High-level production, chemical modification and site-directed mutagenesis of a cephalosporin C acylase from Pseudomonas strain N176.
A cephalosporin acylase from Pseudomonas strain N176 hydrolyses both 7-beta-(4-carboxybutanamido)-cephalosporanic acid (glutarylcephalosporanic acid) and cephalosporin C to 7-amino-cephalosporanic acid. However, its productivity in the original host was low and its activity against cephalosporin C was not sufficient for direct large-scale production of 7-amino-cephalosporanic acid. In order to overcome these problems, we established a high-level expression system for the acylase in Escherichia coli. Tyr270 in the acylase is reported to play an important role in the interaction with glutarylcephalosporanic acid, as determined from the reaction with an affinity-label reagent, 7 beta-(6-bromohexanoylamido) cephalosporanic acid [Ishii, Y., Saito, Y., Sasaki, H., Uchiyama, F., Hayashi, M., Nakamura, S. & Niwa, M. (1994) J. Ferment. Bioeng. 77, 598-603] and modification with tetranitromethane [Nobbs, T. J., Ishii, Y., Fujimura, T., Saito, Y. & Niwa, M. (1994) J. Ferment. Bioeng. 77, 604-609]. From carbamoylation with potassium cyanate and site-directed point mutagenesis of the cephalosporin C acylase, we have deduced that Tyr270 exists at a position where it can interact with a residue (possibly Ser239) corresponding to inactivation by carbamoylation. We mutated Met269 and Ala271 of the acylase and found that mutation of Met269 to Tyr or Phe caused a 1.6-fold and 1.7-fold increase, respectively, of specific activity against cephalosporin C as compared to that of the wild-type enzyme. Kinetic studies of these mutants revealed that their kcat values increased, although their Km values against cephalosporin C were not changed. These data indicate that the mutation of Met269 near Tyr270 induces a minor conformational change to increase the stability of the activated complex with the enzyme and cephalosporin C. In particular, a mutant in which Met269 was replaced by Tyr was 2.5-fold more efficient in converting cephalosporin C to 7-amino-cephalosporanic acid than the wild-type enzyme under conditions similar to those in a bio-reactor system.