Outer membrane mitochondrial

The PBRis distinct from the central BZ receptor although both can be present in the same tissues in differing ratios. PBRs are predominately localized on the outer mitochondrial membrane and are thus intracellular BZ recognition sites. The PBR is composed of three subunits an 18,000 mol wt subunit that binds isoquinoline carboxamide derivatives a 30,000 mol wt subunit that binds BZs and a 32,000 mol wt voltage-dependent anion channel subunit. The porphyrins may be endogenous ligands for the PBR. PBRs are involved in the control of cell proliferation and differentiation and steroidogenesis. 

Long-chain fatty acids (e.g., palmitate Cig) diffuse through pores in the outer mitochondrial membrane, and then form long-chain acyl-CoA esters catalyzed reversibly by palmitoyl-CoA synthase (assumed to be on the inner face of the outer membrane). 

Another pathway is the L-glycerol 3-phosphate shuttle (Figure 11). Cytosolic dihydroxyacetone phosphate is reduced by NADFl to s.n-glycerol 3-phosphate, catalyzed by s,n-glycerol 3-phosphate dehydrogenase, and this is then oxidized by s,n-glycerol 3-phosphate ubiquinone oxidoreductase to dihydroxyacetone phosphate, which is a flavoprotein on the outer surface of the inner membrane. By this route electrons enter the respiratory chain.from cytosolic NADH at the level of complex III. Less well defined is the possibility that cytosolic NADH is oxidized by cytochrome bs reductase in the outer mitochondrial membrane and that electrons are transferred via cytochrome b5 in the endoplasmic reticulum to the respiratory chain at the level of cytochrome c (Fischer et al., 1985). 

While it has been known for many years that the N-terminal presequence is sufficient to promote mitochondrial targeting and assembly, the subsequent interaction of the precursor molecule with the outer mitochondrial membrane and the uptake of the protein is still an area of active research. There seems little doubt, however, that there are proteins on the outer mitochondrial membrane which are required for the import process. The function of these proteins is uncertain, but they may act as receptors with the subsequent transfer through the membrane at proteinous pores located at contact sites between the inner and outer membranes. Several proteins have been identified which seem to play an important role as either receptor proteins or part of the import channel (Pfanner et al., 1991). Again, not all proteins seem to depend on this mechanism. Cytochrome c, which is loosely associated with the outer aspect of the inner mitochondrial membrane, can cross... 

The nuclear-encoded proteins are inserted into both inner and outer mitochondrial membranes, the intermembrane space, and the matrix and there are several different mechanisms involved. As mentioned above there is no apparent requirement for a presequence on proteins which insert specifically into the mitochondrial outer membrane. For proteins destined for the inner mitochondrial membrane, a stop-transfer mechanism is proposed. Thus some information in the peptide must stop the complete transfer of the protein into the mitochondrial matrix, enabling the protein to remain in the inner mitochondrial membrane. For some proteins in the intermembrane space (for example the Rieske iron-sulphur protein associated with the outer face of complex III), a particularly complicated import pathway... 

Murthy, M.S.R. Pande, S.V. (1987). Malonyl-CoA binding site and the overt carnitine palmitoyltransferase activity reside on the opposite sides of the outer mitochondrial membrane. Proc. Nat. Acad. Sci. USA 84,378-382. 

The steps in the subsequent utilization of muscle LCFAs may be summarized as follows. The free fatty acids, liberated from triglycerides by a neutral triglyceride lipase, are activated to form acyl CoAs by the mediation of LCFAcyl-CoA synthetase which is situated on the outer mitochondrial membrane. The next step involves carnitine palmitoyl transferase I (CPT I, see Figure 9) which is also located on the outer mitochondrial membrane and catalyzes the transfer of LCFAcyl residues from CoA to carnitine (y-trimethyl-amino-P-hydroxybutyrate). LCFAcyl... 

Figure 12-14. The creatine phosphate shuttle of heart and skeletal muscle. The shuttle allows rapid transport of high-energy phosphate from the mitochondrial matrix into the cytosol. CKg, creatine kinase concerned with large requirements for ATP, eg, muscular contraction CIC, creatine kinase for maintaining equilibrium between creatine and creatine phosphate and ATP/ADP CKg, creatine kinase coupling glycolysis to creatine phosphate synthesis CK, , mitochondrial creatine kinase mediating creatine phosphate production from ATP formed in oxidative phosphorylation P, pore protein in outer mitochondrial membrane.

Carnitine (p-hydroxy-y-trimethylammonium butyrate), (CHjljN"—CH2—CH(OH)—CH2—COO , is widely distributed and is particularly abundant in muscle. Long-chain acyl-CoA (or FFA) will not penetrate the inner membrane of mitochondria. However, carnitine palmitoyltransferase-I, present in the outer mitochondrial membrane, converts long-chain acyl-CoA to acylcarnitine, which is able to penetrate the inner membrane and gain access to the P-oxidation system of enzymes (Figure 22-1). Carnitine-acylcar-nitine translocase acts as an inner membrane exchange transporter. Acylcarnitine is transported in, coupled with the transport out of one molecule of carnitine. The acylcarnitine then reacts with CoA, cat-... 

