val, and Ser were proven as critical precursors, and strengthening these precursors' supplies via metabolic pathway optimization resulted in a 33.59% increase in the keratinase activity. Furthermore, keratinase activity reached 2210.66 U/mL, up to 56.74-fold from the original activity under the optimized fermentation condition in 3-L fermentor. Finally, the biotransformation process of discarded feathers by the fermented keratinase was optimized, and our results indicated that 90.94% of discarded feathers (16%, w/v) were decomposed in 12 h. Our results suggested that strengthening precursor amino acids' supplies was an efficient strategy for enhanced production of keratinase, and this research provided an efficient strain as well as the biotransformation process for discarded feather re-utilization.

Development of a Growth-Dependent System to Regulate Cell Growth and Keratinase Production in B. subtilis

Keratinases specifically degrade insoluble keratin waste, thus contributing to environmental protection and sustainable biomass development. However, their industrial application is hindered by inefficient enzyme production and poor biomass generation. In this study, the heterologous expression of keratinase was found to have cytotoxicity and might block host cell growth due to its proteolytic property. To address this problem, an autoregulatory expression system based on quorum sensing was developed to synergistically regulate cell growth and keratinase production in Bacillus subtilis. The growth-dependent promoter P_aprE was chosen and shown to be effective in delaying keratinase production while promoting host cell proliferation. Copy number screening and core region mutations further balanced the two states. Carbon supplement optimization indicated that addition of 2% glucose facilitated biomass accumulation during the early stage of fermentation. Cell density increased to 15.6 (OD_600 nm) from 8 with keratinase activity raised to 4200 U·mL^-1 from 1162 U·mL^-1. Keratinase was then utilized in the bioconversion of feather waste to prepare soluble keratins, polypeptides, and amino acids. This study provides a powerful system for efficient production of keratinase and paves the way for keratin waste recycling.

Research progress on the degradation mechanism and modification of keratinase

Keratin is regarded as the main component of feathers and is difficult to be degraded by conventional proteases, leading to substantial abandonment. Keratinase is the only enzyme with the most formidable potential for degrading feathers. Although there have been in-depth studies in recent years, the large-scale application of keratinase is still associated with many problems. It is relatively challenging to find keratinase not only with high activity but could also meet the industrial application environment, so it is urgent to exploit keratinase with high acid and temperature resistance, strong activity, and low price. Therefore, researchers have been keen to explore the degradation mechanism of keratinases and the modification of existing keratinases for decades. This review critically introduces the basic properties and mechanism of keratinase, and focuses on the current situation of keratinase modification and the direction and strategy of its future application and modification. KEY POINTS: •The research status and mechanism of keratinase were reviewed. •The new direction of keratinase application and modification is discussed. •The existing modification methods and future modification strategies of keratinases are reviewed.

Design of 5′-UTR to Enhance Keratinase Activity in Bacillus subtilis

Keratinase is an important industrial enzyme, but its application performance is limited by its low activity. A rational design of 5′-UTRs that increases translation efficiency is an important approach to enhance protein expression. Herein, we optimized the 5′-UTR of the recombinant keratinase KerZ1 expression element to enhance its secretory activity in Bacillus subtilis WB600 through Spacer design, RBS screening, and sequence simplification. First, the A/U content in Spacer was increased by the site-directed saturation mutation of G/C bases, and the activity of keratinase secreted by mutant strain B. subtilis WB600-SP was 7.94 times higher than that of KerZ1. Subsequently, the keratinase activity secreted by the mutant strain B. subtilis WB600-SP-R was further increased to 13.45 times that of KerZ1 based on the prediction of RBS translation efficiency and the multi-site saturation mutation screening. Finally, the keratinase activity secreted by the mutant strain B. subtilis WB600-SP-R-D reached 204.44 KU mL−1 by reducing the length of the 5′ end of the 5′-UTR, which was 19.70 times that of KerZ1. In a 5 L fermenter, the keratinase activity secreted by B. subtilis WB600-SP-R-D after 25 h fermentation was 797.05 KU mL−1, which indicated its high production intensity. Overall, the strategy of this study and the obtained keratinase mutants will provide a good reference for the expression regulation of keratinase and other industrial enzymes.

