Inter-membrane space

Cytochrome c 12.3 kDa 1 C type heme Inter membrane space, loosely associated with inner membrane 0.8-1.02 ... 

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 18.6 Energetics of the ORR at the heme/Cu site of CcO the enzyme couples oxidation of ferroc3ftochrome c (standard potential about —250 mV all potentials are listed with respect to a normal hydrogen electrode) to reduction of O2 (standard potential at pH 7 800 mV). Of the 550 mV difference, only 100 mV is dissipated to drive the reaction 220 mV is expanded to translocate four protons from the basic matrix compartment to the acidic IMS (inter-membrane space). In addition 200 mV is converted into transmembrane electrostatic potential as ferroc3ftochrome is oxidized in the IMS, but the charge-compensating protons are taken from the matrix. The potentials are approximate.

Inhibition of ATP synthase (energy transfer) reduces proton flow from the inter-membrane space to the matrix, which inhibits electron flow in the respiratory chain. Oligomycin, a macrolide antibiotic, prevents phosphoryl group transfer of ATP synthase. Dicyclohexylcarbodimide (DCCD) binds to and inhibits ATP synthase. Similar to the inhibitors of Complexes I, III, and IV, energy transfer inhibitors cause accumulation of reactive electrons and generate ROS. 

Flavocytochromes 2 2-hydroxyacid dehydrogenases found in the inter-membrane space of yeast mitochondria where they couple oxidation of the substrate to reduction of cytochrome c. Examples include the enzymes from Saccharomyces cerevisiae and Hansenula anomala, both of which are l-lactate dehydrogenases (Chapman et al., 1998), and the enzyme from Rhodotorula graminis which is a L-mandelate dehydrogenase (Ilias et al., 1998). This article will concentrate on the flavocytochrome 2 (L-lactate cytochrome c oxidoreductase) from S. cerevisiae (Bakersi yeast), since this is by far the most studied of these enzymes (Chapman et al., 1991). Therefore, throughout this article, the term flavocytochrome 2 will refer specifically to the enzyme from S. cerevisiae unless otherwise stated. 

Cytochrome Cj contains a c-type haem as prosthetic group in its wedge-shaped N-terminal domain located in the inter-membrane space. This extrinsic domain is anchored to membrane by a transmembrane helix at the C-terminal end (residues 204 to 222 in bovine bc ). This helix runs alongside cytochrome b and can be removed by mild protease treatment or gene truncation to produce a protein fragment (Hase et al., 1987 Li et al., 1981). 

Two structural abnormahties in the mitochondria are considered important pathogenetic factors during ischemia. One is characterized by pore formation in the itmer mitochondrial membrane and high amplitude swelling (mitochondrial permeability transition or MPT) [30, 31]. The second involves leakage of cytochrome C from the inter-membrane space into the cytosol [32]. Because of its role as an electron shuttle, dislocation of cytochrome c compromises respiration [33, 34], and as a cytosolic cofactor cytochrome C activates caspase 9, and triggers apoptosis [33-35] (see below). 

Figure 18,11 Structure of Q-cytochrome c ox i do reductase (cytochrome bc. This enzyme is a homodimer with 11 distinct polypeptide chains. Notice that the major prosthetic groups, three hemes and a 2Fe-2S cluster, are located either near the cytoplasmic edge of the complex bordering the inter membrane space (top) or in the region embedded in the membrane (tx helices represented by vertical tubes). They are well positioned to mediate the electron-transfer reactions between quinones in the membrane and cytochrome c in the intermembrane space. [Drawn from IBCC.pdb.J...

Some mitochondrial proteins destined for the inter-membrane space or inner membrane are first imported into the matrix and then redirected others never enter the matrix but go directly to their final location. 

Flavocytochrome hi, the L(+)-lactate cytochrome c oxidoreductase, is a tetrameric enzyme (Mr 235,000) containing one FMN and one protoheme IX per protomer. The enzyme, located in the mitochondrial inter-membrane space, is able to bind one cytochrome c per protomer. Tlie complex is stable at ionic strengths equal to or lowers than 40 mM. 

Both ornithine, which is a homolog of lysine, and citrulline are L-amino acids, but neither has a genetic codon, and both are found only as posttranslational modifications of arginine residues in some proteins such as keratin. Citrulline leaves the mitochondria by the same transport system that facilitates the entry of ornithine from the cytoplasm inter-membrane space. 

Basically, all mitochondria consist of two membranes which surround and enclose an inner compartment containing the mitochondrial matrix. The outer membrane is usually smooth in surface contour, whereas the inner membrane is folded into a series of lamellae, the cristae. The regions of the inner membrane between the cristae are designated collectively as the inner boundary membrane. The space between the outer and inner membranes, known as the inter membrane space, is continuous with the space bounded by the membranes of the cristae. Figure 1 shows a schematic drawing of a mitochondrion. 

Olichon, A., Emorine, L. J., Descoins, E., PeUoquin, L., Brichese, L., Gas, N., Guillou, E., Delettre, C., Valette, A., Hamel, C. P., Ducommun, B., Lenaers, G., and Belenguer, P. (2002). The human dynamin-related protein OPAI is anchored to the mitochondrial inner membrane facing the inter-membrane space. FEBS Lett. 523, 171-176. 

As mentioned above, mitochondria are major cellular sources of oxyradicals. Superoxide anion radical (O ), generated upon autoxidation of ubisemiquinone, is vectorially released into the inter membrane space and the mitochondrial matrix. In the latter compartment, Oj dismutates to HjOj. 

The inner mitochondrial membrane forms the cristae, which are paddle-shaped, double-membrane structures that protrude from the inner membrane into the matrix, as shown in Figure 3.16. The crista membrane is continuous with the inner mitochondrial membrane, and the internal space of the crista is contiguous with the inter-membrane space. However, there is only a relatively narrow stalk connecting the crista to the inter-membrane space, so that the crista space is effectively separate from, albeit communicating with, the inter-membrane space. 

The inter-membrane space contains enzymes involved in nucleotide metabolism, transamination of amino acids (section 9.3.1.2) and a variety of kinases. 

On the outer face of the outer mitochondrial membrane, the fatty acid is transferred from CoA onto carnitine, forming acylcarnitine, which enters the inter-membrane space through an acylcarnitine transporter (Figure 5.22). The structures of CoA and carnitine are shown in Figure 5.23. 

Acylcarnitine can cross only the inner mitochondrial membrane on a countertransport system that takes in acylcarnitine in exchange for free carnitine being returned to the inter-membrane space. Once inside the mitochondrial inner membrane, acylcarnitine transfers the acyl group onto CoA ready to undergo -oxidation. This counter-transport system provides regulation of the uptake of fatty acids into the mitochondrion for oxidation. As long as there is free CoA available in the mitochondrial matrix, fatty acids can be taken up and the carnitine returned to the outer membrane for uptake of more fatty acids. However, if most of the CoA in the mitochondrion is acylated, then there is no need for further fatty uptake immediately and, indeed, it is not possible. 

---