Cytochrome bc1 (mitochondrial complex III), one of the key enzymes of biological energy conversion, is a functional homodimer in which each monomer contains three catalytic subunits: cytochrome c1, the iron–sulfur subunit and cytochrome b. The latter is composed of eight transmembrane α-helices which, in duplicate, form a hydrophobic core of a dimer. We show that two cytochromes b can be fused into one 16-helical subunit using a number of different peptide linkers that vary in length but all connect the C-terminus of one cytochrome with the N-terminus of the other. The fusion proteins replace two cytochromes b in the dimer defining a set of available protein templates for introducing mutations that allow breaking symmetry of a dimer. A more detailed comparison of the form with the shortest, 3 amino acid, linker to the form with 12 amino acid linker established that both forms display similar level of structural plasticity to accommodate several, but not all, asymmetric patterns of mutations that knock out individual segments of cofactor chains. While the system based on a fused gene does not allow for the assessments of the functionality of electron-transfer paths in vivo, the family of proteins with fused cytochrome b offers attractive model for detailed investigations of molecular mechanism of catalysis at in vitro/reconstitution level.
Homodimeric structure of cytochrome bc1, a common component of biological energy conversion
systems, builds
in four catalytic quinone oxidation/reduction sites and four chains
of cofactors (branches) that, connected by a centrally located bridge,
form a symmetric H-shaped electron transfer system. The mechanism
of operation of this complex system is under constant debate. Here,
we report on isolation and enzymatic examination of cytochrome bc1-like complexes containing fused cytochrome b subunits in which asymmetrically introduced mutations
inactivated individual branches in various combinations. The structural
asymmetry of those forms was confirmed spectroscopically. All the
asymmetric forms corresponding to cytochrome bc1 with partial or full inactivation of one monomer retain high
enzymatic activity but at the same time show a decrease in the maximum
turnover rate by a factor close to 2. This strongly supports the model
assuming independent operation of monomers. The cross-inactivated
form corresponding to cytochrome bc1 with
disabled complementary parts of each monomer retains the enzymatic
activity at the level that, for the first time on isolated from membranes
and purified to homogeneity preparations, demonstrates that intermonomer
electron transfer through the bridge effectively sustains the enzymatic
turnover. The results fully support the concept that electrons freely
distribute between the four catalytic sites of a dimer and that any
path connecting the catalytic sites on the opposite sides of the membrane
is enzymatically competent. The possibility to examine enzymatic properties
of isolated forms of asymmetric complexes constructed using the cytochrome b fusion system extends the array of tools available for
investigating the engineering of dimeric cytochrome bc1 from the mechanistic and physiological perspectives.
To address mechanistic questions about the functioning of dimeric cytochrome bc1 new genetic approaches have recently been developed. They were specifically designed to enable construction of asymmetrically-mutated variants suitable for functional studies. One approach exploited a fusion of two cytochromes b that replaced the separate subunits in the dimer. The fusion protein, built from two copies of the same cytochrome b of purple bacterium Rhodobacter capsulatus, served as a template to create a series of asymmetrically-mutated cytochrome bc1-like complexes (B–B) which, through kinetic studies, disclosed several important principles of dimer engineering. Here, we report on construction of another fusion protein complex that adds a new tool to investigate dimeric function of the enzyme through the asymmetrically mutated forms of the protein. This complex (BS–B) contains a hybrid protein that combines two different cytochromes b: one coming from R. capsulatus and the other — from a closely related species, R. sphaeroides. With this new fusion we addressed a still controversial issue of electron transfer between the two hemes bL in the core of dimer. Kinetic data obtained with a series of BS–B variants provided new evidence confirming the previously reported observations that electron transfer between those two hemes occurs on a millisecond timescale, thus is a catalytically-relevant event. Both types of the fusion complexes (B–B and BS–B) consistently implicate that the heme-bL–bL bridge forms an electronic connection available for inter-monomer electron transfer in cytochrome bc1.
HighlightsWe used hybrid fusion bc1 complex to test inter-monomer electron transfer in vivo.Cross-inactivated complexes were able to sustain photoheterotrophic growth.Inter-monomer electron transfer supports catalytic cycle in vivo.bc1 dimer is functional even when cytochrome b subunits come from different species.
Fusing proteins is an attractive genetic tool used in several biochemical and biophysical investigations. Within a group of redox proteins, certain fusion constructs appear to provide valuable templates for spectroscopy with which specific bioenergetic questions can be addressed. Here we briefly summarize three different cases of fusions reported for bacterial cytochrome bc(1) (prokaryotic equivalent of mitochondrial respiratory complex III), a common component of electron transport chains. These fusions were used to study supramolecular organization of enzymatic complexes in bioenergetic membrane, influence of the accessory subunits on the activity and stability of the complex, and molecular mechanism of operation of the enzyme in the context of its dimeric structure. Besides direct connotation to molecular bioenergetics, these fusions also appeared interesting from the protein design, biogenesis, and assembly points of view. This article is part of a Special Issue entitled: 17th European Bioenergetics Conference (EBEC 2012).
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.