NDLI: Spectroscopic Studies of Copper, Silver and Gold-Metallothioneins

Content Provider	PubMed Central
Author	Stillman, Martin J. Presta, Anthony Gui, Ziqi Jiang, De-tong
Copyright Year	1994
Abstract	Metallothionein is a ubiquitous protein with a wide range of proposed physiological roles, includingthe transport, storage and detoxification of essential and nonessential trace metals. The aminoacid sequence of isoform 2a of rabbit liver metallothionein, the isoform used in our spectroscopicstudies, includes 20 cysteinyl groups out of 62 amino acids. Metallothioneins in general representan impressive chelating agent for a wide range of metals. Structural studies carried out by anumber of research groups (using 1H and 113Cd NMR, X-ray crystallography, more recently EXAFS,as well as optical spectroscopy) have established that there are three structural motifs for metalbinding to mammalian metallothioneins. These three structures are defined by metal to proteinstoichiometric ratios, which we believe specifically determine the coordination geometry adoptedby the metal in the metal binding site at that metal to protein molar ratio. Tetrahedral geometry isassociated with the thiolate coordination of the metals in the M7-MT species, for M = Zn(II), Cd(II),and possibly also Hg(II), trigonal coordination is proposed in the M11-12-MT species, for M = Ag(I),Cu(I), and possibly also Hg(II), and digonal coordination is proposed for the metal in the M17-18-MTspecies for M = Hg(II), and Ag(I). The M7-MT species has been completely characterized for M =Cd(II) and Zn(II). 113Cd NMR spectroscopic and x-ray crystallographic data show that mammalianCd7-MT and Zn7-MT have a two domain structure, with metal-thiolate clusters of the form M4(Scys)11 (the α domain) and M3(Scys)9 (the β domain). A similar two domain structure involving Cu6(Scys)11 (α) and Cu6(Scys)9 (β) copper-thiolate clusters has been proposed for the Cu12-MT species.Copper-, silver- and gold-containing metallothioneins luminesce in the 500-600 nm region fromexcited triplet, metal-based states that are populated by absorption into the 260-300 nm region ofthe metal-thiolate charge transfer states. The luminescence spectrum provides a very sensitiveprobe of the metal-thiolate cluster structures that form when Ag(I), Au(I), and Cu(I) are added tometallothionein. CD spectroscopy has been used in our laboratory to probe the formation ofspecies that exhibit well-defined three-dimensional structures. Saturation of the optical signalsduring titrations of MT with Cu(I) or Ag(I) clearly show formation of unique metal-thiolate structuresat specific metal:protein ratios. However, we have proposed that these M=7, 12 and 18 structuresform within a continuum of stoichiometries. Compounds prepared at these specific molar ratioshave been examined by X-ray Absorption Spectroscopy (XAS) and bond lengths have beendetermined for the metal-thiolate clusters through the EXAFS technique. The stoichiometric ratiodata from the optical experiments and the bond lengths from the XAS experiments are used topropose structures for the metal-thiolate binding site with reference to known inorganicmetal-thiolate compounds.
Related Links	http://dx.doi.org/10.1155/mbd.1994.375
Ending Page	394
Page Count	20
Starting Page	375
File Format	PDF
ISSN	07930291
Journal	Metal-Based Drugs
Issue Number	5-6
Volume Number	1
Language	English
Publisher	Hindawi Publishing Corporation
Publisher Date	1994-01-01
Access Restriction	Open
Rights Holder	Hindawi Publishing Corporation
Subject Keyword	Toxicology Inorganic Chemistry Pharmacology Drug Discovery Research in Higher Education
Content Type	Text
Resource Type	Article
Subject	Drug Discovery Toxicology Pharmacology Inorganic Chemistry

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