title: Spectroscopic properties of the cobalt(II)-substituted alpha-fragment of rabbit liver metallothionein.
abstract: The C-terminal segment of rabbit liver metallothionein 1 (alpha-fragment) containing four paramagnetic Co(II) ions was obtained by stoichiometric replacement of the originally bound diamagnetic Cd(II) ions. The latter form was prepared by limited proteolysis with subtilisin as described previously [Winge, D. R., & Miklossy, K. A. (1982) J. Biol. Chem. 257, 3471-3476]. Electronic absorption, magnetic circular dichroism (MCD), and electron paramagnetic resonance (EPR) measurements were employed to monitor the stepwise incorporation of Co(II) ions into the metal-free fragment. Absorption and MCD spectra of the apofragment containing the first 3 Co(II) equiv show the typical features of tetrahedral tetrathiolate Co(II) coordination. However, in the d-d region only small changes in the visible and no apparent change in the near-infrared region are discernible when the fourth Co(II) is bound. This unusual spectral behavior was not seen in Co(II) substitution of native metallothionein [Vasák, M., & Kägi, J. H. R. (1981) Proc. Natl. Acad. Sci. U.S.A. 78, 6709-6713] and may indicate a different cluster geometry. In the charge-transfer region, the binding of all 4 Co(II) equiv is accompanied by characteristic increments of the thiolate S----Co(II) bands. As in the formation of Co(II)7-metallothionein, the development of the charge-transfer and EPR spectral properties upon binding of the first 2 Co(II) equiv to the apofragment is indicative of isolated, noninteracting tetrahedral tetrathiolate Co(II) complexes. The binding of the additional Co(II) ion is accompanied by a red shift in the charge-transfer region and by the dramatic loss of paramagnetism in the EPR spectra, both diagnostic of the formation of metal-thiolate cluster structures.(ABSTRACT TRUNCATED AT 250 WORDS)
mesh_terms: D000596:Amino Acids / Q000032:analysis; D000818:Animals; D002942:Circular Dichroism; D003035:Cobalt / Q000494:pharmacology*; D008099:Liver / Q000378:metabolism*; D008668:Metallothionein / Q000378:metabolism*; D010446:Peptide Fragments / Q000302:isolation & purification / Q000378:metabolism; D011487:Protein Conformation; D011817:Rabbits; D013053:Spectrophotometry; D013381:Subtilisins
chemical_list: D000596:Amino Acids; D010446:Peptide Fragments; D003035:Cobalt; D008668:Metallothionein; D013381:Subtilisins