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High-resolution images capture intricate structure of mitochondrial supercomplexes

Mitochondria are the powerhouses in our cells, producing the energy for all vital processes. Using cryo-electron tomography, researchers at the University of Basel, Switzerland, have now gained insight into the architecture of mitochondria at unprecedented resolution.

The results of the study are in Science.

They discovered that the proteins responsible for energy generation assemble into large "supercomplexes," which play a crucial role in providing the cell's energy.

Most living organisms on our planet—whether plants, animals, or humans—contain mitochondria in their cells. Their main function is to supply energy for nearly all cellular processes.

To achieve this, mitochondria use oxygen from breathing and carbohydrates from food to regenerate ATP, the universal energy currency of cells. This function is performed by proteins known as respiratory complexes, which work together in the energy-generating process.

Although these respiratory complexes were discovered 70 years ago, their exact organization inside mitochondria has remained elusive until now.

Using state-of-the-art cryo-electron tomography, researchers led by Dr. Florent Waltz and Prof. Ben Engel at the Biozentrum of the University of Basel were able to create high-resolution images of the respiratory chain directly inside cells at a resolution never achieved before.

"Our data show that the respiratory proteins organize in specific membrane regions of mitochondria, stick together and form one main type of supercomplex," explains Florent Waltz, SNSF Ambizione Fellow and first author of the study.

"Using the electron microscope, individual supercomplexes were clearly visible—we could directly see their structures and how they work. The respiratory supercomplexes pump protons across the mitochondrial membrane. The ATP production complexes, which act similarly to a watermill, use this flow of protons to drive ATP generation."

Mitochondrial architecture for efficient energy production

The researchers examined mitochondria in living cells of the alga Chlamydomonas reinhardtii. "We were very surprised that all the proteins were actually organized in such supercomplexes," says Waltz. "This architecture might make ATP production more efficient, optimize electron flow, and minimize energy loss."

In addition to the supercomplexes, the researchers were also able to examine the membrane architecture of the mitochondria more closely.

"It's somewhat reminiscent of lung tissue: the inner mitochondrial membranes have many folds that increase the surface area to fit as many respiratory complexes as possible," says Engel.

In the future, the researchers aim to uncover why respiratory complexes are interconnected and how this synergy enhances the efficiency of cellular respiration and energy production. The study may also offer new insights for biotechnology and health.

"By examining the architecture of these complexes in other organisms, we can gain a broader understanding of their fundamental organization," explains Waltz.

"This could not only reveal evolutionary adaptations but also help us understand why disruptions in these complexes contribute to human diseases."