Respiratory chain location

Many prokaryotic organisms such as Escherichia coli have a simplified respiratory chain located in the inner cell membrane (Fig. IB). The E. coli respiratory chain performs a function similar to as its mitochondrial counterpart but lacks Complex III. Instead, electrons are directly transferred from the ubiquinol molecule to Complex IV (ubiquinol oxidase in this case). 

Nagamine et al. used SECM to study S. aureus trapped in nanoliter collagen-gel matrixes localized on a chip. In the presence of Fe(CN)/ , the respiratory chain located within the cytoplasmic membrane of S. aureus cells produces FelCNjg, which can be monitored by the SECM tip. The authors found that the ferricyanide reduction activity in the S. aureus, an osmotolerant bacteria, is enhanced in response to high osmotic stress such as the presence of highly concentrated salts. 

Table 1 shows the effect of several inhibitors. The major result of these inhibitor studies is that an inhibition of proton efflux is possible without impairing respiratory electron transport. This can also be demonstrated advantageously by the lowered H /e ratios given (in percent of control) in the last column of Table 1. These results are in accordance with the hypothesis that an ATP-consuming cytoplasmic-membrane ATPase mediates the proton efflux and contradicts the model of a respiratory chain located on the plasma membrane.

NAD+ and NADP+ are coenzymes of dehydrogenases. NADH and NADPH are intermediate carriers of both hydrogen and electrons. Most NAD-dependent enzymes are located in the mitochondria and deliver H2 to the respiratory chain whereas NADP-dependent enzymes take part in cytosolic syntheses (reductive biosyntheses). 

Mitochondria have their own DNA (mtDNA) and genetic continuity. This DNA only encodes 13 peptide subunits synthesized in the matrix that are components of complexes I, III, IV, and V of the respiratory chain. Most mitochondrial proteins are synthesized on cytoplasmic ribosomes and imported by specific mechanisms to their specific locations in the mitochondrion (see below). 

The cytochromes are iron-containing hemoproteins in which the iron atom oscillates between Fe + and Fe + during oxidation and reduction. Except for cytochrome oxidase (previously described), they are classified as dehydrogenases. In the respiratory chain, they are involved as carriers of electrons from flavoproteins on the one hand to cytochrome oxidase on the other (Figure 12-4). Several identifiable cytochromes occur in the respiratory chain, ie, cytochromes b, Cp c, a, and (cytochrome oxidase). Cytochromes are also found in other locations, eg, the endoplasmic reticulum (cytochromes P450 and h, and in plant cells, bacteria, and yeasts. 

The reduced coenzymes are oxidized by the respiratory chain linked to formation of ATP. Thus, the cycle is the major route for the generation of ATP and is located in the matrix of mitochondria adjacent to the enzymes of the respiratory chain and oxidative phosphorylation. 

Mitochondrial DNA is inherited maternally. What makes mitochondrial diseases particularly interesting from a genetic point of view is that the mitochondrion has its own DNA (mtDNA) and its own transcription and translation processes. The mtDNA encodes only 13 polypeptides nuclear DNA (nDNA) controls the synthesis of 90-95% of all mitochondrial proteins. All known mito-chondrially encoded polypeptides are located in the inner mitochondrial membrane as subunits of the respiratory chain complexes (Fig. 42-3), including seven subunits of complex I the apoprotein of cytochrome b the three larger subunits of cytochrome c oxidase, also termed complex IV and two subunits of ATPase, also termed complex V. 

Electron transfer (ET) is a key reaction in biological processes such as photosynthesis and respiration [1], Photosynthetic and respiratory chain redox proteins contain one or more redox-active prosthetic groups, which may be metal complexes or organic species. Since it is known from crystal structure analyses that the prosthetic groups often are located in the protein interior, it is likely that ET in protein-protein complexes will occur over large molecular distances ( > 10 A) [2-4],... 

