Keratin Waste Recycling Based on Microbial Degradation: Mechanisms and Prospects

Abstract

The severe shortage of protein resources has motivated researchers to seek cheap and sustainable proteins and to produce new functional materials in an environmentally friendly way. Keratin is a ubiquitous and stubborn protein that is attractive for sustainability but difficult to recycle. Structurally, keratin is characterized by a large amount of disulfide bonds, hydrogen bonds and hydrophobic forces and results in considerable hardness. Biorecycling of keratin is more promising than traditional processors because of the green production process, rich products and higher product value. Microbial strains capable of degrading keratin are constantly being developed. However, the mechanism of keratin decomposition by these microorganisms is not fully elucidated, which greatly hinders the sustainable use of keratin waste. In this review, we summarize the underlying mechanisms involved in microbial decomposition of keratin and attempt to discuss prospects and future directions in conjunction with systems biology strategies.

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

“…The process of microbial degradation of keratin involves three essential steps, namely denaturation, decomposition, and transamination [13,36,37]. Denaturation is critical, and involves opening the dense disulfide bond structure of keratin.…”

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

“…Despite the interest in the enzymatic hydrolysis of keratin, mechanisms of keratin degradation in microorganisms are not fully understood. There is evidence that microbial degradation of keratin proceeds via a consortium of enzymes ( Figure 2 [6,7,89]).…”

“…Various mechanisms have been suggested for fungal systems. Disulphide bond reductases and the intracellular cysteine dioxygenase can break the structure-stabilizing disulphide bridges in keratin [6,7,90]. Cysteine dioxygenase in conjunction with aspartate aminotransferase produces the reducing agent, sulphite, from cysteine, which is secreted into the surroundings and contributes to the chemical reduction of the disulphide bond.…”

“…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.…”

“…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.…”