Three-dimensional reconstruction of cytochrome oxidase vesicle crystals prepared by cholate solubilization

Cytochrome c oxidase: structural studies by electron microscopy of two-dimensional crystals

Cytochrome c oxidase is a complex integral membrane protein consisting of 13 different polypeptide chains and four metal centers having a total molecular weight of approximately 200,000 daltons. It can be isolated in two 2-dimensional crystalline forms differing in aggregation state of the enzyme. One crystal form consists of cytochrome oxidase dimers (approximately 400,000 daltons) embedded unidirectionally in the lipid bilayer of a collapsed vesicle while the other form consists of crystalline sheets of cytochrome oxidase monomers. Both crystal forms have been studied by electron microscopy during the past two decades, and this paper summarizes the results of early structural studies as well as more recent results applying techniques of cryoelectron microscopy and digital image processing. The structure of frozen-hydrated cytochrome oxidase dimers at 20 A resolution is discussed as well as the packing of monomers within dimers and the site of cytochrome c binding.

Electron cryo-microscopic analysis of crystalline cytochrome oxidase

The structure of cytochrome oxidase from beef heart mitochondria has been analysed by cryo-electron microscopy of vesicle crystals of the space group p22(1)2(1), with cell dimensions a = 102 A, b = 123 A, gamma = 90 degrees. Several methods of specimen preparation were applied to the vesicular two-dimensional crystals in the electron microscope, to ensure that the structure was preserved to the maximum resolution. The two most informative density maps were from specimens embedded in ice and from negative staining in a 1:1 mixture of glucose and uranyl acetate. The three-dimensional structure of the ice-embedded molecule shows a single, well resolved, but convoluted density, which represents in size and shape one cytochrome oxidase dimer. At the bottom of the molecule, a substantial part of the protein is embedded in the lipid bilayer of the vesicle. The molecule then extends upwards, out of the bilayer, into the internal space within the vesicle. Here, the structure first passes through a region within the molecule containing a hollow cavity that lies roughly at the centre of mass of the dimer, and then branches into two well-resolved halves at some distance from the membrane. The negatively stained structure, in contrast, shows a stain-excluding region in the centre of the vesicle at the level of the cavity in the ice-embedded structure, but otherwise has a similar overall external shape. In addition, there is a small rotation of the whole molecule by approximately 25 degrees relative to the orientation of ice-embedded specimens. We interpret these differences to mean that the central cavity seen in the ice-embedded structure is too small to allow the stain to penetrate during the drying process and that the drying process causes the rotation. The structures described here are consistent with one another and allow an interpretation at higher resolution than from previous work.

Complexity and tissue specificity of the mitochondrial respiratory chain

There is a renewed interest in the structure and functioning of the mitochondrial respiratory chain with the realization that a number of genetic disorders result from defects in mitochondrial electron transfer. These socalled mitochondrial myopathies include diseases of muscle, heart, and brain. The respiratory chain can be fractionated into four large multipeptide complexes, an NADH ubiquinone reductase (complex I), succinate ubiquinone reductase (complex II), ubiquinol oxidoreductase (complex III), and cytochrome c oxidase (complex IV). Mitochondrial myopathies involving each of these complexes have been described. This review summarizes compositional and structural data on the respiratory chain proteins and describes the arrangement of these complexes in the mitochondrial inner membrane. This biochemical information is provided as a framework for the diagnosis and molecular characterization of mitochondrial diseases.

Electron microscopy and image analysis of the complexes I and V of the mitochondrial respiratory chain

The results of Section IV can be summarized in a simple ATP synthase model. This model implies that either the alpha or the beta subunits must be closer to the membrane. The work of Gao and Bauerlein (1987) indicates that the alpha subunits are closer to the membrane. Although the overall structure is more or less clear, important questions need to be clarified. First, the number and the arrangement of the subunits in the F0 part must be known. Second, the exact shape of F1, and particularly the shape of the large subunits needs to be elucidated. On the basis of fluorescence resonance energy transfer measurements by McCarty and Hammes (1987), a model was presented showing large oblong subunits. Such 'banana-shaped' subunits, which are also presented in the many phantasy models (e.g. Walker et al., 1982), are very unlikely in view of the electron microscopical results, although the large subunits do not need to be exactly spherical. The third and most interesting central question is on the changes in the structure that take place during the different steps in the synthesis of ATP. It can now be taken as proven that the energy transmitted to the ATP synthase is used to induce a conformational change in the latter enzyme, in such a way as to bring about the energy-requiring dissociation of already synthesized ATP (Penefsky, 1985 and reviewed in Slater, 1987). But the way in which the three parts of the ATP synthase are involved is completely unknown. It is rather puzzling that such a long distance exists between the catalytic sites, which are on the interface of the alpha and beta subunits and the F0 part where the proton movements occur, which, according to Mitchell's theory (1961), is the driving force for the synthesis of ATP. Perhaps alternative mechanisms such as the collision hypothesis formulated by Herweijer et al. (1985) are more realistic in describing the mechanism of ATP synthesis. It would bring the complexes I and V close together, not only in the artificial way treated in this paper, but in a useful way for energy conversion.