Redox-linked ionization of sulredoxin, an archaeal Rieske-type [2Fe-2S] protein from Sulfolobus sp. strain 7

Abstract

'Sulredoxin' of Sulfolobus sp. strain 7 is an archseal 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 E(m) (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 pK(a(ox1)) of 6.23 ± 0.22 and pK(a(ox2)) of 8.57 ± 0.20. The absorption, CD, and resonance Raman spectra of oxidized sulredoxin are consistent with the proposed S(t)2FeSb2Fe[N(His)](t)2 core structure, and deprotonation of one of the two putative coordinated histidine imidazoles, having the pK(a(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-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 pK(a(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.