Architecture of Active Mammalian Respiratory Chain Supercomplexes

Abstract

In the inner mitochondrial membrane, the respiratory chain complexes generate an electrochemical proton gradient, which is utilized to synthesize most of the cellular ATP. According to an increasing number of biochemical studies, these complexes are assembled into supercomplexes. However, little is known about the architecture of the proposed multicomplex assemblies. Here, we report the electron microscopic characterization of the two respiratory chain supercomplexes I 1 III 2 and I 1 III 2 IV 1 in bovine heart mitochondria, which are also two major supercomplexes in human mitochondria. After purification and demonstration of enzymatic activity, their structures in projection were determined by single particle image analysis. A difference map between the supercomplexes I 1 III 2 and I 1 III 2 IV 1 closely fits the x-ray structure of monomeric complex IV and shows its location in the assembly. By comparing different views of supercomplex I 1 III 2 IV 1 , the location and mutual arrangement of complex I and the complex III dimer are discussed. Detailed knowledge of the architecture of the active supercomplexes is a prerequisite for a deeper understanding of energy conversion by mitochondria in mammals.All living organisms use a series of integral membrane protein complexes for energy conversion and ATP synthesis. In eukaryotes, electrons are transported by the respiratory chain, starting from NADH via complex I (NADH:ubiquinone oxidoreductase) or from succinate via complex II (succinate:ubiquinone oxidoreductase), the membrane integral electron carrier ubiquinol, complex III (ubiquinol:cytochrome c oxidoreductase), the peripheral electron carrier cytochrome c, and complex IV (cytochrome c oxidase) to the terminal acceptor molecular oxygen (1). The electron transport chain generates a proton gradient across the inner mitochondrial membrane, which is used by complex V (F O F 1 -ATP synthase) to synthesize ATP. In the last decade, structures of the individual respiratory chain complexes from various organisms have been determined. Atomic models exist for bovine heart mitochondrial complex III (2) and IV (3). A high resolution structure of complex I is not yet available, but electron microscopy indicates that it is L-shaped in all organisms investigated, and a 2.2-nm resolution map from cryoelectron microscopy exists for the bovine heart complex I (4).Two alternative models for the arrangement of the respiratory chain complexes in the membrane have been proposed. According to the currently favored random collision model (5), all components of the respiratory chain diffuse individually in the membrane, and electron transfer depends on the random, transient encounter of the individual protein complexes and the smaller electron carriers. In the solid state model (6) proposed 50 years ago, the substrate is channeled directly from one enzyme to the next. Recently isolated stoichiometric assemblies, so-called supercomplexes, support this model. Respiratory supercomplexes of different compositions have been described in bact...

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