NMR spectroscopic studies of I = 1/2 metal ions in biological systems

This article reviews the use of nuclear magnetic resonance methods of spin 1/2 metal nuclei to probe the metal binding site(s) in a variety of metalloproteins. The majority of the studies have involved native Zn(II) and Ca(II) metalloproteins where there has been isostructural substitution of these metal ions with the I = 1/2 Cd-111/113(II) ion. Also included are recent studies that have utilized the Ag-109(I) ion to probe Cu(I) sites in yeast metallothionein and Hg-199(II) as a probe of the metal binding sites in mercury resistance proteins. Pertinent aspects for the optimal execution of these experiments along with the procedures for the metal substitution reactions are discussed together with the presentation of a Cd-113 chemical shift correlation map with ligand type and coordination number. Specific examples of protein systems studied using the Cd-111/113 and Ag-109 nuclei include the metallothionein superfamily of Zn(II)- and Cu(I)-binding proteins from mammalian, invertebrate, and yeast systems. In addition to the structural features revealed by these metal ion nuclear magnetic resonance studies, important new information is frequently provided about the dynamics at the active-site metal ion. In an effort for completeness, other less frequently used spin 1/2 metal nuclei are mentioned.