Mitochondrial membranes space

In addition to cytochrome c, other factors such as apoptosis inducing factor (AIF), endonuclease G, and second mitochondria-derived activator of caspases (Smac/DIABLO) may also be released from the inter-mitochondrial membrane space to exert their pro-apoptotic effects (see later). Recently, the endoplasmic reticulum (ER) has been identified as another organelle that can initiate the intrinsic apoptotic cascade in response to cellular stress. The ER is essential for proper... 

The space inside the inner mitochondrial membrane is called the matrix, and it contains most of the enzymes of the TCA cycle and fatty acid oxidation. (An important exception, succinate dehydrogenase of the TCA cycle, is located in the inner membrane itself.) In addition, mitochondria contain circular DNA molecules, along with ribosomes and the enzymes required to synthesize proteins coded within the mitochondrial genome. Although some of the mitochondrial proteins are made this way, most are encoded by nuclear DNA and synthesized by cytosolic ribosomes. 

Cytochrome c, like UQ is a mobile electron carrier. It associates loosely with the inner mitochondrial membrane (in the intermembrane space on the cytosolic side of the inner membrane) to acquire electrons from the Fe-S-cyt C aggregate of Complex 111, and then it migrates along the membrane surface in the reduced state, carrying electrons to cytochrome c oxidase, the fourth complex of the electron transport chain. 

Most of the NADH used in electron transport is produced in the mitochondrial matrix space, an appropriate site because NADH is oxidized by Complex I on the matrix side of the inner membrane. Furthermore, the inner mitochondrial membrane is impermeable to NADH. Recall, however, that NADH is produced in glycolysis by glyceraldehyde-3-P dehydrogenase in the cytosol. If this NADH were not oxidized to regenerate NAD, the glycolytic pathway would cease to function due to NAD limitation. Eukaryotic cells have a number of shuttle systems that harvest the electrons of cytosolic NADH for delivery to mitochondria without actually transporting NADH across the inner membrane (Figures 21.33 and 21.34). 

The mechanism of ATP synthesis discussed here assumes that protons extruded during electron transport are in the bulk phase surrounding the inner mitochondrial membrane (intermembrane and extramitochondrial spaces). An alternative view is that there are local proton circuits within or close to the respiratory chain and complex V, and that these protons may not be in free equilibrium with the bulk phase (Williams, 1978), although this has not been supported experimentally (for references see Nicholls and Ferguson, 1992). The chemiosmotic mechanism is both elegant and simple and explains all the known facts about ATP synthesis and its dependence on the structural integrity of the mitochondria, although the details may appear complex. This mechanism will now be discussed in more detail. 

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... 

The details of how preproteins are translocated have not been fully elucidated. It is possible that the electric potential associated with the inner mitochondrial membrane causes a conformational change in the unfolded preprotein being translocated and that this helps to puU it across. Furthermore, the fact that the matrix is more negative than the intermembrane space may attract the positively charged amino terminal of the preprotein... 

The above describes the major pathway of proteins destined for the mitochondrial matrix. However, certain proteins insert into the outer mitochoiidrial membrane facilitated by the TOM complex. Others stop in the intermembrane space, and some insert into the inner membrane. Yet others proceed into the matrix and then return to the inner membrane or intermembrane space. A number of proteins contain two signaling sequences—one to enter the mitochondrial matrix and the other to mediate subsequent relocation (eg, into the inner membrane). Certain mitochondrial proteins do not contain presequences (eg, cytochrome Cy which locates in the inter membrane space), and others contain internal presequences. Overall, proteins employ a variety of mechanisms and routes to attain their final destinations in mitochondria. 

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.

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].)... 

In addition to forming close contacts with the PM, the SR network also comes into close contact with the mitochondria (Nixon et al 1994, Rizzuto et al 1998), forming yet another diffusionally restricted space (Fig. 4, panel E-G). This space, approximately 60—80 nm wide and sandwiched between the SR and mitochondrial membranes, also appears to be functionally important. As the SR network penetrates deeper into the cell, it inserts into the nuclear membrane such that the lumen of the perinuclear SR network is continuous with the lumen of the nuclear envelope (Somlyo 1985). 

We now turn our attention to how the gradient of protons pumped by Complexes I, III and IV across the inner mitochondrial membrane into the intermembrane space, together with the associated membrane potential, is used to turn the molecular rotor that ensures... 

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...

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 tricarboxylic acid cycle not only takes up acetyl CoA from fatty acid degradation, but also supplies the material for the biosynthesis of fatty acids and isoprenoids. Acetyl CoA, which is formed in the matrix space of mitochondria by pyruvate dehydrogenase (see p. 134), is not capable of passing through the inner mitochondrial membrane. The acetyl residue is therefore condensed with oxaloacetate by mitochondrial citrate synthase to form citrate. This then leaves the mitochondria by antiport with malate (right see p. 212). In the cytoplasm, it is cleaved again by ATP-dependent citrate lyase [4] into acetyl-CoA and oxaloacetate. The oxaloacetate formed is reduced by a cytoplasmic malate dehydrogenase to malate [2], which then returns to the mitochondrion via the antiport already mentioned. Alternatively, the malate can be oxidized by malic enzyme" [5], with decarboxylation, to pyruvate. The NADPH+H formed in this process is also used for fatty acid biosynthesis. 

In the respiratory chain (see p. 140), electrons are transferred from NADH or ubiquinol (QH2) to O2. The energy obtained in this process is used to establish a proton gradient across the inner mitochondrial membrane. ATP synthesis is ultimately coupled to the return of protons from the intermembrane space into the matrix. 

Mitochondria are bacteria-sized organelles (about 1 X 2 im in size), which are found in large numbers in almost all eukaryotic cells. Typically, there are about 2000 mitochondria per cell, representing around 25% of the cell volume. Mitochondria are enclosed by two membranes—a smooth outer membrane and a markedly folded or tubular inner mitochondrial membrane, which has a large surface and encloses the matrix space. The folds of the inner membrane are known as cristae, and tube-like protrusions are called tubules. The intermembrane space is located between the inner and the outer membranes. 

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). 

Oxidizible substrates from glycolysis, fatty acid or protein catabolism enter the mitochondrion in the form of acetyl-CoA, or as other intermediaries of the Krebs cycle, which resides within the mitochondrial matrix. Reducing equivalents in the form of NADH and FADH pass electrons to complex I (NADH-ubiquinone oxidore-ductase) or complex II (succinate dehydrogenase) of the electron transport chain, respectively. Electrons pass from complex I and II to complex III (ubiquinol-cyto-chrome c oxidoreductase) and then to complex IV (cytochrome c oxidase) which accumulates four electrons and then tetravalently reduces O2 to water. Protons are pumped into the inner membrane space at complexes I, II and IV and then diffuse down their concentration gradient through complex V (FoFi-ATPase), where their potential energy is captured in the form of ATP. In this way, ATP formation is coupled to electron transport and the formation of water, a process termed oxidative phosphorylation (OXPHOS). 

A. As electrons pass through complexes I, III, and IV (but not complex II), protons are transported across the inner mitochondrial membrane from the matrix to the intermembrane space, creating a pH gradient that represents a form of stored energy. 

In the third and final step of the carnitine shuttle, the fatty acyl group is enzymatically transferred from carnitine to intramitochondrial coenzyme A by carnitine acyltransferase II. This isozyme, located on the inner face of the inner mitochondrial membrane, regenerates fatty acyl-CoA and releases it, along with free carnitine, into the matrix (Fig. 17-6). Carnitine reenters the intermembrane space via the acyl-camitine/car-nitine transporter. 

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