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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 ...

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Clostridium butyricum, a well known H2 producing bacterium, produces lactate, butyrate, acetate, ethanol, and CO2 as its main by‐products from glucose. The conversion of pyruvate to lactate, butyrate and ethanol involves oxidation of NADH. It was hypothesized that the NADH could be increased if the formation of these by‐products could be eliminated, resulting in enhancing H2 yield. Herein, this study aimed to establish a genetic and metabolic approach for enhancing H2 yield via redirection of metabolic pathways of a C. butyricum strain. The ethanol formation pathway was blocked by disruption of aad (encoding aldehyde‐alcohol dehydrogenase) using a ClosTron plasmid. Although elimination of ethanol formation alone did not increase hydrogen production, the resulting aad‐deficient mutant showed approximately 20% enhanced performance in hydrogen production with the addition of sodium acetate. This work demonstrated the possibility of improving hydrogen yield by eliminating the unfavorable by‐products ethanol and lactate. Biotechnol. Bioeng. 2013; 110: 338–342. © 2012 Wiley Periodicals, Inc.