In addition to its interaction with central BZD receptors, pharmacological concentrations of melatonin can also bind to the peripheral-type BZ receptors (PBRs), which are involved in neurosteroidogenesis (Garcia-Ovejero et al. 2005). PBRs are primarily localized on the outer mitochondrial membrane these sites are therefore also referred to as mitochondrial BZD receptors. Using the isoquinoline carboxamide, PK14105, for photoaffinity labelling, an 18 kDa isoquinoline... 

Figure 12.2 Copper chaperone function, (a) Copper homeostasis in Enterococcus hirae is affected by the proteins encoded by the cop operon. CopA, Cu1+-import ATPase CopB, Cu1+-export ATPase CopY, Cu1+-responsive repressor copZ, chaperone for Cu1+ delivery to CopY. (b) The CTR family of proteins transports copper into yeast cells. Atxlp delivers copper to the CPx-type ATPases located in the post Golgi apparatus for the maturation of Fet3p. (c) Coxl7p delivers copper to the mitochondrial intermembrane space for incorporation into cytochrome c oxidase (CCO). (d) hCTR, a human homologue of CTR, mediates copper-ion uptake into human cells. CCS delivers copper to cytoplasmic Cu/Zn superoxide dismutase (SOD1). Abbreviations IMM, inner mitochondrial membrane OMM, outer mitochondrial membrane PM, plasma membrane PGV, post Golgi vessel. Reprinted from Harrison et al., 2000. Copyright (2000), with permission from Elsevier Science.

Akt is inactivated, and Bad is no longer phosphorylated and, as a consequence, relocalizes in the outer mitochondrial membrane, initiating apoptosis. 

GMBS or sulfo-GMBS have been used for studying carnitine palmitoyltransferase-1 in its formation of a complex within the outer mitochondrial membrane (Faye et al., 2007), for investigating protein organization of the postsynaptic density (Liu et al., 2006), and in studying the structure and dynamics of rhodopsin (Jacobsen et al., 2006). 

Faye, A., Esnous, C., Price, N.T., Onfray, M.A., Girard, J., and Prip-Buus, C. (2007) Rat liver carnitine palmitoyltransferase 1 forms an oligomeric complex within the outer mitochondrial membrane.. Biol. Cbem. 10.1074/jbc.M705418200. 

NO may react with superoxide to yield the highly reactive peroxynitrite, ONOO-. Superoxide may also be converted into H202 and the reactive hydroxyl radical, OH. In this way excessive activation of glutamate receptors leads to oxidative damage. The calcium influx has a major effect on mitochondria and causes them to depolarize and swell. This leads to a pore being formed in the outer mitochondrial membrane, which allows the escape of cytochrome c and procaspases from the mitochondria into the cytosol. Cytochrome c activates the caspase cascade, which leads to apoptotic cell death (Ch. 35). 

FIGURE 31-7 Mitochondrial carriers. Ions and small molecules enter the intermembrane space, since the outer mitochondrial membrane is not a significant permeability barrier. However, the inner mitochondrial membrane is impermeable to ions except those for which there are specific carriers. Most of the carriers are reversible, as indicated by two-headed arrows. Compounds transported in one direction are indicated in red. The ATP/ADP translocase and the aspartate-glutamate carrier are both electrophoretic their transport is driven in the direction of the mitochondrial membrane potential, as indicated by red arrows. Glutamine is carried into the matrix by an electroneutral carrier. The unimpaired functioning of mitochondrial carriers is essential for normal metabolism. (Adapted with permission from reference [70].)... 

The biogenic amines are the preferred substrates of MAO. The enzyme comes in two flavors, MAO-A and MAO-B, both of which, like FMO, rely on the redox properties of FAD for their oxidative machinery. The two isoforms share a sequence homology of approximately 70% (81) and are found in the outer mitochondrial membrane, but they differ in substrate selectivity and tissue distribution. In mammalian tissues MAO-A is located primarily in the placenta, gut, and liver, while MAO-B is predominant in the brain, liver, and platelets. MAO-A is selective for serotonin and norepinephrine and is selectively inhibited by the mechanism-based inhibitor clorgyline (82). MAO-B is selective for /1-phcncthylaminc and tryptamine, and it is selectively inhibited by the mechanism-based inhibitors, deprenyl and pargyline (82) (Fig. 4.32). Recently, both MAO-A (83) and MAO-B (84) were structurally characterized by x-ray crystallography. 

Kinner, a. and Rolling, R. The yeast deubiquitinating enzyme Ubpl6 is anchored to the outer mitochondrial membrane, FEES Lett, 2003, 549, 135-40. 

