Mitochondrial membrane transporters, table

The clinical syndrome of acute neonatal hyper-ammonemic encephalopathy described in the case report represents the classical presentation of a patient with a urea cycle disorder (UCD). It is important to note that this neonatal course represents only the most common and severe presentation of a UCD. This holds true for all the diseases listed in Table 18-1, with the exceptions of arginase (ARG-1) deficiency, which results in progressive spasticity of the lower limbs, and of the mitochondrial membrane transporters citrin and ornithine transporter 1 (ORNT-1). Deficiency of citrin results in adult-onset encephalopathy deficiency of... 

To date, twelve transporters with different substrate specificities have been demonstrated in mammalian mitochondrial membranes - (cf.. Table 8.1). Plant mitochondria have a slightly different set [11]. Early studies of metabolite transport in mitochondria dating from the middle 1960s were concerned with identifying these transporters. More recently, research has been directed toward elucidation of molec-... 

Thus, Og and cytochrome c oxidase are the final destination for the electrons derived from the oxidation of food materials. In concert with this process, cytochrome c oxidase also drives transport of protons across the inner mitochondrial membrane. These important functions are carried out by a transmembrane protein complex consisting of more than 10 subunits (Table 21.2). 

Critical for predictivity in a recent comprehensive study was the number and choice of parameters measured [4]. Early, sublethal effects on cell proliferation, cell morphology and mitochondria occurred consistently and ubiquitously with toxicity and when used collectively were most diagnostic. It is noteworthy that the toxicity of many drugs is attributable to various mitochondrial targets, including oxidative phosphorylation, fatty acid oxidation, Krebs cycling, membrane transport, permeability transition pore, proliferation and oxidative stress (Table 14.4). 

The lipid-soluble ubiquinone (Q) is present in both bacterial and mitochondrial membranes in relatively large amounts compared to other electron carriers (Table 18-2). It seems to be located at a point of convergence of the NADH, succinate, glycerol phosphate, and choline branches of the electron transport chain. Ubiquinone plays a role somewhat like that of NADH, which carries electrons between dehydrogenases in the cytoplasm and from soluble dehydrogenases in the aqueous mitochondrial matrix to flavoproteins embedded in the membrane. Ubiquinone transfers electrons plus protons between proteins within the... 

Inhibition of the electron transport chain in coupled mitochondria can occur at any of the three constituent functional processes electron transport per se, formation of ATP, or antiport translocation of ADP/ATP (Table 16-1). The best known inhibitor of the ADP/ATP translocase is atractyloside in the presence of which no ADP for phosphorylation is transported across the inner membrane to the ATP synthase and no ATP is transported out. In the absence of ADP phosphorylation the proton gradient is not reduced allowing other protons to be extruded into the intermembrane space because of the elevated [H+], and thus electron transfer is halted. Likewise the antibiotic oligomycin directly inhibits the ATP synthase, causing a cessation of ATP formation, buildup of protons in the intermembrane space, and a halt in electron transfer. Similarly, a blockade of complex I, III, or IV that inhibits electron flow down the chain to would also stop both ATP formation and ADP/ATP translocation across the inner mitochondrial membrane. 

For a proper understanding of the role of the translocators in the regulation of metabolism, knowledge of their kinetic constants is indispensable. Table 1 summarises these parameters. Because of technical difficulties, in most cases it has not been possible to determine these parameters at 37°C. It is also important to stress that the kinetic constants have been determined in isolated mitochondria. It is likely that the kinetic constants in the intact cell are different, one reason being that there is an inhibitory interaction of cytosolic anions with the various translocators. Some of these effects are given in Table 2. With the exception of a few cases (the a-oxoglutarate translocator in heart [18] and the carnitine and aspartate translocators see Sections If, iii and iv), little is known about the values of the metabolites to be transported from the matrix side of the mitochondrial membrane. In the case of citrate and ATP transport such information is difficult to obtain because most of the intramitochondrial citrate and ATP is chelated with Mg " and only the free anions are transported. Likewise, little is known about possible competition between metabolites present in the matrix for export out of the mitochondria. The complexity of the complete kinetic analysis of a translocator, in which both the external and internal concentrations have been taken into account, is illustrated by the studies of Sluse et al. [18] on a-oxoglutarate transport in heart mitochondria. 

Most studies with isolated hepatocytes or perfused liver indicate the existence of a ApH across the mitochondrial membrane, ranging from 0-0.6 [33,34,37-39], but under some conditions the calculated ApH depends on the metabolite chosen. This indicates disequilibrium in one or more of the transport steps. Under most conditions it is unlikely that the phosphate translocator is out of equilibrium because of its very high (Table 1). Problems do arise, however, when the dicarboxylate translocator is out of equilibrium because it connects the movement of citrate, malate and a-oxoglutarate with that of H. In that case the above equation cannot... 

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). 

Yeast cytochrome oxidase is built up from seven different subunits, of which four are synthesized in the cytosol, and three in the mitochondria (25). R. 0. Poyton (26) recently reported evidence suggesting that the four cytosolic subunits are synthesized in one peptide chain. This precursor protein might be split by limited proteolysis in connection with or after transport from the cytosol to the inner mitochondrial membrane. Here seems to be an example of a proteolytic process in yeast participating in subcellular translocation of subunits and assembly of an enzyme (see Table V). V/hich proteinase is involved in this process, and whether specific inhibitors also play a part, is as present unknown. 

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). 

The discussion to this point has focused on the isolation of intact mitochondria. By various chemical and physical treatments, mitochondria may be separated into their four components. This allows biochemists to study the biological functions of each component. For example, by measuring enzyme activities in each fraction, one can assign the presence of a particular enzyme to a specific region of the mitochondria. Studies of mitochondrial subfractions have resulted in a distribution analysis of enzyme activities in the four locations (Table E10.1). This type of study is often referred to as an enzyme profile or enzyme activity pattern and the enzyme may be considered a marker enzyme. For example, cytochrome oxidase, which is involved in electron transport, is a marker enzyme for the inner membrane. 

Table 7.1 shows that one mitochondrial, two hydrogenosomal, and one mitosomal ADP/ATP transporter all possess different Km values and have different requirements for a membrane potential. Also, the spectrum of nucleotides which can be transported by the various proteins is quite peculiar at present, the transporter of Entamoeba is the only ADP/ATP transporter that also transports AMP, just like its relative (brittle-1) in the plastids of Solanum tuberosum (Leroch et al. 2005 Table 7.2). In addition, the different ADP/ATP transporters exhibit a peculiar spectrum of sensitivity, or resistance,...

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