Porins mitochondrial

Both mitochondrial membranes are very rich in proteins. Porins (see p. 214) in the outer membrane allow small molecules (< 10 kDa) to be exchanged between the cytoplasm and the intermembrane space. By contrast, the inner mitochondrial membrane is completely impermeable even to small molecules (with the exception of O2, CO2, and H2O). Numerous transporters in the inner membrane ensure the import and export of important metabolites (see p. 212). The inner membrane also transports respiratory chain complexes, ATP synthase, and other enzymes. The matrix is also rich in enzymes (see B). 

Mitochondria are surrounded by an inner and an outer membrane (see p. 210). The outer membrane contains porins, which allow smaller molecules up to 10 kDa in size to pass. By contrast, the inner membrane is also impermeable to small molecules (with the exception of water and the gases O2, CO2, and NH3). All of the other substrates of mitochondrial metabolism, as well as its products, therefore have to be moved through the inner membrane with the help of special transporters. 

Mukherjee M, Seivers SA, Brown MT, Johnson PJ (2006b) Identification and biochemical characterization of serine hydroxymethyl transferase in the hydrogenosome of Trichomonas vaginalis. Eukaryot Cell 5 2072-2078 Muller A, Rassow J, Grimm J, Machuy N, Meyer TF, Rudel T (2002) VDAC and the bacterial porin PorB of Neisseria gonorrhoeae share mitochondrial import pathways. EMBO J 21 1916-1929... 

The most abundant protein in the E. coli outer membrane is OmpA. It appears to form a transmembrane helical bundle. Although it is regarded primarily as a structural protein it too acts, in monomeric form, as an inefficient diffusion pore.350 Mitochondrial outer membranes contain nonspecific pores (mitochondrial porins) that allow passage of sucrose and other saccharides of molecular mass up to 2 to 8 kDa.351 352 Similar pore-forming proteins have been found in plant peroxisomes.353... 

The outer membranes of mitochondria can be removed from the inner membranes by osmotic rupture.13 Analyses on separated membrane fractions show that the outer membrane is less dense (density — 1.1 g / cm3) than the inner (density 1.2 g / cm3). It is highly permeable to most substances of molecular mass 10 kDa or less because of the presence of pores of 2 nm diameter. These are formed by mitochondrial porins,14-17 which are similar to the outer membrane porins of gram-negative bacteria (Fig. 8-20). The ratio of phospholipid to protein ( 0.82 on a weight basis) is much higher than in the inner membrane. Extraction of the phospholipids by acetone destroys the membrane. Of the lipids present, there is a low content of cardiolipin, a high content of phosphatidylinositol and cholesterol, and no ubiquinone. 

Solutes enter mitochondria through pores in thousands of molecules of the voltage-gated anion-selective channel VDAC, also known as mitochondrial porin.1516 288 289 In the absence of a membrane potential these pores allow free diffusion to molecules up to 1.2 kDa in mass and may selectively permit passage of anions of 3- to 5-kDa mass. However, a membrane potential greater than 20 mV causes the pores to close. NADH also decreases permeability. 

Outer membrane. Because it contains a large channel-forming protein (called porin), the outer membrane is permeable to all molecules 5000 daltons or less. Other proteins in this membrane include enzymes involved in mitochondrial lipid synthesis and enzymes that convert lipid substrates into forms that are subsequently metabolized in the matrix. 

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. 

VDAC plays a role in the regulated flux of metabolites—usually anionic species such as phosphate, chloride, organic anions, and the adenine nucleotides—across the outer membrane. VDAC appears to form an open p -barrel structure similar to that of the bacterial porins (Section 12.5.2). although mitochondrial porins and bacterial porins may have evolved independently. Some cytoplasmic kinases bind to VDAC, thereby obtaining preferential access to the exported ATP. In contrast, the inner membrane is intrinsically impermeable to nearly all ions and polar molecules. A large family of transporters shuttles metabolites such as ATP, pyruvate, and citrate across the inner mitochondrial membrane. The two faces of this membrane will be referred to as the matrix side and the cytosolic side (the latter because it is freely accessible to most small molecules in the cytosol). They are also called the N and P sides, respectively, because the membrane potential is negative on the matrix side and positive on the cytosolic side. 

A steady flow of metabolites both in and out of the mitochondrial matrix space is necessary for mitochondria to perform functions which involve the participation of enzymes inside the membrane permeability barrier. These functions include oxidative phosphorylation and therefore O2, ADP, phosphate and electron-rich substrates such as pyruvate, fatty acids and ketone bodies must enter the mitochondria, and the products, HjO, CO2 and ATP must leave. Although Oj, HjO and CO2 are permeable to the inner mitochondrial membrane [1,2], most metabolites are not, because of their highly hydrophiUc nature. The outer mitochondrial membrane does not present a significant barrier to hydrophilic metabolites because of the presence of large unregulated channels composed of the membrane protein, porin [3]. The inner mitochondrial membrane has a much larger surface area [4] than the outer membrane and a much higher ratio of protein to lipid [5]. It is composed not only of proteins involved in electron transport and oxidative phosphorylation but also specialized proteins which facilitate, and in many cases provide, directionality to the transport of metabolites [6]. 

