Inner mitochondrial membrane composition

Biological membranes consist of lipids, proteins, and carbohydrates (see p. 214). These components occur in varying proportions (left). Proteins usually account for the largest proportion, at around half. By contrast, carbohydrates, which are only found on the side facing away from the cytoplasm, make up only a few percent. An extreme composition is seen in myelin, the insulating material in nerve cells, three-quarters of which consists of lipids. By contrast, the inner mitochondrial membrane is characterized by a very low proportion of lipids and a particularly high proportion of proteins. 

A second important feature worth emphasizing is that the proton leak is affected by the lipid composition of the inner mitochondrial membrane. In general, the fatty acyl chains of phospholipid components of membranes can vary in their degree of unsaturation. The proton leak increases with the incorporation of polyunsaturated fatty acids (FAs) the greater the number of double bonds, the greater the proton leak (Hulbert and Else, 2000). As we shall note in the chapter on temperature relations, fatty acyl composition of membranes, including the inner mitochondrial membranes, changes in various... 

The mitochondrial permeability transition (MPT) is the loss of the inner mitochondrial membrane impermeability to solutes caused by opening of the MPT pore (MPTP). In turn, this action results in a loss of mitochondrial function and provides a common mechanism implicated in activation of mi-tophagy/autophagy, apoptosis, and necrosis in different cell systems. Although the composition of MPTP is not fully settled, multiple studies suggest involvement of adenine nucleotide translocase (ANT) in the inner mitochondrial membrane, voltage-dependent anion channel (VDAC or porin) in the outer membrane, and cyclophilin D (CypD) in the matrix. 

The essential role of cytochrome c release from injured mitochondria in the activation of caspase 9 has been alluded to above. This pathway is especially important in proapoptotic stimuli that are not initiated by surface receptors for apoptosis, such as UV irradiation, and may involve mitochondrial dependent pathways [83]. Continued respiration in the presence of an open mitochondrial pore may result in the generation of reactive oxygen species. Release of cytochrome c may be mediated by the opening of the mitochondrial FT pore, a non-selective channel whose composition is only partially defined [84]. Inhibitors of FT pore opening, such as cyclosporine, which binds to the adenine nucleotide translocator (ANT), a component of the FT pore, and bongkrekic acid, as well as Bcl-2, prevent cytochrome c release and inhibit apoptosis [85] whereas activators of the FT pore, such as atractyloside and Bax induce it [86]. Oxidants can rupture the outer membrane of mitochondria and release caspase-activating proteins [87]. Some studies have shown cytochrome c release before collapse of the mitochondrial membrane potential [83] suggesting alternate control of the FT pore. Many, but not all, of the members of the Bd-2 family of proteins reside in the inner mitochondrial membrane, form ionic channels in hpid membranes and increase rates of proton extrusion in mitochondria [88] and thus may control the FT pore. The antiapoptotic and mitochondrial affects of Bd-2 are independent of caspase activity as they occur in the presence of caspase inhibitors and also in yeast that lack caspases [86]. 

FIGURE 11-2 Lipid composition of the plasma membrane and organelle membranes of a rat hepatocyte. The functional specialization of each membrane type is reflected in its unique lipid composition. Cholesterol is prominent in plasma membranes but barely detectable in mitochondrial membranes. Cardiolipin is a major component of the inner mitochondrial membrane but not of the plasma membrane. Phosphatidylserine, phosphatidylinositol, and phosphatidylglycerol are relatively minor components (yellow) of most membranes but serve critical functions phosphatidylinositol and its derivatives, for example, are important in signal transductions triggered by hormones. Sphingolipids, phosphatidylcholine, and phosphatidylethanolamine are present in most membranes, but in varying proportions. Clycolipids, which are major components of the chloroplast membranes of plants, are virtually absent from animal cells. 

