Improvement of expression level of keratinase Sfp2 from Streptomyces fradiae by site-directed mutagenesis of its N-terminal pro-sequence

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.

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.

Streptomyces as Microbial Chassis for Heterologous Protein Expression

Heterologous production of recombinant proteins is gaining increasing interest in biotechnology with respect to productivity, scalability, and wide applicability. The members of genus Streptomyces have been proposed as remarkable hosts for heterologous production due to their versatile nature of expressing various secondary metabolite biosynthetic gene clusters and secretory enzymes. However, there are several issues that limit their use, including low yield, difficulty in genetic manipulation, and their complex cellular features. In this review, we summarize rational engineering approaches to optimizing the heterologous production of secondary metabolites and recombinant proteins in Streptomyces species in terms of genetic tool development and chassis construction. Further perspectives on the development of optimal Streptomyces chassis by the design-build-test-learn cycle in systems are suggested, which may increase the availability of secondary metabolites and recombinant proteins.

Advances in research on calf rennet substitutes and their effects on cheese quality

Milk coagulation is an important step in cheese production, and milk-clotting enzymes (MCEs) play a major role in this process. Calf rennet is the most widely used MCE in the cheese industry. The use of calf rennet substitutes is becoming necessary due to the limited availability of calf rennet and the increase in cheese consumption. The objective of this review is to summarize the latest findings on calf rennet substitutes (animal MCEs, plant-derived MCEs, recombinant MCEs and microbial MCEs) and their application in cheese production. Special emphasis has been placed on aspects of the effects of these substitutes on hydrolysis, functional peptides, cheese variety and cheese yield. The advantages and disadvantages of different calf rennet substitutes are discussed, in which microbial MCEs have the advantages of less expensive production, greater biochemical diversity, easier genetic modification, etc. In particular, some of these MCEs have suitable characteristics for cheese production and are considered to be the most potential calf rennet substitutes. Moreover, challenges and future perspectives are presented to provide inspiration for the development of excellent calf rennet substitutes.

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.

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.

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

Keratinases are becoming biotechnologically important since they have shown potential in hydrolysis of recalcitrant keratins with highly rigid and strongly cross-linked structures. However, the large-scale application of keratinases has been limited by the inefficient expression level and low enzyme activity. In this work, we employed pro-peptide engineering and saturation mutagenesis to construct excellent keratinase variants with improved activities. It turned out that amino acid substitutions at the pro-peptide cleavage site (P1) could accelerate the release of active mature enzymes, resulting in a 3-fold activity increase. Eighteen sites of the pro-peptide area were targeted for codon mutagenesis, and a multisite saturation mutagenesis library of the six potential sites was generated, achieving a significant improvement of keratinase activity from 179 to 1114 units/mL. Also, the mutants exhibited alterant catalytic properties. Finally, fermentation for keratinase production in a 15 L fermenter was carried out, and the enzyme activity reached up to over 3000 units/mL. Our results demonstrated that pro-peptide engineering played a crucial role in high expression and engineering of proteases. This study provides a universal route toward improvement of industrial enzymes that were first synthesized as precursors in the form of pre-pro-protein.

Expression in Pichia pastoris and characterization of Rhizomucor miehei lipases containing a new propeptide region

A large number of propeptide regions from various proteins have been identified which function as intramolecular chaperones and assist the folding of the respective functional domains. The same polypeptide can fold into an altered conformation because of a mutated intramolecular chaperone and can maintain the "memory" of the folding process (new physicochemical properties). Two new kinds of Rhizomucor miehei lipase (RML) were constructed by replacing its propeptide region with that from either Rhizopus chinensis lipase (RCL) or Rhizopus oryzae lipase (ROL). The enzymatic properties were also analyzed and compared between wild-type RML and the mutants. The results indicated that the same polypeptide can fold into different conformations because of changes in the propeptide region.

Genome and secretome analyses provide insights into keratin decomposition by novel proteases from the non-pathogenic fungus Onygena corvina

Poultry processing plants and slaughterhouses produce huge quantities of feathers and hair/bristle waste annually. These keratinaceous wastes are highly resistant to degradation. Onygena corvina, a non-pathogenic fungus, grows specifically on feathers, hooves, horn, and hair in nature. Hence, the proteases secreted by O. corvina are interesting in view of their potential relevance for industrial decomposition of keratinaceous wastes. We sequenced and assembled the genome of O. corvina and used a method called peptide pattern recognition to identify 73 different proteases. Comparative genome analysis of proteases in keratin-degrading and non-keratin-degrading fungi indicated that 18 putative secreted proteases from four protease families (M36, M35, M43, and S8) may be responsible for keratin decomposition. Twelve of the 18 predicted protease genes could be amplified from O. corvina grown on keratinaceous materials and were transformed into Pichia pastoris. One of the recombinant proteases belonging to the S8 family showed high keratin-degrading activity. Furthermore, 29 different proteases were identified by mass spectrometry in the culture broth of O. corvina grown on feathers and bristle. The culture broth was fractionated by ion exchange chromatography to isolate active fractions with five novel proteases belonging to three protease families (S8, M28, and M3). Enzyme blends composed of three of these five proteases, one from each family, showed high degree of degradation of keratin in vitro. A blend of novel proteases, such as those we discovered, could possibly find a use for degrading keratinaceous wastes and provide proteins, peptides, and amino acids as valuable ingredients for animal feed.