Pan European Networks - Horizon 2020 - page 59

A
team of research groups of the Frankfurt Cluster of
Excellence ‘Macromolecular Complexes’, the Nijmegen
Center for Mitochondrial Disorders and the BIOSS Cluster
of Excellence at the University of Freiburg has solved the X-ray
structure of mitochondrial complex I, a very large membrane
protein complex with a key function in energy metabolism.
Respiratory complex I
Mitochondria produce ATP, the energy currency of the cell.
Respiratory complex I is a very large membrane protein complex
that plays a critical role in biological energy conversion. A number
of human diseases have been associated with complex I
dysfunction. Volker Zickermann started his work on complex I as a
postdoctoral fellow at the University of Helsinki when he
investigated complex I mutations that cause blindness associated
with atrophy of the optic nerve in young adults. The results of this
study suggested that exchange of a specific amino acid interfered
with the turnover of the substrate ubiquinone. However,
interpretation of the results was at the time controversial because
structural information was lacking and complex I was essentially a
‘black box’.
Structure determination
To understand the function of a macromolecular complex in detail,
it is imperative to determine its 3D structure at the best possible
resolution. In the case of mitochondrial complex I this turned out
to be an enormous challenge. The structure of macromolecular
complexes can be solved by X-ray crystallography, provided that
well-diffracting crystals can be obtained. Unfortunately, membrane
proteins like complex I are extremely difficult to purify and
crystallise. To make things worse, complex I contains no less than
eight iron-sulphur centres that render crystals highly sensitive to
radiation damage when exposed to the X-ray beam. Above all, with
a mass of 1MDa and more than 40 subunits, mitochondrial
complex I is among the largest protein complexes known, thus
rendering structure determination a formidable task.
Success by a collaboration of research teams
The Frankfurt Cluster of Excellence is a research centre dedicated
to the study of the structure and function of macromolecular
complexes. Research groups led by Volker Zickermann, Ulrich
Brandt – who moved to the Nijmegen Center for Mitochondrial
Disorders in 2012 – and Harald Schwalbe developed purification
of complex I and achieved generation of suitable crystals for X-ray
diffraction experiments. Moreover, inhibitors synthesised by Hamid
Nasiri permitted to determine the position of the active site in the
structure. The Frankfurt researchers closely collaborated with
crystallographers Carola Hunte and Christophe Wirth from the
University of Freiburg. X-ray diffraction data were collected at the
Swiss Light Source, Villigen, and the European Synchrotron
Radiation Facility, Grenoble. The allocation of synchrotron beam
time and the constant support of beam line scientists, as well as
technological progress, were crucial to the success of the project.
After more than ten years of hard work the combined efforts finally
led to a breakthrough.
New insights from the structure and future
experiments
Understanding complex I function is a goal of great significance.
Looking back to the beginning of the story we now have a much
deeper understanding of mitochondrial complex I. The results of
extensive mutagenesis studies make perfect sense in light of the
structural data. The disease mutation with which Volker
Zickermann started his work on complex I many years ago is now
seen to obstruct the narrow entrance of an access channel for
ubiquinone to the active site, explaining why activity of the enzyme
is compromised and damage is inflicted to neurons. The structure
lends support to a conceptually new view of the catalytic cycle of
complex I, but a number of issues remain unresolved and require
new experimental approaches and future work.
Volker Zickermann
Goethe-University Frankfurt, Medical School
Institute of Biochemistry II
Structural Bioenergetics Group
te l :
+49 69 79829575
The 3D structure of the large membrane protein complex with a key function in
energy metabolism was solved by a collaboration of research groups
Mitochondrial complex I
H O R I Z O N 2 0 2 0 P R O J E C T S : P O R TA L
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