Cassette mutagenesis of Met121 in azurin from Pseudomonas aeruginosa.

Cassette mutagenesis was used to exchange the suggested copper ligand Met121 in azurin to all other amino acids, and a stop codon. The mutant proteins were characterized by optical absorption spectroscopy and EPR. At low pH, all mutants exhibit the characteristics of a blue type 1 copper protein, indicating that methionine is not needed to create a blue copper site. At high pH, the Glu121 and the Lys121 mutants constitute a new form of protein-bound copper that is outside the range of type 1 copper.