Intermembrane space, mitochondria

The mitochondrion has an outer and an inner membrane (Figure 1). The outer membrane contains pores formed from a protein, porin, which allow exchange of molecules with molecular weights up to about 2,000 between the cytosol and the intermembrane space. The inner membrane is extensively invaginated to increase its surface area. It has a different lipid composition from the outer membrane and is rich in the acidic phospholipid cardiolipin (diphosphatidyl-glycerol) which is only found in animal cells in mitochondria. Cardiolipin confers good electrical insulating properties on the inner membrane which is impermeable... 

What do I mean by a proton concentration gradient Simply, there is a higher concentration of protons in the space between the inner and outer membranes of the mitochondrion than in the mitochondrial interior. The gradient is formed from the energy released in the transfer of electrons down the electron transport chain. Put another way, the released energy is employed to pump protons across the inner mitochondrial membrane into the intermembrane space. 

The ATP, which is transported out of the mitochondrion, immediately phosphorylates creatine, catalysed by the creatine kinase in the intermembrane space. 

Fig. 7.3.1 The heme synthesis pathway starts in the mitochondrion. The next four steps proceed in the cytosol. Coproporphyrinogen oxidase is in the intermembrane space of the mitochondrion, and the last two enzymes reside at the mitochondral matrix side of the inner membrane. The product heme represses the first and rate-limiting enzyme -aminolevulinic acid (5-ALA) synthase at transcription, during the translation step, and by its transport into the mitochondrion... 

Mitochondria are intracellular centers for aerobic metabolism. They are cell organelles that are identified by well-defined structural and biochemical properties. In morphological terms, mitochondria are relatively large particles that are characterized by the presence of two membranes, a smooth outer membrane that is permeable to most important metabolites and an inner membrane that has unique transport properties. The inner membrane is highly folded, which serves to increase its surface area. Figure E10.1, which shows the structure of a typical mitochondrion, divides the organelle into four major components inner membrane, outer membrane, intermembrane space, and the matrix. These regions are associated with different and... 

In eukaryotes, most of the reactions of aerobic energy metabolism occur in mitochondria. An inner membrane separates the mitochondrion into two spaces the internal matrix space and the intermembrane space. An electron-transport system in the inner membrane oxidizes NADH and succinate at the expense of 02, generating ATP in the process. The operation of the respiratory chain and its coupling to ATP synthesis can be summarized as follows ... 

Besides its role as the energy-generating organelle, the mitochondrion has recently emerged as the center of conversion of cellular life and death signals. This organelle contains, in its intermembrane space, apoptogenic... 

These organelles are the sites of energy production of aerobic cells and contain the enzymes of the tricarboxylic acid cycle, the respiratory chain, and the fatty acid oxidation system. The mitochondrion is bounded by a pair of specialized membranes that define the separate mitochondrial compartments, the internal matrix space and an intermembrane space. Molecules of 10,000 daltons or less can penetrate the outer membrane, but most of these molecules cannot pass the selectively permeable inner membrane. By a series of infoldings, the internal membrane forms cristae in the matrix space. The components of the respiratory chain and the enzyme complex that makes ATP are embedded in the inner membrane as well as a number of transport proteins that make it selectively permeable to small molecules that are metabolized by the enzymes in the matrix space. Matrix enzymes include those of the tricarboxylic acid cycle, the fatty acid oxidation system, and others. 

Mitochondria release not only cytochrome c but also many pro-apoptotic factors (Table 17.1). They are normally localized in the intermembrane space of mitochondria. However, except for cytochrome c and the apoptosis inducing factor (AIF), their functions in the mitochondria have not been determined or they may have no function under normal conditions. Because they are larger than 5kD, they remain inside the mitochondrion. Once the mitochondrial outer membrane is permeabi-... 

Almost all cells have an active transport system to maintain nonequilibrium concentration levels of substrates. For example, in the mitochondrion, hydrogen ions are pumped into the intermembrane space of the organelle as part of producing ATP. Active transport concentrates ions, minerals, and nutrients inside the cell that are in low concentration... 

The complete amino acid sequence of S. cerevisiae flavocytochrome 2 has been determined (26). The mature form of the enzyme is composed of 511 amino acids (Fig. 4). The DNA sequence has also been determined (27), revealing the presence of an 80-residue N-terminal presequence. This N-terminal extension directs the enzyme into the mitochondrion, where it is processed in two proteolytic steps that result in mature flavocytochrome 62, which is located in the intermembrane space (1, 33-36). 

The nucleus, mitochondrion, and chloroplast are bounded by two bIlayer membranes separated by an intermembrane space. All other organelles are surrounded by a single membrane. 

In the mitochondrion, the proton-motive force is generated by coupling electron flow from NADH and FADH2 to O2 to the uphill transport of protons from the matrix across the inner membrane to the Intermembrane space. 

