Over 50 years ago, Britton Chance proposed that the electron-transport complexes might assemble into large supercomplexes. Doing so would bring the complexes into close and highly organized proximity, which might improve the speed and efficiency of the overall electron-transport process. Indeed, genetic, biochemical, and biophysical studies have provided very strong evidence for the existence of electron-transport chain supercomplexes. These studies in volved polyacrylamide gel electrophoretic methods called blue native (BN)-PAGE and colorless native (CN)-PAGE, which permit separation of very large macromolecular protein complexes, and electron microscopic analysis of their three-dimensional structures. One such supercomplex contains one copy of complex I, a dimer of complex III (III2), and one or more copies of complex IV (Figure 1). When this I/III2/IV supercomplex was isolated with ubiquinone (CoQ) and cytochrome c from BN-PAGE gels, it was shown to transfer electrons from NADH to O2; in other words, this supercomplex can respire—it is a respirasome. The precise function of supercomplex formation in the context of the very high protein concentration in the inner mitochondrial membrane remains to be established with certainty, but is thought to involve improving the speed and efficiency of electron transport, stabilizing individual multiprotein complexes, or preventing inappropriate protein aggregates.
Fig1. Electrophoresis and electron microscopic imaging have identified an electron-transport chain supercomplex containing complexes I, III, and IV. (a) Membrane proteins in isolated bovine heart mitochondria were solubilized with a detergent, and the complexes and supercomplexes were separated by gel electrophoresis using the blue native (BN)-PAGE method. Each blue-stained band within the gel represents the indicated protein com plex or supercomplex. The intensity of the blue stain is approximately proportional to the amount of complex or supercomplex present. (b) Supercomplex I/III2/IV was extracted from a BN-PAGE gel, frozen, and visualized by cryoelectron tomography. The left image shows the three-dimensional surface structure viewed from an orientation parallel to the presumptive plane of the membrane. The right image is the same structure into which were fit models of the structures of the individual complexes: complex I (blue), dimer of complex III (III2, orange), and complex IV (green). Colored dashed lines represent the approximate outlines of these complexes. The complex I structure is based on essentially the entire complex I from the yeast Y. lipolytica, not just the 14 core subunits. [Part (a) from Schafer, E., et al., “Architecture of active mammalian respiratory chain supercomplexes,” J. Biol. Chem. 2006 Jun 2; 281(22):15370-5. Epub 2006 Mar 20. Part (b) from Proc. Natl. Acad. Sci. USA 2011. 108(37):15196-15200, Fig. 2A and 3A, “Interaction of complexes I, III, and IV within the bovine respirasome by single particle cryoelectron tomography,” by Dudkina et al.]
The unique phospholipid cardiolipin (diphosphatidyl glycerol) appears to play an important role in the assembly and function of these supercomplexes.
Generally not observed in other membranes of eukaryotic cells, cardiolipin has been observed to bind to integral membrane proteins of the inner mitochondrial mem brane (e.g., complex II). Genetic and biochemical studies in yeast mutants in which cardiolipin synthesis is blocked have established that cardiolipin contributes to the formation and activity of mitochondrial supercomplexes; thus it has been called the glue that holds together the electron-transport chain, though the precise mechanism by which it does so remains to be defined. In addition, there is evidence that cardiolipin may influence the inner membrane’s binding and permeability to protons and consequently the proton-motive force. Barth’s syndrome is a human X-linked genetic disease caused by defects in an enzyme that determines the structures of the acyl chains on cardiolipin. The reduction in the amounts of cardiolipin in patients with Barth’s syndrome and its abnormal structure result in heart and skeletal muscle defects, growth retardation, and other abnormalities.
