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Abstract

The smallest molecular weight subunit (subunit IV), which contains no redox prosthetic group, is the only supernumerary subunit in the four-subunit Rhodobacter sphaeroides bc1 complex. This subunit is involved in Q binding and the structural integrity of the complex. When the cytochrome bc1 complex is photoaffinity labeled with [3H]azido-Q derivative, radioactivity is found in subunits IV and I (cytochrome b), indicating that these two subunits are responsible for Q binding in the complex. When the subunit IV gene (fbcQ) is deleted from the R. sphaeroides chromosome, the resulting strain (RSdeltaIV) requires a period of adaptation before the start of photosynthetic growth. The cytochrome bc1 complex in adapted RSdeltaIV chromatophores is labile to detergent treatment (60-75% inactivation), and shows a four-fold increase in the Km for Q2H2. The first two changes indicate a structural role of subunit IV; the third change supports its Q-binding function. Tryptophan-79 is important for structural and Q-binding functions of subunit IV. Subunit IV is overexpressed in Escherichia coli as a GST fusion protein using the constructed expression vector, pGEX/IV. Purified recombinant subunit IV is functionally active as it can restore the bc1 complex activity from the three-subunit core complex to the same level as that of wild-type or complement complex. Three regions in the subunit IV sequence, residues 86-109, 77-85, and 41-55, are essential for interaction with the core complex because deleting one of these regions yields a subunit completely or partially unable to restore cytochrome bc1 from the core complex.

Inhibitor-complexed Structures of the Cytochrome bc1 from the Photosynthetic Bacterium Rhodobacter sphaeroides

“…It is not uncommon to lose a supernumerary subunit during crystallization of mitochondrial bc 1 complexes (17,27). To test whether subunit IV is indirectly required for the crystallization of Rsbc 1 , we purified the ⌬-subIV mutant (32) and subjected it to the same crystallization conditions. Triclinic crystals grew readily, displaying the same plate-like morphology as the monoclinic ones, and diffracted x-rays to 3.1-Å resolution.…”

The Role of Extra Fragment at the C-terminal of Cytochrome b (Residues 421–445) in the Cytochrome bc1 Complex from Rhodobacter sphaeroides

“…These extra fragments are modeled into the structure of the Rhodobacter sphaeroides bc 1 complex by using coordinates of mitochondrial supernumerary subunits (5). These findings encouraged us to suggest that these extra fragments may possess mitochondrial supernumerary subunit function in stabilizing the structure of the core subunits in the bacterial complex.…”

“…Sequence alignment of cytochrome b, cytochrome c 1 , and ISP in bacterial complexes with their counterparts in mitochondrial complexes reveals four extra fragments in bacterial cytochrome b and one each in bacterial cytochrome c 1 and ISP (5). These extra fragments are modeled into the structure of the Rhodobacter sphaeroides bc 1 complex by using coordinates of mitochondrial supernumerary subunits (5).…”

“…In the structure model of R. sphaeroides cytochrome bc 1 complex (5), the four extra fragments of cytochrome b are located at the N terminus (residues 2 to 12), the connecting loop between helices D and E (residues 232 to 239), the connecting loop between helices E and F (residues 309 to 326), and the C terminus (residues 421-445) (see Fig. 1).…”

The competition between chemistry and biology in assembling iron–sulfur derivatives. Molecular structures and electrochemistry. Part III. {[Fe2S2](Cys)3(X)} (X=Asp, Arg, His) and {[Fe2S2](Cys)2(His)2} proteins

“…It crystallizes as a symmetric dimer in which each monomer contains the three redox active subunits: cyt b, cyt c 1 , the Rieske center (known as core subunits) and a redox inactive supernumerary subunit IV (which enhances the activity of the core subunits, but is not essential for the function of the complex or the survival of the organism [43]). In particular the crystal structure of the monomeric Rieske iron-sulfur subunit and that of its mutants Tyr156Phe, Tyr156Trp, Ser154Ala, Ser154Thr, Ser154Cys have been solved [44].…”