Mitochondria are also described as being the cell s biochemical powerhouse, since—through oxidative phosphorylation (see p. 112)—they produce the majority of cellular ATP. Pyruvate dehydrogenase (PDH), the tricarboxylic acid cycle, p-oxidation of fatty acids, and parts of the urea cycle are located in the matrix. The respiratory chain, ATP synthesis, and enzymes involved in heme biosynthesis (see p. 192) are associated with the inner membrane. 

The reactive thiol/thioketene produced by the (3-lyase is an alkylating fragment, which binds to protein, DNA, and GSH. The fact that one of the locations of C-S lyase is in mitochondria may explain why this organelle seems to be damaged. Damage to the respiratory chain will lead to depletion and a shortage of ATP, which is vitally necessary for the activity of the kidney in terms of active uptake and secretion. 

This hypothesis presumes that early free-living prokaryotes had the enzymatic machinery for oxidative phosphorylation and predicts that their modern prokaryotic descendants must have respiratory chains closely similar to those of modern eukaryotes. They do. Aerobic bacteria carry out NAD-linked electron transfer from substrates to 02, coupled to the phosphorylation of cytosolic ADP. The dehydrogenases are located in the bacterial cytosol and the respiratory chain in the plasma membrane. The electron carriers are similar to some mitochondrial electron carriers (Fig. 19-33). They translocate protons outward across the plasma membrane as electrons are transferred to 02. Bacteria such as Escherichia coli have F0Fi complexes in their plasma membranes the F portion protrudes into the cytosol and catalyzes ATP synthesis from ADP and P, as protons flow back into the cell through the proton channel of F0. 

Figure 18-19 The ammonia oxidation system of the bacterium Nitrosomonas. Oxidation of ammonium ion (as free NH3) according to Eq. 18-17 is catalyzed hy two enzymes. The location of ammonia monooxygenase (step a) is uncertain but hydroxylamine oxidoreductase (step b) is periplas-mic. The membrane components resemble complexes I, III, and IV of the mitochondrial respiratory chain (Fig. 18-5) and are assumed to have similar proton pumps. Solid green lines trace the flow of electrons in the energy-producing reactions. This includes flow of electrons to the ammonia monoxygenase. Complexes HI and IV pump protons out but complex I catalyzes reverse electron transport for a fraction of the electrons from hydroxylamine oxidoreductase to NAD+. Modified from Blaut and Gottschalk.315...

Oxidative phosphorylation is ATP synthesis linked to the oxidation of NADH and FADH2 by electron transport through the respiratory chain. This occurs via a mechanism originally proposed as the chemiosmotic hypothesis. Energy liberated by electron transport is used to pump H+ ions out of the mitochondrion to create an electrochemical proton (H+) gradient. The protons flow back into the mitochondrion through the ATP synthase located in the inner mitochondrial membrane, and this drives ATP synthesis. Approximately three ATP molecules are synthesized per NADH oxidized and approximately two ATPs are synthesized per FADH2 oxidized. 

Oxidative phosphorylation is the name given to the synthesis of ATP (phosphorylation) that occurs when NADH and FADH2 are oxidized (hence oxidative) by electron transport through the respiratory chain. Unlike substrate level phosphorylation (see Topics J3 and LI), it does not involve phosphorylated chemical intermediates. Rather, a very different mechanism was proposed by Peter Mitchell in 1961, the chemiosmotic hypothesis. This proposes that energy liberated by electron transport is used to create a proton gradient across the mitochondrial inner membrane and that it is this that is used to drive ATP synthesis. Thus the proton gradient couples electron transport and ATP synthesis, not a chemical intermediate. The evidence is overwhelming that this is indeed the way that oxidative phosphorylation works. The actual synthesis of ATP is carried out by an enzyme called ATP synthase located in the inner mitochondrial membrane (Fig. 3). 

The energy saved in the electron carriers NADH and FADH2 is transferred into the respiratory chain. The enzyme complexes of the respiratory chain are physically located in the inner membrane (Figure 17.2). The unique characteristic (impermeability to almost all molecules) of the inner membrane is essential for this reaction as described below. The reaction in the respiratory chain is very simple. 

As implied by the above description, cytochrome oxidase is the terminal member of the so-called respiratory chain and functionally located at the confluence of the electron current... 

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