A combination of bioinformatics analysis and rational design strategies to enhance keratinase thermostability for efficient biodegradation of feathers

Keratinase has shown great significance and application potentials in the biodegradation and recycle of keratin waste due to its unique and efficient hydrolysis ability. However, the inherent instability of the enzyme limits its practical utilization. Herein, we obtained a thermostability-enhanced keratinase based on a combination of bioinformatics analysis and rational design strategies for the efficient biodegradation of feathers. A systematical in silico analysis combined with filtering of virtual libraries derived a smart library for experimental validation. Synergistic mutations around the highly flexible loop, the calcium binding site and the non-consensus amino acids generated a dominant mutant which increased the optimal temperature of keratinase from 40 °C to 60 °C, and the half-life at 60 °C was increased from 17.3 min to 66.1 min. The mutant could achieve more than 66% biodegradation of 50 g/L feathers to high-valued keratin product with a major molecular weight of 36 kDa. Collectively, this work provided a promising keratinase variant with enhanced thermostability for efficient conversion of keratin wastes to valuable products. It also generated a general strategy to facilitate enzyme thermostability design which is more targeted and predictable.

Directed evolution driving the generation of an efficient keratinase variant to facilitate the feather degradation

Keratinases can specifically degrade keratins, which widely exist in hair, horns, claws and human skin. There is a great interest in developing keratinase to manage keratin waste generated by the poultry industry and reusing keratin products in agriculture, medical treatment and feed industries. Degradation of keratin waste by keratinase is more environmentally friendly and more sustainable compared with chemical and physical methods. However, the wild-type keratinase-producing strains usually cannot meet the requirements of industrial production, and some are pathogenic, limiting their development and utilization. The main purpose of this study is to improve the catalytic performance of keratinase via directed evolution technology for the degradation of feathers. We first constructed a mutant library through error-prone PCR and screened variants with enhanced enzyme activity. The keratinase activity was further improved through fermentation conditions optimization and fed-batch strategies in a 7-L bioreactor. As a result, nine mutants with enhanced activity were identified and the highest enzyme activity was improved from 1150 to 8448 U/mL finally. The mutant achieved efficient biodegradation of feathers, increasing the degradation rate from 49 to 88%. Moreover, a large number of amino acids and soluble peptides were obtained as degradation products, which were excellent protein resources to feed. Therefore, the study provided a keratinase mutant with application potential in the management of feather waste and preparation of protein feed additive.

Bioconversion of Keratin Wastes Using Keratinolytic Microorganisms to Generate Value-Added Products

The management of keratinous wastes generated from different industries is becoming a major concern across the world. In each year, more than a billion tons of keratin waste is released into the environment. Despite some trials that have been performed and utilize this waste into valuable products, still a huge amount of keratin waste from different sources is a less explored biomaterial for making valuable products. This indicates that the huge amount of keratin waste is neither disposed properly nor converted into usable products rather thrown away to the environment that causes environmental pollution. Due to the introduction of this waste associated with different pathogenic organisms into soil and water bodies, human beings and other small and large animals are affected by different diseases. Therefore, there is a need for modern and ecofriendly approaches to dispose and convert this waste into usable products. Hence, the objective of this review is to give a concise overview regarding the degradation of keratin waste by biological approaches using keratinase producing microorganisms. The review also focuses on the practical use of keratinases and the economical importance of bioconverted products of keratinous wastes for different applications. Various researches have been studied about the source, disposal mechanisms, techniques of hydrolysis, potential use, and physical and chemical properties of keratin wastes. However, there is negligible information with regard to the use of keratin wastes as media supplements for the growth of keratinolytic microorganisms and silver retrieval from photographic and used X-ray films. Hence, this review differs from other similar reviews in the literature in that it discusses these neglected concerns.

