Integration of Structures, Spectroscopies and Mechanisms981
A structural analysis of the transient interaction
between the cytochrome bc1complex and its
substrate cytochrome c
Ajeeta Nyola* and Carola Hunte†‡1
*Department of Molecular Membrane Biology, Max Planck Institute of Biophysics, Max-von-Laue-Strasse 3, D-60438 Frankfurt am Main, Germany, †Institute
of Membrane and Systems Biology, University of Leeds, Leeds LS2 9JT, U.K., and ‡Center of Excellence ‘Macromolecular Complexes’, Johann Wolfgang
Goethe-Universit¨ at, D-60590 Frankfurt am Main, Germany
In cellular respiration, cytochrome c transfers electrons from the cytochrome bc1complex to cytochrome c
oxidase by transiently binding to the membrane proteins. The first X-ray structure of the yeast cytochrome
bc1complex with bound cytochrome c revealed the general architecture of the electron-transfer complex.
The interface of the complex is small. The haem moieties are centrally located in a mainly non-polar
contact site, which includes a cation–π interaction and is surrounded by complementary charged residues.
Only one cytochrome c1-docking site of the dimeric complex is occupied with cytochrome c. The recent
1.9 Å (1 Å=0.1 nm) resolution structure of the complex showed that the interface is highly hydrated. With
cytochrome c bound, a higher number of interfacial water molecules are present on the cytochrome c1
interface, whereas its protein surface is not affected. Remarkably, the dimer structure is slightly asymmetric.
Univalent cytochrome c binding coincides with conformational changes of the Rieske head domain and
subunit QCR6p. Pronounced hydration and a mobility mismatch at the interface with disordered charged
residues on the cytochrome c side are favourable for transient binding. Comparison with a new structure
of the complex with bound isoform-2 cytochrome c led to the definition of a core interface, which refers to
four common interaction pairs including the cation–π interaction. They encircle the haem groups and are
surrounded by variable interactions. The core interface may be a feature to gain specificity for formation of
the reactive complex. The consistency in the binding interaction despite differences in primary sequence,
redox state and crystal contacts, together with crystallization at physiological ionic strength, clearly suggest
that the structures show the native bound state of the electron-transfer complex.
Electron-transfer processes are of great importance in many
metabolic pathways of living organisms. They are essential
for photosynthesis and cellular respiration, in which small
large membrane-embedded enzymes. The intermolecular
partners [1,2]. These interactions are of low affinity, with
equilibrium dissociation constants in the micromolar to
milliomolar range . To promote high turnover and
efficiency of the energy-converting machinery, binding of
the mobile electron-carrier proteins has to be not only
transient, but also specific. Transient complexes are difficult
to crystallize and their structures are poorly represented
in the RCSB PDB database (http://www.rcsb.org). For the
respiratory chain, only crystal structures of the QCR (cyto-
chrome bc1complex) with bound CYC (cytochrome c) from
Saccharomyces cerevisiae have been determined to date [4,5].
Key words: cytochrome bc1complex, cytochrome c, electron-transfer complex, interfacial water
molecule, mobility mismatch, protein–protein interface.
Abbreviations used: COX, cytochrome c oxidase; CYC, cytochrome c; CYT1, cytochrome c1; QCR,
1To whom correspondence should be addressed (email email@example.com).
In the present article, the structural information about this
transient electron-transfer complex is reviewed, with a focus
on the recent 1.9 A˚(1 A˚=0.1 nm) resolution structure .
The mitochondrial respiratory chain
Oxidative phosphorylation produces the majority of energy
equivalents in eukaryotic cells. In this final step of nutrient
catabolism, the mitochondrial respiratory chain couples elec-
with vectorial proton translocation, thereby generating the
protonmotive force that drives ATP synthesis. Electron
and proton transfer in the respiratory chain employs four
multisubunit enzymes (complexes I–IV), which are embed-
ded in the inner mitochondrial membrane. Electron transfer
between complex III (QCR) and complex IV [COX (cyto-
chrome c oxidase)] is facilitated by reversible binding of the
tions are highly transient in nature, enabling turnover num-
bers of 150 s−1and 600 s−1for QCR and COX respectively
[7,8], which is essential for the continuous flow of electrons
across the different components of the respiratory chain.
Three crystal structures of QCR with bound CYC from
Biochem. Soc. Trans. (2008) 36, 981–985; doi:10.1042/BST0360981
C ?The Authors Journal compilation
C ?2008 Biochemical Society