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.

Biological Applications of Perturbed Angular Correlations of γ-Ray Spectroscopy

“…Thus, one function of the methionine might be to prevent a water molecule from entering the metal ion binding site. 27,33 methionine does not take part in bonding to the metal but appears to stabilize the metal geometry was also supported by 199m Hg studies of azurin. 45,46 For references to studies of other mutants of azurin, please refer to Table 3.…”

“…This was later expanded by a PAC study using 111 Ag on wild-type azurin as well as Met121Leu-azurin of P. aeruginosa. 33 In Figure 8, the Fouriertransformed PAC spectra of Ag-azurin, Cd-azurin, Ag-Met121Leu-azurin, and Cd-Met121Leu-azurin are compared. A comparison clearly shows that for wildtype azurin metal substitution with Ag or Cd results in almost the same structure.…”

Effects of the nuclear transformation111Ag(I)to111Hansen1, Bukrinsky2, Hemmingsen3 et al. 1999Phys. Rev. BSelf Cite91100View full textAdd to dashboardCite

“…A relaxation of a Ag͑I͒ geometry in to a Cd͑II͒ geometry has also been observed in the copper protein, azurin, within a time scale of about 100 ns. 1 The present study is the first time that the full NQI tensor has been determined for a Cd͑II͒ complex experimentally as well as by ab initio calculations. Perspectives includes PAC experiments on single crystals of metal containing proteins, providing the full NQI tensor, thereby increasing the information content.…”

“…Similar investigation performed on the copper protein azurin showed that the coordination geometry for Ag͑I͒ relaxes to the geometry of Cd͑II͒ on a time scale of about 100 ns at 25°C. 1 Applying perturbed angular correlation of ␥-ray ͑PAC͒ spectroscopy to a single crystal allows for the determination of the full nuclear quadrupole interaction ͑NQI͒ tensor in contrast to experiments on randomly oriented molecules, where only the eigenvalues of the tensor are obtained. Previous work on the determination of the NQI tensor has been performed with various experimental techniques including PAC, NMR, and electron-nuclear double resonance ͑EN-DOR͒ spectroscopy.…”

“…Ag͑I͒ was prepared as described in Bauer et al 1 Small crystals of Ag͑imidazole͒ 2 NO 3 were obtained from aqueous solution by mixing 1.2 g ͑0.007 mol͒ AgNO 3 in 10 ml water with 2.0 g ͑0.03 mol͒ imidazole in 10 ml of water. A white precipitate of silver oxide/silver hydroxide immediately appeared upon mixing of the two solutions.…”