In muscle, most of the fatty acids undergoing beta oxidation are completely oxidized to C02 and water. In liver, however, there is another major fate for fatty acids this is the formation of ketone bodies, namely acetoacetate and b-hydroxybutyrate. The fatty acids must be transported into the mitochondrion for normal beta oxidation. This may be a limiting factor for beta oxidation in many tissues and ketone-body formation in the liver. The extramitochondrial fatty-acyl portion of fatty-acyl CoA can be transferred across the outer mitochondrial membrane to carnitine by carnitine palmitoyltransferase I (CPTI). This enzyme is located on the inner side of the outer mitochondrial membrane. The acylcarnitine is now located in mitochondrial intermembrane space. The fatty-acid portion of acylcarnitine is then transported across the inner mitochondrial membrane to coenzyme A to form fatty-acyl CoA in the mitochondrial matrix. This translocation is catalyzed by carnitine palmitoyltransferase II (CPTII Fig. 14.1), located on the inner side of the inner membrane. This later translocation is also facilitated by camitine-acylcamitine translocase, located in the inner mitochondrial membrane. The CPTI is inhibited by malonyl CoA, an intermediate of fatty-acid synthesis (see Chapter 15). This inhibition occurs in all tissues that oxidize fatty acids. The level of malonyl CoA varies among tissues and with various nutritional and hormonal conditions. The sensitivity of CPTI to malonyl CoA also varies among tissues and with nutritional and hormonal conditions, even within a given tissue. Thus, fatty-acid oxidation may be controlled by the activity and relative inhibition of CPTI. 

How does brown fat generate heat and burn excess calories For the answer we must turn to the mitochondrion. In addition to the ATP synthase and the electron transport system proteins that are found in all mitochondria, there is a protein in the inner mitochondrial membrane of brown fat tissue called thermogenin. This protein has a channel in the center through which the protons (H ) of the intermembrane space could pass back into the mitochondrial matrix. Under normal conditions this channel is plugged by a GDP molecule so that it remains closed and the proton gradient can continue to drive ATP synthesis by oxidative phosphorylation. 

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. 

One puzzling question is how heme A is incorporated into CcO, the final and only known target of heme A. It was previously demonstrated that the assembly of Coxl in the mitochondrion is a heme A-dependent process (22). Mutations on COXIO or COX15 in humans cause CcO deficiency, and no subassembly complexes accumulate, indicating that heme A is required not only for functional CcO, but also for the stable assembly of the CcO complex (25). The identification of a heme A-Coxl assembly intermediate and the lack of a Coxl-Cox4-Cox5 subassembly complex in CQA7( -deficient human cells support the notion that heme A insertion is the early step of CcO assembly (22-26). The crystal structure of CcO reveals that heme A is deeply buried inside the inner mitochondrial membrane. It was subsequently hypothesized that heme A may enter the hydrophobic pocket of Coxl from the IMS (intermembrane space) side of the IM (inner membrane) (27, 28),... 

Interestingly, Cox 17 is localized both in the cytoplasm and in the intermembrane space (IMS), perhaps suggesting a secondary function of delivering cytoplasmic copper to the mitochondrion (55). However, Cox 17 is still functional when tethered to the inner mitochondrial membrane by a heterologous transmembrane sequence, and the mitochondrial copper concentration is not decreased in the coxl7 null mutants (36). One possible explanation for these results is that cytosolic Cox 17 localization simply results from incomplete mitochondrial uptake. A second possibility is that the primary function of Cox 17 is shuttling copper ions to Scol and Cox 11 inside the mitochondrion, and that transporting copper ions from the cytosol to the mitochondrion is a secondary fimction in which Cox 17 plays a redundant role. 

A quick review of some aspects of mitochondrial structure is in order here because we shall want to describe the exact location of each of the components of the citric acid cycle and the electron transport chain. Recall from Chapter 1 that a mitochondrion has an inner and an outer membrane (Figure 19.2). The region enclosed by the inner membrane is called the mitochondrial matrix, and an intermembrane space exists between the inner and outer membranes. The inner membrane is a tight barrier between the matrix and the cytosol, and very few compounds can cross this barrier without a specific transport protein (Section 8.4). The reactions of the citric acid cycle take place in the matrix, except for the one in which the intermediate electron acceptor is FAD. The enzyme that catalyzes the FAD-linked reaction is an integral part of the inner mitochondrial membrane and is linked direcdy to the electron transport chain (Chapter 20). 

How are fatty acids transported to the mitochondrion for oxidation After an initial activation step in the cytosol, with formation of an acyl-CoA corresponding to each fetty acid, each acyl group is transesterified to carnitine for transport across the intermembrane space of the mitochondrion. The acyl group is again transesterified to form an acyl-GoA. 

The outer membrane of the mitochondrion contains a large number of pores, so that molecules with molecular weights less than 1,000 can pass from the cytoplasm into the intermembrane space without any specialized transport mechanisms. This means, for example, that NADH, ADP, and inorganic phosphate can reach the intermembrane space from the cytoplasm while NAD and ATP can reach the cytoplasm. The inner membrane is much less permeable, in part due to the presence of a specialized membrane lipid known as cardiolipin (meaning heart lipid—cardiac cells have a large number of mitochondria). The inner membrane of the mitochondrion is highly folded into cristae, so that it has an interleaved appearance in the electron microscope. 

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