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Production of Proteolytic Enzymes by a Keratin-Degrading Aspergillus niger

DOI: 10.4061/2011/487093

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A fungal isolate with capability to grow in keratinous substrate as only source of carbon and nitrogen was identified as Aspergillus niger using the sequencing of the ITS region of the rDNA. This strain produced a slightly acid keratinase and an acid protease during cultivation in feather meal. The peak of keratinolytic activity occurred in 48?h and the maximum proteolytic activity in 96?h. These enzymes were partly characterized as serine protease and aspartic protease, respectively. The effects of feather meal concentration and initial pH on enzyme production were evaluated using a central composite design combined with response surface methodology. The optimal conditions were determined as pH 5.0 for protease and 7.8 for keratinase and 20?g/L of feather meal, showing that both models were predictive. Production of keratinases by A. niger is a less-exploited field that might represent a novel and promising biotechnological application for this microorganism. 1. Introduction Aspergillus niger is one of the most important microorganisms in biotechnology. It has been already used to produce extracellular enzymes such as glucose oxidase, pectinase, α-amylase and glucoamylase, organic acids, and recombinant proteins. In addition, A. niger is used for biotransformations and waste treatment [1–3]. Among the various enzymes produced by the fungus are included proteases. The major extracellular proteolytic activities in A. niger appear to be due to acid proteases [4]. Acid proteases [E.C.3.4.23] are endopeptidases that depend on aspartic acid residues for their catalytic activity and show maximal activity at low pH. These enzymes offer a variety of applications in the food, beverage industry, and medicine [5]. Keratin is a fibrous protein that occurs in vertebrates and exerts protective and structural functions. It is the major component of feathers, wool, scales, hair, stratum corneum, horns, scalps, and nails [6]. Keratin is insoluble and presents high mechanic resistance, as well as recalcitrance to common proteolytic enzymes like pepsin, trypsin, and papain [7]. This resistance is because of the tight folding of protein chain in α-helix (α-keratin) and β-sheets (β-keratin) in a super-coiled polypeptide chain, kept by strong association by disulfide bonds [8, 9]. Keratinases [EC 3.4.21/24/99.11] are specific proteases that display the capability of keratin degradation. These enzymes are gaining importance in the last years, with many applications associated with hydrolysis of keratinous substrates, mainly byproducts of agroindustrial processes [10].


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