New structural insights reveal how human respiratory chain complexes assemble
A new study in Nature Communications shows how one of the cell’s most important energy-producing machines is built. Researchers at Karolinska Institutet have mapped late steps in the formation of the human respirasome, a large protein assembly that drives mitochondrial respiration.
The respirasome is made up of several protein complexes that work together to transfer electrons and support the production of ATP, the cell’s main energy source. Although scientists have known that these complexes can join into larger structures, it has remained unclear whether they assemble as finished units or form step by step.
Using high-resolution cryo-electron microscopy, the research team at the Department of Medical Biochemistry and Biophysics captured previously unknown intermediates of the respirasome. Their findings suggest that the final stages of assembly occur while one of the key components, complex IV, is still maturing. This indicates that the respirasome may act as a platform that helps guide the correct order of assembly.
The study also identifies a protein called HIGD2A as a temporary “placeholder” within complex IV. This protein occupies a critical position until the final subunit, NDUFA4, is ready to be incorporated. Only then is the respirasome able to reach its functional form.
“This placeholder mechanism acts like a molecular timer. By delaying the addition of the final subunit, the cell ensures that assembly happens in a controlled sequence”, says Joanna Rorbach, Principal researcher at the Department of Medical Biochemistry and Biophysics at Karolinska Institutet.
Understanding the structure and timing
Defects in complex IV assembly are known to cause severe mitochondrial disorders, including neurological diseases. By revealing how the final steps of assembly occur, the study provides new clues to how such conditions may develop.
“Mitochondrial diseases often arise from small errors in how these complexes are built. Understanding the structure and timing of assembly helps us get closer to identifying where those errors occur”, says Minh Duc Nguyen, lead author of the study and researcher at the Department of Medical Biochemistry and Biophysics at Karolinska Institutet.
Collaborations
The study was led by Karolinska Institutet in collaboration with international partners from the University of Miami and was supported, among others, by the Swedish Research Council, the Knut and Alice Wallenberg Foundation, and additional European and international funding bodies.
Publication
Structural basis for late maturation steps of mitochondrial respiratory chain complex IV within the human respirasome.
Nguyen MD, Sierra-Magro A, Singh V, Khawaja A, Timón-Gómez A, Barrientos A, Rorbach J
Nat Commun 2026 Jan;():