Purification, characterisation and salt-tolerance molecular mechanisms of aspartyl aminopeptidase from Aspergillus oryzae 3.042.

A salt-tolerant aspartyl aminopeptidase (approximately 57kDa) from Aspergillus oryzae 3.042 was purified and identified. Specific inhibitor experiments indicated that it was an aminopeptidase containing Zn2+. Its optimal and stable pH values and temperatures were 7 and 50°C, respectively. Its relative activity remained beyond 30% in 3M NaCl solution for 15d, and its Km and Vmax were slightly affected in 3M NaCl solution, indicating its excellent salt-tolerance. A comprehensive analysis including protein homology modelling, molecular dynamics simulation, secondary structure, acidic residues and hydrophobicity of interior residues demonstrated that aspartyl aminopeptidase had a greater stability than non-salt-tolerant protease in high salinity. Higher contents of ordered secondary structures, more salt bridges between hydrated surface acidic residues and specific basic residues and stronger hydrophobicity of interior residues were the salt-tolerance mechanisms of aspartyl aminopeptidase.