Identification of copper ligands in Aspergillus oryzae tyrosinase by site-directed mutagenesis

Copper ligands of the recombinant tyrosinase from the fungus Aspergillus oryzae expressed in Saccharomyces cerevisiae or Escherichia coli were identified by site-directed mutagenesis. The recombinant protyrosinases expressed in S. cerevisiae were assayed for catalytic activities of mono-oxygenase and L-dopa oxidase at pH 5.5 after acid shock at pH 3.0. Replacements of His-63, His-84, His-93, His-290, His-294, His-332 or His-333 with asparagine resulted in mutant enzymes exhibiting no activities. The site-directed mutant Cys82Ala showed that Cys-82 was also an essential residue for the activity. We obtained homogeneous preparations of activated tyrosinases from mutated thioredoxin fusion gene products expressed in E. coli by acid shock. The copper contents of engineered mutants and wild-type enzyme expressed in E. coli were determined by atomic absorption spectrophotometry. The wild-type enzyme contained 2 g-atoms of copper/mol of the subunit. The His63Asn, His84Asn, His93Asn, His290Asn, His294Asn, His332Asn, His333Asn or Cys82Ala substitution decreased copper binding by approx. 50%, indicating that the mutants contain only approx. 1 g-atom of copper/mol of the subunit. The five mutants His63Asn, His93Asn, His290Asn, His294Asn and Cys82Ala contain only one copper ion, which is fully detectable by EPR. From the correlation of g( parallel) and (Cu)A( parallel), we deduced that the nitrogen or sulphur donors in the copper ligands should be in a square or a distorted tetrahedral geometric environment. In further atomic absorption spectrophotometry experiments, no copper atom was observed in the seven double mutants His63Asn/His290Asn, His63Asn/His294Asn, His63Asn/His332Asn, His63Asn/His333Asn, Cys82Ala/His290Asn, His84Asn/His333Asn and His93Asn/His290Asn. We propose a new structure of active sites of tyrosinase from A. oryzae: the most likely binding sites of tyrosinase for Cu(A) are His-63, His-84 and His-93, with the remaining conserved Cys-82 providing the fourth ligand. Cu(B) liganded by four histidine residues, His-290, His-294, His-332 and His-333, is identified as new binding motif of Cu(B).