Proapototic signals direct these proteins to mitochondria where they compete with antiapoptotic members of the Bcl-2 family to regulate the cytochrome c release and to determine the fate of the cell life or death (Cosulich et al., 1999). Unlike proapoptotic proteins, the antiapoptotic Bcl-2 proteins reside in the outer mitochondrial membrane, anchored by a hydrophobic stretch of amino acids located at the COOH-termini,... 

Figure 3. Possible mechanisms of actions of Bcl-2 members. Two prevailing models through which Bcl-2 membas trigger cytochrome c release have been suggested. In both models phospholipids in the bilayer stnicture either individually and/or collectively induce a conformational change in Bcl-2 members, allowing them to insert into the outer mitochondrial membrane. In model 1 proapoptotic proteins destabilize the outer mitochondrial membrane, oligomerize and form channels through which cytochrome c and other proteins of the intermembrane space can escape.BcI-2 proteins such as Bax or tBid act in concert with other proteins of the BcI-2 family to form channels. In model 2 Bcl-2 members such as Bax interact with residoit proteins in the outer membrane (OM) such as the voltage-dependent anion...

In mammalian cells, the final stage of PS biosynthesis occurs in ER and MAM (Trotter and Voelker, 1994 Daum and Vance, 1997 Voelker, 2000). The other membranes in the ceU, such as mitochondria, nucleus, and plasma membrane, are therefore assembled from PS exported from ER and MAM (Figure 2). Phospholipid synthesis in mitochondria is restricted to the formation ofphosphatidylglycerol, cardiolipin, and PE, and other lipids such as PC and PS must be imported from sites of cellular lipid synthesis, ER or MAM (Daum, 1985 Vance, 1991). PS imported to the outer mitochondrial membrane is then translocated to the inner mitochondrial membrane, where it is converted to PE by PS decarboxylase (PSD) (Dennis and Kennedy, 1972 Voelker, 1990). It has been shown that the translocation of PS to mitochondria followed by its decarboxylation is a major pathway for the synthesis of PE in some cultured mammahan cells (Voelker, 1984 Kuge et al, 1986 Voelker and Frazier, 1986), suggesting that significant amounts of PE found in cell membranes are derived from mitochondria. 

Since the enzymes involved in PS synthesis are located in ER or MAM, and PSD is exclusively located at the inner mitochondrial membrane, the conversion of PS to PE by PSD has been used as an indicator of PS translocation into the inner mitochondrial membrane (Dennis and Kennedy, 1972 Voelker, 1990). Recent studies have shown that the transport of newly synthesized PS to the outer mitochondrial membrane requires no cytosolic proteins and is probably mediated by direct contact region between MAM and mitochondria (Voelker, 1989 Voelker, 1993 Shiano et al., 1995). It is also suggested that the translocation of PS from the outer to iimer mitochondrial membrane occurs through the contact sites where the two mitochondrial membranes are closely apposed and linked in a stable manner, since agents that dismpt the contact sites such as 1,4-dinitrophenol and adriamycin inhibit the PS transport (Hovius et al., 1992 Voelker, 1991). 

Long-chain fatty acids must be activated and transported into the mitochondria. Fatty acyl CoA synthetase, on the outer mitochondrial membrane, activates the fatty adds by attaching CoA. The fetty acyl portion is then transferred onto carnitine by carnitine aqdtransferase-I for transport into the mitochondria. The sequence of events is shown in Figure 1-16-2 and indudes the following steps ... 

Fatty acyl synthetase activates the fatty add (outer mitochondrial membrane). 

Answer C. CAT-1 (CPT-1) and fatty acyl synthetase are among the few enzymes associated with the outer mitochondrial membrane. 

The process of oxidative deamination is the most important mechanism whereby all monoamines are inactivated (i.e. the catecholamines, 5-HT and the numerous trace amines such as phenylethylamine and tryptamine). Monoamine oxidase occurs in virtually all tissues, where it appears to be bound to the outer mitochondrial membrane. Whereas there are several specific and therapeutically useful monoamine oxidase inhibitors, inhibitors of catechol-O-methyltransferase have found little application. This is mainly due to the fact that at most only 10% of the monoamines released from the nerve terminal are catabolized by this enzyme. The main pathways involved in the catabolism of the catecholamines are shown in Figure 2.16. 

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). 

A nucleotide transporter (located in the outer mitochondrial membrane) that mediates one-for-one transloca-tion/exchange of cytosolic ADP for mitochondrial ATP. This translocase is potently inhibited by atractyloside and bonkregic acid. 

Figure 8-3. The carnitine shuttle. A long-chain fatty acyl CoA (LCFA CoA) can diffuse across the outer mitochondrial membrane but must be carried across the inner membrane as acyl-carnitine. The active sites of CPT-I and CPT-II are oriented toward the interiors of their respective membranes. CPT, carnitine palmitoyltransferase.

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