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(Received for publication, March 11, 1996, and in revised form, June 21, 1996)
From the ``Sulredoxin'' of Sulfolobus sp.
strain 7 is an archaeal soluble Rieske-type [2Fe-2S] protein and was
initially characterized by several spectroscopic techniques (Iwasaki,
T., Isogai, T., Iizuka, T., and Oshima, T. (1995) J. Bacteriol. 177, 2576-2582). It appears to have tightly linked
ionization affecting the redox properties of the protein, which is
characteristic of the Rieske FeS proteins found as part of the
respiratory chain. Sulredoxin had an Em(low pH)
value of +188 ± 9 mV, and the slope of pH dependence of the
midpoint redox potential indicated two ionization equilibria in the
oxidized form with pKa(ox1) of
6.23 ± 0.22 and pKa(ox2) of
8.57 ± 0.20. The absorption, CD, and resonance Raman spectra of
oxidized sulredoxin are consistent with the proposed
St2FeSb2Fe[N(His)]t2
core structure, and deprotonation of one of the two putative
coordinated histidine imidazoles, having the
pKa(ox2) of 8.57 ± 0.20, causes a
decrease in the midpoint redox potential, the change in the optical and
CD spectra, and the appearance of a new Raman transition at 278 cm
Volume 271, Number 44,
Issue of November 1, 1996
pp. 27659-27663
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.
§
,
,
and''
Department of Life Science, Tokyo Institute
of Technology, Nagatsuta, Yokohama 226, the § Department of
Biochemistry and Molecular Biology, Nippon Medical School, Sendagi,
Tokyo 113, the 
Department of Chemistry, Rikkyo (St. Paul's)
University, Toshima-ku, Tokyo 171, and the '' Department of Molecular
Biology, Tokyo University of Pharmacy and Life Science, Hachioji,
Tokyo 192-03, Japan
1, without major structural rearrangement of the
[2Fe-2S] cluster as well as the overall protein conformation. The
redox-linked ionization of sulredoxin is also contributed by local
changes involving another ionizable group having the
pKa(ox1) of 6.23 ± 0.22, which is
probably attributed to a certain positively charged amino acid residue
that may not be a ligand by itself but located very close to the
cluster. We suggest that sulredoxin provides a new tractable model of
the membrane-bound homologue of the respiratory chain, the Rieske FeS
proteins of the cytochrome
bc1-b6f
complexes.
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