Harnessing the Keratinolytic Activity of Bacillus licheniformis Through Random Mutagenesis Using Ultraviolet and Laser Irradiations

Keratinase is one of the important proteases, which is widely used for converting keratin of the keratinaceous materials into various value-added products. In this study, a popular keratinase producer, Bacillus licheniformis PWD-1, was exposed to ultraviolet (UV) and He-Ne laser irradiations to develop high keratinase-producing mutants. Laser irradiation showed a higher lethality of cells (94%) than UV treatment (92%), whereas laser treatment required a longer time (75 min) than UV treatment (20 min). A total of 58 mutants were selected from 176 isolates to study protein and keratinase production capability of the mutants. The highest keratin-to-casein (K:C) ratio (1.43) was exhibited by LU11 mutant, which was obtained from the combined laser and UV irradiations. The purified keratinase (65 kDa) of LU11 showed 40% yield 1.7-fold purity, while the respective value for wild enzyme was 29% and 1.3-fold. Both enzymes showed optimal activity at 55 ℃ and pH 8, with a Z value of 15.78 ℃ for LU11 and 19.72 ℃ for wild strain. The V_max and specific constant (K_cat/K_m) of the mutant enzyme were 357.17 U/ml and 33.11 min^-1 mM^-1, respectively, which were significantly higher than the respective values of wild enzyme (102.04 U/ml and 28.36 min^-1 mM^-1).

Mutations in the regulatory regions result in increased streptomycin resistance and keratinase synthesis in Bacillus thuringiensis

Keratinases are a group of proteases of great industrial significance. To take full advantage of Bacillus species as an inherent superior microbial producer of proteases, we performed the ribosome engineering to improve the keratinase synthesis capacity of the wild-type Bacillus thuringiensis by inducing streptomycin resistance. Mutant Bt(Str-O) was identified as a stable keratinase overproducer. Comparative characterization of the two strains revealed that, although the resistance to Streptomycin increased by eight-fold in MIC, the mutant's resistance to other commonly used antibiotics was not affected. Furthermore, the mutant exhibited an enhanced keratinase synthesis (1.5-fold) when cultured in a liquid LB medium. In the whole feather degradation experiment, the mutant could secret twofold keratinase into the medium, reaching 640 U/mL per 10⁷ CFU. By contrast, no significant differences were found in the scanning electron microscopic analysis and spore formation experiment. To understand the genetic factors causing these phenotypic changes, we cloned and analyzed the rpsL gene. No mutation was observed. We subsequently determined the genome sequences of the two strains. Comparing the rpsL gene revealed that the emergence of streptomycin resistance was not necessarily dependent on the mutation(s) in the generally recognized "hotspot." Genome-wide analysis showed that the phenotypic changes of the mutant were the collective consequence of the genetic variations occurring in the regulatory regions and the non-coding RNA genes. This study demonstrated the importance of genetic changes in regulatory regions and the effectiveness of irrational ribosome engineering in creating prokaryotic microbial mutants without sufficient genetic information.

Enhanced activity of Rhizomucor miehei lipase by directed saturation mutation of the propeptide

The propeptide is a short sequence that facilitates protein folding. In this study, four highly active Rhizomucor miehei lipase (RML) mutants were obtained through saturation mutagenesis at three propeptide positions: Ser8, Pro35, and Pro47. The enzyme activities of mutants P35 N, P47 G, P47 N, and S8E/P35S/P47A observed at 40 °C, and pH 8.0 were 10.19, 7.53, 6.15, and 8.24 times of that wild-type RML, respectively. The S8E/P35S/P47A mutant showed good thermostability. After incubation at 40 °C for 1 h, 98.98 % of its initial activity remained, whereas wild-type RML retained only 78.76 %. This result indicated that the enhancement of hydrophilicity of 35- and 47- amino-acid residues could promote the interaction between the propeptide and the mature peptide and the enzyme activity and expression level. Highly conserved sites had a more significant impact on enzyme performance than did other sites, similar to the Pro35 and Pro47 mutants showed in this study. This study provides a new idea for protein modification: enzyme performance can be improved through propeptide regulation.

