Interplay between Oxidation State and Coordination Geometry of Metal Ions in Azurin

Abstract

The small electron transporting copper protein, azurin, has been studied in order to investigate the interplay between the oxidation states of the metal and its coordination geometry. The results show that the metal coordination geometry for Ag(I) in Ag(I) substituted wild type azurin is only slightly different from the geometry for Cd(II) in cadmium substituted azurin both being similar to the geometry for copper in native azurin. Furthermore, the coordination geometry for Ag(I) in the Met121 to Leu substituted mutant of azurin is also similar to the geometry of copper in native azurin. In contrast, previously published results show that Cd(II) substituted Met121Leu-azurin exhibits two different coordination geometries for Cd(II), one again similar to the wild type geometry and another very flexible and distinctly different from wild type azurin. These results have been obtained by Perturbed Angular Correlation of γ-rays spectroscopy using the two radioactive isotopes 111Ag(I) and 111mCd(II) as probes of a monovalent and a divalent ion, respectively. The technique also revealed that the metal-coordination geometry for Ag(I) in wild type azurin relaxes to the coordination geometry of Cd(II) on a time scale of 100 ns after the decay from 111Ag(I) to 111Cd(II). We suggest that the role of Met121 is to maintain the rigid tricoordinated metal coordination geometry independent of the oxidation state of the metal.