A new carboxypeptidase from Aspergillus niger with good thermostability, pH stability and broad substrate specificity

A new serine carboxypeptidase gene, capA, was identified in Aspergillus niger CBS 513.88 by reading genomic information and performing sequence alignment, and the gene was cloned and expressed in Pichia pastoris GS115. In a shake flask, the enzyme activity of the recombinant strain GS115 (pPIC9K-capA) reached 209.3 U mg^-1. The optimal temperature and pH for enzyme activity were determined to be 45 °C and 6.0, respectively. After incubation at 40-50 °C or at pH 4.0-8.0 for 1 h, the enzyme retained more than 80% or 60% of its initial activity. The presence of 1-10 mmol L^-1 Mg²⁺ enhanced the activity of CapA, whereas 1-10 mmol L^-1 Cu²⁺, Fe²⁺, or Co²⁺, 10 mmol L^-1 Mn²⁺, or 1-10 mmol L^-1 phenylmethylsulfonyl fluoride (PMSF) significantly inhibited its activity. CapA had a broad substrate specificity and preferred the hydrophobic amino acids Leu and Lys at the C terminus of proteins, and N-benzyloxycarbonyl-L-phenylalanyl-L-leucine (Cbz-Phe-Leu) was the optimal substrate, for which CapA exhibited K_m 0.063 mmol L^-1 and k_cat/K_m 186.35 mmol L^-1 s^-1. The good thermostability, pH stability and hydrolysis characteristics of CapA provide a solid foundation for application in the food and biotechnology fields.