Structure, Application, and Biochemistry of Microbial Keratinases

Keratinases belong to a class of proteases that are able to degrade keratins into amino acids. Microbial keratinases play important roles in turning keratin-containing wastes into value-added products by participating in the degradation of keratin. Keratin is found in human and animal hard tissues, and its complicated structures make it resistant to degradation by common proteases. Although breaking disulfide bonds are involved in keratin degradation, keratinase is responsible for the cleavage of peptides, making it attractive in pharmaceutical and feather industries. Keratinase can serve as an important tool to convert keratin-rich wastes such as feathers from poultry industry into diverse products applicable to many fields. Despite of some progress made in isolating keratinase-producing microorganisms, structural studies of keratinases, and biochemical characterization of these enzymes, effort is still required to expand the biotechnological application of keratinase in diverse fields by identifying more keratinases, understanding the mechanism of action and constructing more active enzymes through molecular biology and protein engineering. Herein, this review covers structures, applications, biochemistry of microbial keratinases, and strategies to improve its efficiency in keratin degradation.

Engineered pro-peptide enhances the catalytic activity of keratinase to improve the conversion ability of feather waste

Keratinase is an attractive industrial enzyme that can specifically catalyze keratin waste to obtain value-added products. A challenge to the application of keratinase is improving catalytic capacity to achieve efficient hydrolysis. In this study, we effectively expressed the keratinase gene from Bacillus licheniformis BBE11-1 in Bacillus subtilis WB600 based on pro-peptide engineering. Partial deletion of the pro-peptide sequence and the substitution of amino acid at the pro-peptide cleavage site (P1) suggested that the "chaperone effect" and "cleavage efficiency" of the pro-peptide determine the activity of the mature enzyme. Subsequently, seven target sites that can increase the activity of the mature enzyme by 16%-66% were obtained through the multiple sequence alignment of pro-peptides and site-directed mutation. We further performed combinatorial mutations at six sites based on the design principle of three-codon saturation mutations and obtained mutant 2-D12 (236.8 KU/mg) with a mature enzyme activity of 186% of the original (127.6 KU/mg). Finally, continuous fermentation was carried out in a 5-L bioreactor for 22 h, and the activity of the 2-D12 mature enzyme was increased to 391.6 KU/mg. Most importantly, 2-D12 could degrade more than 90% of feather waste into amino acids and peptides within 12 h with the aid of sulfite.

A Novel Thermostable Keratinase from Deinococcus geothermalis with Potential Application in Feather Degradation

Keratinase can specifically attack disulfide bridges in keratin to convert them from complex to simplified forms. Keratinase thermal stability has drawn attention to various biotechnological industries. In this study, a keratinase DgeKer was identified from a slightly thermophilic species, D. geothermalis. The in silico analysis showed that DgeKer is composed of signal peptide, N-terminal propeptide, mature domain, and C-terminal extension. DgeKer and its C-terminal extension-truncated enzyme (DgeKer-C) were cloned and expressed in E. coli. The purified DgeKer and DgeKer-C showed maximum activity at 70 °C and pH 9–The thermal stability assay (60 °C) showed that the half-life value of DgeKer and DgeKer-C were 103.45 min and 169.10 min, respectively. DgeKer and DgeKer-C were stable at the range of pH from 9 to 11 and showed good tolerance to some metal ions, surfactants and organic solvent. Furthermore, DgeKer could degrade feathers at 70 °C for 60 min. However, the medium became turbid with obvious softening of barbules after being treated with DgeKer-C, which might be due to C-terminal extension. In summary, a thermostable keratinase DgeKer with high efficiency degradation of feathers may have great potential in industry.

