Unveiling the Complex Assembly of the Human Respiratory Chain
A groundbreaking study published in Nature Communications reveals the intricate process of constructing one of the cell's most vital energy-producing machinery, the respirasome. Researchers from Karolinska Institutet have delved into the late stages of the respirasome's formation, a crucial protein complex that drives mitochondrial respiration in humans.
The respirasome is a powerhouse, comprising various protein complexes that collaborate to transfer electrons and facilitate ATP production, the cell's primary energy source. While scientists were aware of these complexes' ability to form larger structures, the question of whether they assemble as complete units or step-by-step remained unanswered.
Through cutting-edge high-resolution cryo-electron microscopy, the research team uncovered previously unknown intermediates in the respirasome's assembly. Their findings suggest that the final assembly stages occur while complex IV, a key component, is still maturing. This discovery implies that the respirasome might act as a guiding platform, ensuring the correct sequence of assembly.
The study also identified a fascinating protein, HIGD2A, as a temporary placeholder within complex IV. This protein takes up a critical position until the final subunit, NDUFA4, is ready for integration. Only then does the respirasome attain its functional form.
"This placeholder mechanism functions like a molecular timer," explains Joanna Rorbach, Principal Researcher at Karolinska Institutet's Department of Medical Biochemistry and Biophysics. "By delaying the addition of the final subunit, the cell ensures a controlled assembly sequence."
Understanding the Structure and Timing
Defects in complex IV assembly are linked to severe mitochondrial disorders, including neurological diseases. By elucidating the final assembly steps, the study offers valuable insights into the development of such conditions.
"Mitochondrial diseases often stem from minor errors in complex construction," says Minh Duc Nguyen, lead author and researcher at Karolinska Institutet's Department of Medical Biochemistry and Biophysics. "Grasping the structure and timing of assembly brings us closer to pinpointing the locations of these errors."
Collaborative Efforts
The study was a collaborative effort led by Karolinska Institutet, with contributions from international partners at the University of Miami. It received support from various funding bodies, including the Swedish Research Council, the Knut and Alice Wallenberg Foundation, and additional European and international sources.
Publication
The research is published in the journal Nature Communications under the title: "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;(): https://pubmed.ncbi.nlm.nih.gov/41519940/
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