Inhibitor binding changes domain mobility in the iron-sulfur protein of the mitochondrial bc1 complex from bovine heart

CD-monitored redox titration of the Rieske Fe-S protein of Rhodobacter sphaeroides: pH dependence of the midpoint potential in isolated bc1 complex and in membranes

The redox potential of the Rieske Fe-S protein has been investigated using circular dichroism (CD)-spectroscopy. The CD features characteristic of the purified bc1 complex and membranes of Rhodobacter sphaeroides were found in the region between 450 and 550 nm. The difference between reduced and oxidized CD-spectra shows a negative band at about 500 nm with a half of width 30 nm that corresponds to the specific dichroic absorption of the reduced Rieske protein (Fee, J.A. et al. (1984) J. Biol. Chem. 259, 124-133; Degli Esposti, M. et al. (1987) Biochem. J. 241, 285-290; Rich, P.R. and Wiggins, T.E. (1992) Biochem. Soc. Trans. 20, 241S). It was found that the redox potential at pH 7.0 for the Rieske center in the isolated bc1 complex and in chromatophore membranes from the R-26 strain of Rh. sphaeroides is 300 +/- 5 mV. In chromatophores from the BC17C strain of Rh. sphaeroides, the Em value measured for the Rieske iron-sulfur protein (ISP) was higher (315 +/- 5 mV), but the presence of carotenoids made measurement less accurate. The Em varied with pH in the range above pH 7, and the pH dependence was well fit either by one pK at approximately 7.5 in the range of titration, or by two pK values, pK1 = 7.6 and pK2 = 9.8. Similar titrations and pK values were found for the Rieske Fe-S protein in the isolated bc1 complex and membranes from the R-26 strain of Rb. sphaeroides. The results are discussed in the context of the mechanism of quinol oxidation by the bc1 complex, and the role of the iron sulfur protein in formation of a reaction complex at the Qo-site.

Flexibility of the neck region of the rieske iron-sulfur protein is functionally important in the cytochrome bc1 complex

The crystal structure of the mitochondrial cytochrome bc1 complex suggests that movement of the extramembrane (head) domain of the Rieske iron-sulfur protein (ISP) is involved in electron transfer. Such movement requires flexibility in the neck region of ISP. To test this hypothesis, Rhodobacter sphaeroides mutants expressing His-tagged cytochrome bc1 complexes with altered ISP necks (residues 39-48) were generated and characterized. Mutants with increased rigidity of the neck, generated by a double-proline substitution at Ala-46 and Ala-48 (ALA-PLP) or by a triple-proline substitution of ADV at residues 42-44 (ADV-PPP), have retarded (50%) or no photosynthetic growth, respectively. However, the mutant with a shortened neck, generated by deleting ADV (DeltaADV), has a photosynthetic growth rate comparable to that of complement cells, indicating that the length of the ISP neck is less critical than its flexibility in support of photosynthetic growth. The DeltaADV and ALA-PLP mutant membranes have 10 and 30% of the cytochrome bc1 complex activity found in the complement membrane, respectively, whereas the ADV-PPP mutant membrane contains no cytochrome bc1 complex activity. The loss of cytochrome bc1 complex activity in the DeltaADV membrane is attributed to improper docking of the head domain of ISP on cytochrome b, as indicated by a drastic change in the EPR characteristics of the Rieske iron-sulfur cluster. The loss of cytochrome bc1 complex activity in the ALA-PLP and ADV-PPP mutant membranes results from the decreased mobility of the ISP head domain due to the increased rigidity of the ISP neck. The ALA-PLP mutant complex has a larger activation energy than the wild-type complex, suggesting that movement of the head domain decreases the activation energy barrier of the cytochrome bc1 complex. Using the conditions developed for the isolation of the His-tagged complement cytochrome bc1 complex, a two-subunit complex (cytochromes b and c1) was obtained from the DeltaADV and ADV-PPP mutants, indicating that mutations at the neck region of ISP weaken the interactions among cytochrome b, ISP, and subunit IV.