The tale of a versatile enzyme: Molecular insights into keratinase for its industrial dissemination

Keratinases are unique among proteolytic enzymes for their ability to degrade recalcitrant insoluble proteins, and they are of critical importance in keratin waste management. Over the past few decades, researchers have focused on discovering keratinase producers, as well as producing and characterizing keratinases. The application potential of keratinases has been investigated in the feed, fertilizer, leathering, detergent, cosmetic, and medical industries. However, the commercial availability of keratinases is still limited due to poor productivity and properties, such as thermostability, storage stability and resistance to organic reagents. Advances in molecular biotechnology have provided powerful tools for enhancing the production and functional properties of keratinase. This critical review systematically summarizes the application potential of keratinase, and in particular certain newly discovered catalytic capabilities. Furthermore, we provide comprehensive insight into mechanistic and molecular aspects of keratinases including analysis of gene sequences and protein structures. In addition, development and current advances in protein engineering of keratinases are summarized and discussed, revealing that the engineering of protein domains such as signal peptides and pro-peptides has become an important strategy to increase production of keratinases. Finally, prospects for further development are also proposed, indicating that advanced protein engineering technologies will lead to improved and additional commercial keratinases for various industrial applications.

Enzymatic Extraction of Bioactive and Self-Assembling Wool Keratin for Biomedical Applications

Keratin is widely recognized as a high-quality renewable protein resource for biomedical applications. Despite their extensive existence, keratin resources such as feathers, wool, and hair exhibit high stability and mechanical properties because of their high disulfide bond content. Consequently, keratin extraction is challenging and its application is greatly hindered. In this work, a biological extraction strategy is proposed for the preparation of bioactive keratin and the fabrication of self-assembled keratin hydrogels (KHs). Based on moderate and controlled hydrolysis by keratinase, keratin with a high molecular weight of approximately 45 and 28 kDa that retain its intrinsic bioactivities is obtained. The keratin products show excellent ability to promote cell growth and migration and are conferred with significant antioxidant ability because of their intrinsically high cysteine content. In addition, without the presence of any cross-linking agent, the extracted keratin can self-assemble into injectable hydrogels. The KHs exhibit a porous network structure and 3D culture ability, showing potential in promoting wound healing. This enzyme-driven keratin extraction strategy opens up a new approach for the preparation of keratin that can self-assemble into injectable hydrogels for biomedical engineering.

Recombinant expression and molecular engineering of the keratinase from Brevibacillus parabrevis for dehairing performance

Keratinase is capable of distinctive degradation of keratin, which provides an eco-friendly approach for keratin waste management towards sustainable development. In this study, the recombinant keratinase (KERBP) from Brevibacillus parabrevis was successfully expressed in Escherichia coli. The purified KERBP had the specific activity of 6005.3 U/mg. It showed remarkable tolerance to various surfactants and also no collagenolytic activity. However, the moderate thermal stability limited its further application. Thus, protein engineering was further adopted to improve its stability. The variants of T218S, S236C and N181D were constructed by site-directed mutagenesis and combinatorial mutagenesis. Compared with the wild type, the t_1/2 at 60 °C for the variants T218S, S236C and N181D were 3.05-, 1.18- and 1-fold increase, respectively. Moreover, the double variants N181D-T218S and N181D-S236C significantly improved thermostability with 5.1 and 2.9 °C increase of T₅₀, and prolonging t_1/2 at 60 °C with 4.09 and 1.54-fold, respectively. And the catalytic efficiency of the T218S and N181D-T218S variants was also significantly improved. Furthermore, the keratinase displayed favorable ability to dehair wool from skin within 7 h, which showed potential in leather dehairing. Our work contributes to a further insight into the thermostability of keratinase and offers a promising alternative for industrial leather application.

