Cadmium-thiolate clusters in metallothionein: spectrophotometric and spectropolarimetric features.
Cd-thiolate cluster formation in rabbit liver metallothionein 1 (MT) has been followed at pH 8.4 by monitoring spectroscopic features below 300 nm as a function of increasing Cd-to-apometallothionein (apoMT) ratio. The emerging absorption profiles form a family of closely similar spectra attributable to tetrahedral Cd-tetrathiolate coordination previously established for Cd7-MT [Vasák, M., Kägi, J.H.R., & Hill, H.A.O. (1981) Biochemistry, 20, 2852-2856]. However, there is a 6-nm red shift of the unresolved lowest energy absorption band when greater than 3 equiv of Cd(II) is incorporated. This shift is paralleled by a changeover in the circular dichroism (CD) features of MT from a broad monophasic positive CD profile with ellipticity bands near 240 and 220 nm to a biphasic CD spectrum characterized by positive ellipticity bands at 260 and 224 nm and an interposed negative band at 240 nm. Both features can be attributed to a changeover from separate Cd-tetrathiolate units formed at low metal-to-apoMT ratio to Cd-thiolate clusters when the supply of cysteine ligands becomes limiting. A comparable red shift signaling the transition from the mononuclear to a trinuclear tetrahedral Cd-tetrathiolate complex is also observed upon titration of the synthetic tetrathiol dodecapeptide N-Ac-Pro-Cys-Orn-Cys-Pro-Glu-Cys-Glu-Cys-Arg-Arg-Val with Cd(II). The latter studies also provide evidence for the predominantly ligand (sulfur) character of the lowest energy Cd-tetrathiolate ligand-metal charge-transfer transition. As a corollary it is inferred that the biphasic CD profile arises from excitonic coupling of these sulfur-centered transition dipole moments dissymmetrically oriented within the Cd(II)-thiolate clusters.
Willner
H
H
Biochemisches Institut der Universität Zürich, Switzerland.
Vasák
M
M
Kägi
J H
JH
D000818:Animals; D002104:Cadmium / Q000378:metabolism*; D002942:Circular Dichroism; D008099:Liver / Q000378:metabolism; D008668:Metallothionein / Q000302:isolation & purification / Q000378:metabolism*; D009942:Organometallic Compounds / Q000378:metabolism*; D011485:Protein Binding; D011487:Protein Conformation; D011817:Rabbits; D013056:Spectrophotometry, Ultraviolet
D009942:Organometallic Compounds; D002104:Cadmium; D008668:Metallothionein