Specific Metal Ion Recognition Mechanism by Protein Molecule

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The UV resonance Raman spectrum of pseudoazurin gives a copper coordinated histidine imidazole band at 1385 cm^<-1>. A characteristic histidine imidazolium Raman band at 1408 cm^<-1> is detected in the UVRR spectrum of Cu(I) pseudoazurin at acidic pH. This supports the protonation of the His81 ligand in the reduced protein at low pH values. The imidazolium Pseudoazurin substituted with Zn^<2+> gave characteristic Raman Bands for the metal ion coordinated imidazole at 1388 cm^<-1> at neutral pH. The histidine imidazole band diminishes in intensity in Zn^<2+> pseudoazurin upon lowering the pH. In all of the metal-substituted pseudoazurins an imidazolium band at 1408 cm^<-1> appears at pH < 4.0. Pseudoazurin substituted with Zn^<2+>, Cd^<2+>, and Ni^<2+> give very similar pH dependence of their X-Pro Raman bands as apo-pseudoazurin. These findings indicate that the metal ion is released from the active site of Zn^<2+>, Cd^<2+>, and Ni^<2+> pseudoazurin at acidic pH accompanied by a change in the protein's secondary structure. This behavior is clearly absent in both the Cu(I) and CU(II) proteins indicating that pseudoazurin discriminates between copper and the other metal ions studied.