Biotransformation of keratin waste to amino acids and active peptides based on cell-free catalysis

BACKGROUND

Keratin is the primary constituent of the vertebrate epidermis and epidermal appendages, as well as the main waste product generated during poultry processing from feathers, hair, scales, nails, etc. Keratin is generally hard, stubborn and difficult to hydrolyze; however, it is also inexpensive and contains more than 85% protein. Currently, tens of millions of tons of keratin waste are produced each year worldwide; however, no effective methods for the recovery of keratin waste have been reported thus far, making such research urgent. Keratinase has been reported to be useful for keratin waste recovery; however, nearly all keratinases are unable to hydrolyze keratin after they are detached from living cell systems. This may be due to low keratinase activity and lack of synergistic factors.

RESULTS

Herein, the keratinase gene from Bacillus licheniformis BBE11-1 was successfully expressed in Bacillus subtilis WB600, allowing for improved activity of the recombinant keratinase KerZ1 to 45.14 KU/mL via promoter substitution and screening of the ribosome-binding sites. Further, real-time control of temperature, pH, dissolved oxygen, and feed strategy allowed the activity of KerZ1 to reach 426.60 KU/mL in a 15-L fermenter, accounting for a 3552-fold increase compared to the wild-type keratinase (120.1 U/mL). Most importantly, we proposed a method based on the synergistic action of keratinase KerZ1 and sodium sulfite, to hydrolyze feathers into amino acids. In specific, 100 g/L of feather waste can be successfully converted into 56.6% amino acids within 12 h, while supporting the production of dozens of bioactive peptides.

CONCLUSIONS

The activity of recombinant keratinase can be greatly enhanced via transcription and translational regulation in Bacillus subtilis. The synergistic action of keratinase and sulfite can rapidly degrade feather waste and produce amino acids and polypeptides.

Efficient keratinase expression via promoter engineering strategies for degradation of feather wastes

Keratinases are promising alternatives over ordinary proteases in several industrial applications due to their unique properties compared with their counterparts in the protease categories. However, their large-scale industrial application is limited by the low expression and poor fermentation efficiency of keratinase. Here, we demonstrate that the expression level of keratinase can be improved by constructing a more efficient enzyme expression system hereby enables the highest production titer as regarding recombinant keratinase production to date. Specially, ten promoters were evaluated and the aprE promoter exhibits a significant promotion of keratinase (kerBv) titer from 165 U/mL to 2605 U/mL in Bacillus subtilis. The batch fermentation mode resulted in a maximum keratinase activity of 7176 U/mL at 36 h in a 5-L fermenter. Furthermore, the extracellular keratinase activity attained up to 16,860 U/mL via fed-batch fermentation within 30 h. The combination of keratinase with l-cysteine brings about 66.4 % degree of degradation of feather. Our work provides a new insight into the development of efficient keratinase fermentation processes with B. subtilis cell factory.

Progress in Microbial Degradation of Feather Waste

Feathers are a major by-product of the poultry industry. They are mainly composed of keratins which have wide applications in different fields. Due to the increasing production of feathers from poultry industries, the untreated feathers could become pollutants because of their resistance to protease degradation. Feathers are rich in amino acids, which makes them a valuable source for fertilizer and animal feeds. Numerous bacteria and fungi exhibited capabilities to degrade chicken feathers by secreting enzymes such as keratinases, and accumulated evidence shows that feather-containing wastes can be converted into value-added products. This review summarizes recent progress in microbial degradation of feathers, structures of keratinases, feather application, and microorganisms that are able to secrete keratinase. In addition, the enzymes critical for keratin degradation and their mechanism of action are discussed. We also proposed the strategy that can be utilized for feather degradation. Based on the accumulated studies, microbial degradation of feathers has great potential to convert them into various products such as biofertilizer and animal feeds.