Directed evolution of N-carbamyl-D-amino acid amidohydrolase for simultaneous improvement of oxidative and thermal stability

Directed evolution of N-carbamyl-D-amino acid amidohydrolase from Agrobacterium tumefaciens NRRL B11291 was attempted in order to simultaneously improve oxidative and thermal stability. A mutant library was generated by DNA shuffling, and positive clones with improved oxidative and thermal stability were screened on the basis of the activity staining method on a solid agar plate containing pH indicator (phenol red) and substrate (N-carbamyl-D-p-hydroxyphenylglycine). Two rounds of directed evolution resulted in the best mutant 2S3 with a significantly improved stability. Oxidative stability of the evolved enzyme 2S3 was about 18-fold higher than that of the wild type, and it also showed an 8-fold increased thermostability. The K-m value of 2S3 was comparable to that of wild-type enzyme, but k(cat) was slightly decreased. DNA sequence analysis revealed that six amino acid residues (Q23L, V40A, H58Y, G75S, M184L, and T262A) were substituted in 2S3. From the mutational analysis, four mutations (Q23L, H58Y, M184L, and T262A) were found to lead to an improvement of both oxidative and thermal stability. Of them, T262A had the most significant effect, and V40A and G75S only increased the oxidative stability.