Separation, biochemical characterization and salt-tolerant mechanisms of alkaline protease from Aspergillus oryzae

BACKGROUNDThe salt tolerance of proteases secreted by Aspergillus oryzae 3.042 closely relates to the utilization of raw materials and the quality of soy sauce. However, little is known about the salt-tolerant proteases and their salt-tolerant mechanisms. RESULTSIn this study, we isolated and identified a salt-tolerant alkaline protease (AP, approximately 29kDa) produced by A. oryzae 3.042. It was considered as a metal-ion-independent serine protease. The optimum and stable pH values were both pH9.0 and the optimum temperature was 40 degrees C. Over 20% relative activity of AP remained in the presence of 3.0molL(-1) NaCl after 7days, but its K-m and V-max were only mildly influenced by the presence of 3.0molL(-1) NaCl, indicating its outstanding salt tolerance. Furthermore, AP was more stable than non-salt-tolerant protease at high salinity. The salt-tolerant mechanisms of AP could be due to more salt bridges, higher proportion of ordered secondary structures and stronger hydrophobic amino acid residues in the interior. CONCLUSIONSThe above results are vital for maintaining, activating and/or modulating the activity of AP in high-salt environments. They would also provide theoretical guidance for the modification of AP and the engineering of A. oryzae 3.042 so as to secrete more AP. (c) 2018 Society of Chemical Industry