Metallothioneins EXAFS

Fig. 1. X-ray absorption spectrum (XAS) of Cu—Zn metallothionein at the Cu and Zn K-edges. The structure near the edge, referred to as XANES is dominated by multiple scattering events while the extended structure, referred to as EXAFS, at photoelectron energies greater than 30-50 eV is primarily due to single scattering events...

Metallothioneins are an ubiquitous class of proteins with which the important roles of metal storage (Cu, Zn) and detoxification (Cd, Pb, Hg) are associated EXAFS studies have been carried out on a variety of metallothioneins including Zttj, ZUjCuj, ZiigCuj, Cd, and to show that metal atoms are exclusively... 

Table 1. Structural parameters deduced from EXAFS for a number of metallothioneins . R denotes the distance of scattering atoms from the absorbing atom is a Debye-Waller factor and...

Fig 1. The X-ray absorption spectrum of Cu,Zn-metallothionein, showing the Cu and Zn K-edges, the demarcation between the X-ray absorption near-edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS), and a pictorial representation of the origin of EXAFS. 

Metallothioneins are a unique and widely distributed group of proteins. They are characterized by their low molecular weight (—6000), high cysteinyl content, and the ability to bind substantial numbers of metal ions (43). The proteins bind copper and zinc, thereby providing a mobile pool as part of the normal metabolism of these elements, and offer protection from the invasion of inorganic forms of the toxic elements cadmium, lead, and mercury. In addition, other metals, such as iron and cobalt, can be induced to bind. XAS is ideally suited to probe the environment of these different metal atoms (see Fig. 1), and the structural interpretations obtained from an analysis of the EXAFS data obtained in several such studies are summarized in Table 1(44). Thus, in each case, the data are consistent with the primary coordination of the metal deriving from the cysteinyl residues. 

Structural Parameters Deduced from EXAFS OF Metallothioneins"... 

The first EXAFS studies of a cadmium environment in a protein were achieved for CdsZna- and Cdy-Metallothionein from rat liver (45). The two samples manifest identical EXAFS and the data are consistent with a shell of four sulfur atoms at —2.51 A. These results demonstrate that the inequivalence of cadmium atoms observed by ii Cd NMR studies of metallothioneins (46) does not arise from marked variations in atom type, coordination number, or metal-ligand distances within the metals first coordination sphere. 

Clear evidence for the close approach of metal atoms in Cdy-metal-lothionein is well established by Cd NMR spectroscopy. Indications of the intermetallic distances involved for cadmium and other metals have been sought from EXAFS. Although some indications of back-scattering of metals bound within metallothioneins have been obtained (31, 47), the principal conclusion is that these metal-metal separations are not coherent. Therefore, the backscattered waves, especially with their high frequency at distances >3 A, engage in destructive interference, which effectively renders them silent in the EXAFS. 

Another important application of the EXAFS technique is the study of transition metal ions involved in biological macromolecules. The analysis of EXAFS data from gold metallothionein suggests that the gold atoms are twofold coordinated whereas the zinc atoms, which gold replaces, are fourfold coordinated. Eidsness and Elder have studied several Au—S and Au—P type structures as models for the biological systems. The results of these studies are summarized in Table 3. 

Figure 4.35 X-ray Absorption Spectrum of Cu,Zn-metallothionein. The copper and zinc K-edges are observed and the XANES and EXAFS regions are shown for the zinc K-edge.

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