One of the best-studied examples of protein incorporation is the mitochondrial outer membrane. In yeast and Neurospora crassa none of the outer membrane proteins studied thus far are made as larger precursors. All of these proteins are made on free ribosomes and are incorporated into the outer membrane post-translationally [35,69,99]. The import of the porin (a pore-forming protein) is time and temperature dependent but does not require energy [35,99]. The incorporation of porin in vitro was found to be membrane specific [35]. How is this membrane specificity determined, and what anchors the outer membrane protein to the outer membrane ... 

Fig. 12.6. The onset of synthesis of various mitochondrial polypeptides upon transferring anaerobically grown yeast cells to aerobic conditions. Yeast cells were grown overnight under anaerobic conditions. At time zero they were transferred to aerobic conditions, and at the indicated time periods samples of cells were removed and lysed in the presence of NaOH and mercaptoethanol. Samples containing about 50 /ig of protein were electrophoresed in a sodium dodecyl sulfate-polyacrylamide gel. The proteins were electrotransferred to nitrocellulose sheets and decorated with specific antibodies and l-labelled protein A. Paper pieces corresponding to the labelled protein spots were cut out from the immune blot and counted in a y counter. The amount of counts obtained in the samples of 8 h aerobic conditions was taken as 100%. The antibodies used were directed against the following polypeptides porin of the mitochondrial outer membrane (29 k) /8 subunit of the proton-ATPase (iS-F,) subunit IV of cytochrome c oxidase (OxIV) and subunit V of cytochrome c oxidase (OxV).

The outer membrane is quite permeable to most small molecules and ions because it contains many copies of mitochondrial porin, a 30 to. iSkd pore-forming protein also known as VDAC, tor voltage-dependent anion channel. VDAC plays a role in the regulated flux of metabolites—... 

The outer membrane contains mitochondrial porin, a transmembrane channel protein similar In structure to... 

The osmotic stress method was first applied to the large voltage dependent anion channel (VDAC), also known as mitochondrial porin , from the outer membrane of mitochondria. The protein was placed in an artificial... 

The data base PORINS consisted of seven porins and two defensins all with known or proposed transmembrane P-strand structure. The porins with known X-ray structure were porin from Rhodobacter capsulatus [10,28] and porins PhoE and OmpF from Escherichia coli [39,11]. Porins with proposed transmembrane P-barrel topology were anion-selective porin Omp32 from Comamonas acidovorans [40], outer membrane protein OmpA from Escherichia coli K12 membrane (membrane-embedded fragment residues 1 to 177, [41,42]), and mitochondrial outer membrane porin from human B-lymphocytes [43] and from Neurospora crassa [44]. Two defensins of known structure were HNP-3 [45] and defensin from larvae of the dragonfly Aeschna cyanea [46]. 

The inner mitochondrial membrane is highly impermeable, and the proton gradient that is built up across this membrane during oxidative phosphorylation is essential for ATP generation from ADP and phosphate. The transport of ions occurs principally through facilitative transporters in a type of secondary active transport powered by the proton gradient established by the electron transport chain. The outer membrane contains pores made from proteins called porins and is permeable to molecules with a molecular weight up to about 1000 g/mole. 

Mitochondria and cell death Although oxidative phosphorylation is a mitochondrial process, most ATP utilization occurs outside of the mitochondrion. ATP synthesized from oxidative phosphorylation is actively transported from the matrix to the intermembrane space by adenine nucleotide translocase (ANT). Porins form voltage-dependent anion channels (VDAC) through the outer mitochondrial membrane for the diffusion of H2O, ATP metabolites, and other ions. Under certain types of stress, ANT, VDAC, and other proteins form a nonspecific open channel known as the mitochondrial permeability transition pore. This pore is associated with events that lead rapidly to necrotic cell death. 

Whereas the inner mitochondrial membrane is highly impermeable, the outer mitochondrial membrane is permeable to compounds with a molecular weight up to approximately 6,000 daltons because it contains large nonspecific pores called volt-age-dependent anion channels (VDAC) that are formed by mitochondrial porins (see Fig. 21.13). Unlike most transport proteins, which are membrane-spanning helices with specific binding sites, VDACs are composed of porin homodimers that form a p-barrel with a relatively large nonspecific water-filled pore through the center. These channels are open at low transmembrane potential, with a preference for anions such as phosphate, chloride, pyruvate, citrate, and adenine nucleotides. 

Hexokinases, the enzymes that catalyze the phosphorylation of glucose, are a family of tissue-specific isoenzymes that differ in their kinetic properties. The isoenzyme found in liver and p cells of the pancreas has a much higher than other hexokinases and is called glucokinase. In many cells, some of the hexokinase is bound to porins in the outer mitochondrial membrane (voltage-dependent anion channels see Chapter 21), which gives these enzymes first access to newly synthesized ATP as it exits the mitochondria. 

Solutes enter mitochondria through pores in thousands of molecules of the voltage-gated anion-selective channel VDAC, also known as mitochondrial porin. 46/288,289. j g absence of a membrane... 

Ono, H. Tuboi, S. (1987) Eur. J. Biochem., 168, 509-514. Integration of porin synthetized in vitro into outer mitochondrial membranes. 

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