To mitigate the climate change caused by CO2 emission, the global incentive to the low-carbon alternatives as replacement of fossil fuel-derived products continuously expands the need for renewable feedstock. There will be accompanied by the generation of enormous protein waste as a result. The economical viability of the biorefinery platform can be realized once the surplus protein waste is recycled in a circular economy scenario. In this context, the present review focuses on the current development of biotechnology with the emphasis on biotransformation and metabolic engineering to refine protein-derived amino acids for production of fuels and chemicals. Its scope starts with the explosion of potential feedstock sources rich in protein waste. The availability of techniques is applied for purification and hydrolysis of various feedstock proteins to amino acids. Useful lessons are leaned from the microbial catabolism of amino acids and lay a foundation for the development of the protein-based biotechnology. At last, the future perspective of the biorefinery scheme based on protein waste is discussed associated with remarks on possible solutions to overcome the technical bottlenecks.

Background Keratins, insoluble proteins with a robust structure, are a major component of epidermal tissue and appendages such as hair, feathers, nails, and walls. Keratinous waste mainly emanates from poultry and leather industries, thereby severely contaminating the environment. Keratinase can lyse proteins with robust cross-linked structures, such as keratin, and can hence be used in animal feed, fertilizer, detergent, leather, pharmaceutical, and cosmetic industries. Bacillus polyfermenticus B4, isolated from feather compost, secretes keratinase to metabolize feathers. Hence, this study aimed to investigate the enzymatic characteristics and recombinant production of keratinase from B. polyfermenticus B4. Methods A novel keratinase KerP was isolated from B. polyfermenticus B4 and overexpressed in B. subtilis PT5, via the T7 promoter. Results The highest keratinolytic activity of recombinant KerP was observed at pH 9.0 and 60 °C. Enzyme activity was enhanced with Fe²⁺, Mn²⁺, and SDS, and inhibited by Zn²⁺, Ni²⁺, EDTA, PMSF, and β-mercaptoethanol. KerP production was the highest at 473 ± 20 U/mL with B. subtilis aprE signal peptide using LB broth. Conclusions The novel keratinase KerP has potential industrial applications, particularly in the treatment of keratinous waste.