Space, intermembrane matrix

FIGURE 21.11 The structure of UQ-cyt c reductase, also known as the cytochrome hci complex. The alpha helices of cytochrome b (pale green) define the transmembrane domain of the protein. The bottom of the structure as shown extends approximately 75 A into the mitochondrial matrix, and die top of the structure as shown extends about 38 A into the intermembrane space. (Photograph kindly provided by Di Xia and Johann Deismhofer [From Xia, D., Yn, C.-A., Kim, H., Xia,J-Z., Kachnrin, A. M., Zhang, L., Yn,... 

Complex IV consists of 13 peptides, two heme A groups (cytochrome a and a3> and two or three Cu atoms (Table 2). It spans the inner membrane and protrudes into the intermembrane space. Complex IV catalyzes the reduction of dioxygen by oxidized cytochrome c, and four protons derived from the matrix are consumed in the reaction. 

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... 

Mitochondria have an outer membrane that is permeable to most metabohtes, an inner membrane that is selectively permeable, and a matrix within (Figure 12-1). The outer membrane is characterized by the presence of various enzymes, including acyl-CoA synthetase and glycerolphosphate acyltransferase. Adenylyl kinase and creatine kinase are found in the intermembrane space. The phospholipid cardiolipin is concentrated in the inner membrane together with the enzymes of the respiratory chain. 

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. 

Adenylate kinase (AK) is a ubiquitous monomeric enzyme that catalyzes the interconversion of AMP, ADP, and ATP. This interconversion of the adenine nucleotides seems to be of particular importance in regulating the equilibrium of adenine nucleotides in tissues, especially in red blood cells. AK has three isozymes (AK 1,2, and 3). AK 1 is present in the cytosol of skeletal muscle, brain, and red blood cells, and AK 2 is found in the intermembrane space of mitochondria of liver, kidney, spleen, and heart. AK 3, also called GTP AMP phosphotransferase, exists in the mitochondrial matrix of liver and heart. 

Figure 2.8 The structure of the dimeric cytochrome bcomplex of the respiratory chain, (a) The cave for chemistry constituted by the hollow between the two monomers (the essential dimer ) in a cartoon representation. Reprinted with permission from Smith, 1998. Copyright (1998), American Association for the Advancement of Science, (b) The structure viewed perpendicular to the twofold axis and parallel to the membrane. All of the eleven subunits are completely traced and their sequences assigned. The top of the molecule extends 3.8 nm into the intermembrane space, the middle spans the membrane (4.2 nm), and the bottom extends some 7.5 nm into the matrix. Reprinted with permission from Iwata et al., 1998. Copyright (1998) American Association for the Advancement of 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].)... 

Mitochondria (45-56) are organelles possessing a double membrane, the inner of which is invaginated as cristae. An intermembrane space exists between the inner and outer membranes. The inner membrane consists of an unusually high amount of protein and possesses spherically shaped particles approx 9 nm in diameter. These particles appear to be equivalent to F0, Fb and adenosine triphosphatase. In contrast to the inner membrane, the outer membrane is smooth and appears to be connected to the smooth er. This membrane is permeable to all molecules of 10,000 Dalton or less. A mitochondrial matrix is enclosed by the inner membrane and consists of a ground substance of particles, nucleoids, ribosomes, and electron-transparent regions containing DNA. 

Most of the mitochondrial proteins are nuclear encoded and thus must be targeted into mitochondria and sorted into some of their components after their synthesis at the cytosol. Because mitochondria have two membranes, there are four localization sites the matrix, the inner membrane, the intermembrane space, and the outer membrane (Fig. 6). Although there has been considerable progress in our understanding of these processes, some questions still remain. Moreover, the total picture is rather complicated and contains many exceptions. A simplified view is presented here based mainly on the view of Pfanner and Mihara (Mihara and Omura, 1996 Pfanner et al., 1997 Pfanner, 1998). There are also a number of other excellent reviews on this subject (Schatz, 1996 Stuart and Neupert, 1996 Neupert, 1997 Roise, 1997). 

Bovine heart cytochrome bci (PDB 1BE3 and PDB IBGY) as studied by Iwata et al. exists as a dimer in the asymmetric unit cell. Each monomer consists of 11 different polypeptide subunits (SU) with a total of -2165 amino acid residues and a molecular mass of -240 kDa. The protein subunits of the complex occupy three separate regions (1) the intermembrane space (p side) occupied by cytochrome Ci (subunit 4, SU4), the iron-sulfur protein (ISP, SU5) and subunit 8 (2) the transmembrane region occupied by cytochrome b (SU3), the transmembrane helices of cytochrome Ci and the ISP, and subunits 7,10, and 11 and (3) the matrix space (n side) occupied by two large core proteins (subunits 1 and 2) as well as subunits 6 and 9. Subunit 8 is often called the hinge protein and is thought to be essential for proper complex formation between cytochrome c (the exit point for some bci complex electrons) and... 

The proton-motive Q-cycle model, put forward by Mitchell (references 80 and 81) and by Trumpower and co-workers, is invoked in the following manner (1) One electron is transferred from ubiquinol (ubiquinol oxidized to ubisemi-quinone see Figure 7.27) to the Rieske [2Fe-2S] center at the Qo site, the site nearest the intermembrane space or p side (2) this electron can leave the bci complex via an attached cytochrome c or be transferred to cytochrome Ci (3) the reactive ubisemiquinone reduces the low-potential heme bL located closer to the membrane s intermembrane (p) side (4) reduced heme bL quickly transfers an electron to high-potential heme bn near the membrane s matrix side and (5) ubiquinone or ubisemiquinone oxidizes the reduced bn at the Qi site nearest the matrix or n side. Proton translocation results from the deprotonation of ubiquinol at the Qo site and protonation of ubisemiquinone at the Qi site. Ubiquinol generated at the Qi site is reoxidized at the Qo site (see Figure 7.27). Additional protons are transported across the membrane from the matrix (see Figure 7.26 illustrating a similar process for cytochrome b(6)f). The overall reaction can be written... 

A third, clearer explanation of the electron transfer, proton translocation cycle is given by Saratse. Each ubiquinol (QH2) molecule can donate two electrons. A hrst QH2 electron is transferred along a high-potential chain to the [2Fe-2S] center of the ISP and then to cytochrome Ci. From the cytochrome Cl site, the electron is delivered to the attached, soluble cytochrome c in the intermembrane space. A second QH2 electron is transferred to the Qi site via the cytochrome b hemes, bL and bn. This is an electrogenic step driven by the potential difference between the two b hemes. This step creates part of the proton-motive force. After two QH2 molecules are oxidized at the Qo site, two electrons have been transferred to the Qi site (where one ubiquinone (Qio) can now be reduced, requiring two protons to be translocated from the matrix space). The net effect is a translocation of two protons for each electron transferred to cytochrome c. Each explanation of the cytochrome bci Q cycle has its merits and its proponents. The reader should consult the literature for updates in this ongoing research area. 

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