Petite mutants spontaneous mutants, chiefly yeasts, with chemical or physical defects in the respiratory chain. P. m. grow very slowly and form small ( petite ) colonies on nutrient agar. The same phenotype can be produced by a chromosomal mutation (segregational petite), or a mutation in the mitochondrial DNA (vegetative or neutral petite). In the latter case, mitochondrial structure is considerably altered, largely due to changes in the amino acid composition of the structural proteins of the inner mitochondrial membrane. Since these structural proteins are important for the correct arrangement and conformation of the respiratory chain enzymes, the effect of petite mutation on the respiratory chain is probably secondary. 

Stoffel, W. Schiefer, H.G. (1968) Hoppe-Seylers Z. Physiol. Chem., M9, 1017-1026. Biosynthesis and composition of phosphatides in outer and inner mitochondrial membranes. 

In a review on membrane proteins, Guidotti (1972) has classified membranes into three types on the basis of their protein content. The first class is the simple, inert membrane represented by myelin. It consists primarily of lipid with little protein, acts as a permeability barrier and insulator, and has only three known enzymatic activities (Beck et al., 1968 Olafson et al., 1969 Kurihara and Tsukada, 1967 Gammer et al., 1976 Yandrasitz et al., 1976). The large second class of membranes which have a protein-to-lipid ratio of about 1 1 (w w) are typified by most mammalian plasma membranes. They have many enzymatic activities and sophisticated transport systems associated with them, in addition to the permeability factor. The third class of membranes has bacterial and inner mitochondrial membranes as its models. These membranes have proportionately larger amounts of protein than lipid and have added functions such as oxidative phosphorylation and nucleic acid synthesis. In general, the specialization and enzyme function of the membrane increases in proportion to its protein content. Table 4 gives the amino acid composition of some isolated membrane proteins. Total membrane protein (intrinsic + extrinsic) often has an amino acid composition which falls into the range of other nonmembrane, "soluble" proteins (Vanderkooi and Capaldi, 1972). 

Analysis of the amino acid composition of the mitochondrially synthesized subunits of cytochrome oxidase of the rutamycin-sensitive ATP-ase (another membrane component which requires mitochondrial protein synthesis for some of its component polypeptides) and cytochrome b displayed a high proportion of nonpolar amino acid residues. The resulting unusually hydrophobic composition explains their insolubility in aqueous solutions. This hydrophobic character of the mitochondrially translated polypeptides may have relevance for speculations on the existence of mitochondrial protein synthesis. It has been suggested that their hydro-phobic properties make it necessary that these polypeptides be delivered to the inner mitochondrial membrane from the matrix side, since they cannot be transported through the cytoplasm and the intercristae space. 

The most important membranes in animal cells are the plasma membrane, the inner and outer nuclear membranes, the membranes of the endoplasmic reticulum (ER) and the Golgi apparatus, and the inner and outer mitochondrial membranes. Lysosomes, peroxisomes, and various vesicles are also separated from the cytoplasm by membranes. In plants, additional membranes are seen in the plastids and vacuoles. All membranes show polarity—e., there is a difference in the composition of the inner layer (facing toward the cytoplasm) and the outer layer (facing away from it). 

To determine the basis for this regular variation in rate of proton leakage, several characteristics of the mitochondria were measured, including inner membrane surface area per unit of matrix volume and fatty acid composition of mitochondrial membrane phospholipids. The largest share (about 70%) of the variation in proton flux rate appears to be due to dififer-... 

In the plasma membrane of animals (1), the amount of cholesterol is usually around 20-30 mol%. The rest of the lipids are mainly PC, PE, and sphingomyelin (SM) lipids, with smaller amounts of PS, PI, and glycolipids. These lipids are distributed asymmetrically across the membrane, because most cholesterol, PC, and glycolipids are located in the extracellular (outer) leaflet, whereas PS and PE lipids are located mainly in the intracellular (inner) monolayer. The lipid composition can be highly different in other organelles, however, as is the case in mitochondria (1), in which the mitochondrial membrane is composed of two (inner and outer) membranes. There, the amounts of cholesterol, SM, and PS are negligible most lipids are PC and PE. The major difference compared with plasma membrane is the concentration of cardiolipins. They are actually found only in bacterial and in mitochondrial membranes, where their numbers are significant even in mitochondria they are located mainly on the iimer membrane. 

