Proteinases are largely used in several industries for biotechnological applications involving the hydrolysis of protein substrates. Among them, bacterial keratinases are of particular interest because of their action on insoluble keratin substrates and, generally, on a broad range of protein substrates. The animal proteases are mixtures of trypsin, chymotrypsin, and various peptidases that may contain amylase or lipase as secondary enzymes. For the most part and for economic reasons, enzymes from microorganisms have come to play a significant role in recent years and enzyme products of microbial origin are already being produced on a wide scale.
Since microorganisms can be made to propagate rapidly and freely, they are an ideal source for enzymes. Generally, neutral and alkaline proteases are obtained from bacteria, which differ in their pH activity range. Fungal proteases are also classified according to the pH activity range: fungal acid proteases act between pH 2.5 and 6.0. Fungal alkaline proteases belong to the same group of serine proteases as alkaline bacterial proteases. However, these are more heat sensitive and are quickly deactivated above 60 degrees Celsius. Most studies have shown that some representatives of Fusarium, Acremonium, and Geotrichum are the most active. However, when strains are cultivated in submerged conditions in a medium with feather broth as the sole source of carbon and nitrogen, other fungi have been proved to be potent, including species from the genus Aspergillus and Trichurus. In addition, the metabolism of sulfur is very important in keratinophilic fungi because keratin is a sulfur-rich substrate and numerous disulphide bridges are the main source of its high resistance to digestion. Furthermore, in hard keratins, the rate of hydrolysis corresponds roughly to the “hardness,” i.e., cystine content. For example, feathers are therefore cleaved more easily than human hair.
Feather hydrolysates produced by bacterial keratinases have been used as additives for animal feed, and several species of bacteria with high keratinolytic activity has been isolated from feather meal broth. In fact, the production of keratinase and capabilities for complete degradation of raw feathers has also been demonstrated (Brandelli and Riffel 2005). Moreover, in several new studies it has been established that pepsin digestibility and amino acid content of fermented feather meal can be largely improved over commercial feather meal. Indeed, since the microbial cells can also supply carotenoid pigments to fermented feather meal, the results suggest that bacteria enriched feather meal may be useful in animal feeding not just as protein, but also as a pigment source.
Microbial keratinases may have other novel uses in improving value of by-products. Microbial keratinases have become biotechnologically important since recent advances in recombinant DNA technology and the ability to selectively exchange amino acids by site-directed mutagenesis have allowed the introduction of predesigned changes into the gene for the synthesis of a protein with an altered function that is desired for the application. Identification of the gene and knowledge of the three-dimensional structure of the protein in question are the two main prerequisites for protein engineering.
The x-ray crystallographic structures of several proteases have been determined. Proteases from bacteria, fungi, and viruses have been engineered to improve their properties to suit their particular applications. Since it has been confirmed that bacterial keratinase from at least one species of the genus Bacillus can cause enzymatic breakdown of prion proteins, their potential application in the challenging field of prion degradation most likely will transform the protease world in the very near future (Gupta and Ramnani 2006).
References
Brandelli, A., and A. Riffel. 2005. Production of an extracellular keratinase from Chryseobacterium sp. growing on raw feathers. Electronic Journal of Biotechnology. Vol. 8, No. 1.
Gupta, R., and P. Ramnani. 2006. Microbial keratinases and their prospective applications: an overview. Applied Microbiology and Biotechnology. 70:1-13.
Tech Topics - June 2006 Render