The Supercomplexes in the Crista Membrane

Cardiolipin is a phospholipid that acts as molecular glue, organizing different combinations of individual electron transport chain complexes into respiratory supercomplexes or megacomplexes. This organization reduces the distance between neighboring complexes, allowing mobile electron carriers to diffuse quickly between them and improving electron transfer and proton-pumping efficiencies.

In cells with high-energy demand, respiratory supercomplexes generate a large proton-motive force by enhancing electron transfer efficiency through reduced distances between complexes. This increased proton gradient drives upregulation of ATP production, allowing cells to meet their elevated energy requirements more effectively than discrete individual complexes could.

A respirasomes is the most abundant supercomplex type, containing complex I monomer, complex III dimer, and one or more units of complex IV. It is unique because it can autonomously carry out respiration in the presence of ubiquinone and cytochrome c, functioning as a self-contained respiratory unit within the inner mitochondrial membrane.

Supercomplexes insulate reactive sites, such as iron-sulfur clusters, within the protein environment, making them inaccessible to oxygen. This prevents toxic superoxide radicals from forming when these reactive sites would otherwise remain exposed, thereby reducing excessive reactive oxygen species production and protecting cellular components from oxidative damage.

Complex I has an inherently unstable structure and may dissociate into individual protein subunits without stabilizing interactions. Its stability depends on association with other complexes, particularly complex III dimer in supercomplex SC I+III2. Genetic mutations causing complex III loss correlate with loss of complex I and associated supercomplexes.

Megacomplexes, also called respiratory strings, are even larger supramolecular structures formed from multiple supercomplexes. For example, human respiratory SC I+III2+IV can form a circular megacomplex MC I2+III2+IV2. The specific function of these high-order complexes remains an active area of research.

Supercomplex abundance and composition vary among organisms and tissues depending on metabolic and physiological conditions. Organisms like Saccharomyces cerevisiae that lack complex I mainly comprise SC III2+IV1 and III2+IV2 supercomplexes, with complex II serving as the only entry point for electrons into the electron transport chain.