Although a great deal is known about the chemical composition of the mitochondrial membrane and it is established that the membrane contains a number of catalytic proteins e.g., the ATPase synthetase system, an ion transport molecular machinery and electron transport chain), the topological distribution of these proteins in the membrane is not known. All topological models proposed are at present hypothetical [177]. However, it is accepted that the mitochondrial membrane, like most if not all biological membranes, is of the fluid mosaic model and is composed of a lipid bilayer traversed by proteins (see plasma membrane in Chapter 16). Electron microscopic studies of the freeze-edge fractured faces of the outer and the inner membrane [178] indicate that the proteins are asymmetrically distributed not only when the inner is compared to the outer membrane, but also when the inner and outer faces of each of the fractured membranes are compared (Table 1-3). 

By definition, the uncoupling effect of certain flavonoids should be independent of their inhibitory effects on mitochondrial respiration or FoFi-ATPase, suggesting an additional mode of action of flavonoids against mitochondrial function. A collapse of the transmembrane potential is likely under conditions in which the permeability barrier created by the mitochondrial inner membrane is compromised (as occurs in the presence of ionophores). Calcium, phosphate, oxidative stress, adenine nucleotide depletion, and membrane depolarization can induce such a nonspecific increase in the permeability of the inner membrane, in an event called the mitochondrial permeability transition (MPT) [30,34]. The MPT can be selectively inhibited by cyclosporin A and is believed to involve the assembly of a multiprotein complex to form a nonspecific pore that spans the inner and outer mitochondrial membranes. The latter assembly is referred to as the permeability transition pore complex (PTPC) (Fig. 1). Its exact composition is unknown, but appears to comprise cyclophilin D, ANT, the voltage-dependent anion channel (porin), and a benzodiazepinebinding site [10,30,34]. 

Membrane Composition. Biological membranes consist mostly of protein and lipid in roughly comparable quantities by weight. The overall composition of a variety of different membranes is given in Table 1.1. Values range from the relatively protein-rich inner mitochondrial and bacterial membranes which contain 70-80%... 

Defects of nuclear DNA also cause mitochondrial diseases. As mentioned above, the vast majority of mitochondrial proteins are encoded by nDNA, synthesized in the cytoplasm and imported into the mitochondria through a complex series of steps. Diseases can be due to mutations in genes encoding respiratory chain subunits, ancillary proteins controlling the proper assembly of the respiratory chain complexes, proteins controlling the importation machinery, or proteins controlling the lipid composition of the inner membrane. All these disorders will be transmitted by mendelian inheritance. From a biochemical point of view, all areas of mitochondrial metabolism can be affected (see below). 

The comprehensive analyses of the mitochondrial inner membrane by McCarty et al. (1973) raise several interesting points. Here PI is again found to be a highly saturated lipid. Again, PC and PE have similar fatty acid compositions, but again PC is more unsaturated than PE. The analysis for lyso-... 

Each membrane in a cell has its distinct set of proteins and lipids. The most common membrane lipids are phospholipids. Phospholipids are diglycerides. Two of the three hydroxyls of glycerol are linked to long-chain fatty acids by ester bonds. The third position is occupied by phosphate. A number of different polar substituents are linked to the phosphate by anhydride bonds. The phospholipid composition of the mitochondrial inner membrane is virtually the same in plant mitochondria as in animal mitochondria and resembles that in the plasma membrane of some bacteria. The lipids in chloroplast membranes are very distinctive. The phospholipid content is unusually low and about 80% of the membrane lipids in thylakoids are diglycerides that have one or two galactose (a six-... 

---