Respiratory chains

The respiratory chain is one of the pathways involved in oxidative phosphorylation (see p. 122). It catalyzes the steps by which electrons are transported from NADH+H or reduced ubiquinone (QH2) to molecular oxygen. Due to the wide difference between the redox potentials of the donor (NADH+H or QH2) and the acceptor (O2), this reaction is strongly exergonic (see p. 18). Most of the energy released is used to establish a proton gradient across the inner mitochondrial membrane (see p. 126), which is then ultimately used to synthesize ATP with the help of ATP synthase. 

The electron transport chain consists of three protein complexes (complexes I, III, and IV), 

All of the complexes in the respiratory chain are made up of numerous polypeptides and contain a series of different protein bound redox coenzymes (see pp. 104, 106). These include flavins (FMN or FAD in complexes I and II), iron-sulfur clusters (in I, II, and III), and heme groups (in II, III, and IV). Of the more than 80 polypeptides in the respiratory chain, only 13 are coded by the mitochondrial genome (see p. 210). The remainder are encoded by nuclear genes, and have to be imported into the mitochondria after being synthesized in the cytoplasm (see 

Electrons enter the respiratory chain in various different ways. In the oxidation of NADH+H by complex I, electrons pass via FMN and Fe/S clusters to ubiquinone (Q). Electrons arising during the oxidation of succinate, acyl CoA, and other substrates are passed to ubiquinone by succinate dehydrogenase or other mitochondrial dehydrogenases via en- 

Proton transport via complexes I, III, and IV takes place vectorially from the matrix into the intermembrane space. When electrons are being transported through the respiratory chain, the concentration in this space increases—i. e., the pH value there is reduced by about one pH unit. For each H2O molecule formed, around 10 H ions are pumped into the intermembrane space. If the inner membrane is intact, then generally only ATP synthase (see p. 142) can allow protons to flow back into the matrix. This is the basis for the coupling of electron transport to ATP synthesis, which is important for regulation purposes (see p. 144). 

MODELING NONENZYMATIC SYSTEMS OF ELECTRON TRANSFER IN THE INITIAL PART OF THE RESPIRATORY CHAIN OF MITOCHONDRIA 

The two main processes of providing the organisms with energy— photosynthesis and respiration—are localized in the membranes of chloroplasts, mitochondria, and bacteria.8-10 The energy coming from the environment can be accumulated in organic substances liable to oxidation—substrates—or could be delivered by light quanta. 

The process of the oxidation of substrates is a multistage one and the energy is used by organisms during transfer of electrons along the so-called electron transport chain—a complex of closely bound enzymes inserted in the membrane (Fig. 1). 

When investigating the electron transport processes in mitochondria and chloroplasts and bacteria, it is simplest to assume that the interaction of the chain components follows the mass action law.11-14 That would mean that the free movement of individual elements of the chain in the membrane is possible and also transfer of the charge by accidental collisions. However, transfer of electrons during both respiration and photosynthesis passes along the electron transfer chain organized into definite structural complexes. Consequently a molecule possessing an electron can donate it to a 

A significant difference between the free collisions model and the structural model lies in the fact that the latter predicts a higher reaction order with respect to donor or acceptor concentration under certain conditions. The degree of reduction (or oxidation) of the carrier depends on how far it is located from the beginning of the chain. Using the structural approach, it is possible to find the place of each element in the transport chain by determining the dependence of the rate of the process on the degree of reduction of this element.14,18 

Another important feature of the electron transport chain is that the macromolecular protein complexes responsible for electron transfer along the electron transport chain have a dynamic structure. That was clearly shown, for example, in the investigation of electron transfer in the reaction centers of purple bacteria Rhodospirileum rubrum in oxidizing conditions.  

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L(+)-lactate dehydro-genase lactate pymvate IFMN Iheme yt b[) respiratory chain... 

ATP results from the movement of approximately three protons from the cytosol into the matrix through Fg. Altogether this means that approximately four protons are transported into the matrix per ATP synthesized. Thus, approximately one-fourth of the energy derived from the respiratory chain (electron transport and oxidative phosphorylation) is expended as the electrochemical energy devoted to mitochondrial ATP-ADP transport. 

Walker, J. E., 1992. The NADH ubiquinone oxidoreducta.se (Complex I) of respiratory chains. Quarterly Reviews of Biophysics 25 253-324. 

Wefss, H., Friedrich, T, Hofliaus, G., and Preis, D., 1991. The respiratory-chain NADH dehydrogena.se (Complex I) of mitochondria. European Journal of Biochemistry 197 563—576. 

Figure 5.5 Respiratory chains in A. niger. SHAM = salicyl-hydroxamic acid Fp = flavoprotein -= inhibition x and y are unidentified components.

The thiazolidinediones have also been reported to act as inhibitors of the respiratory chain at high concentrations, and this appears to account for their ability to activate AMGPK in cultured cells. However, the primary target of the thiazolidinediones appears to be the peroxisome proliferator-activated receptor-y ( PPAR-y), a member of the nuclear receptor superfamily expressed in adipocytes. One of the major effects of stimulation of PPAR-y in adipocytes is the release ofthe... 

Atovaquone, a hydroxynaphthoquinone, selectively inhibits the respiratory chain of protozoan mitochondria at the cytochrome bcl complex (complex III) by mimicking the natural substrate, ubiquinone. Inhibition of cytochrome bcl disrupts the mitochondrial electron transfer chain and leads to a breakdown of the mitochondrial membrane potential. Atovaquone is effective against all parasite stages in humans, including the liver stages. 

Mitochondrial permeability transition involves the opening of a larger channel in the inner mitochondrial membrane leading to free radical generation, release of calcium into the cytosol and caspase activation. These alterations in mitochondrial permeability lead eventually to disruption of the respiratory chain and dqDletion of ATP. This in turn leads to release of soluble intramito-chondrial membrane proteins such as cytochrome C and apoptosis-inducing factor, which results in apoptosis. 

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). 

I These dehydrogenases are lim-j ited to the respiratory chain at 7 the level of complex III by ETF (5) dehydrogenase (6) and ubiqui-... 

NADH and reduced substrate dehydrogenase-flavoproteins (FPH2) must be continually reoxidized for mitochondrial oxidations to proceed. This is achieved by the electron transport chain (respiratory chain) which is a series of redox carriers of graded redox potential in the inner mitochondrial membrane (Appendix 1) that catalyzes the net reactions ... 

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. 

Another pathway is the L-glycerol 3-phosphate shuttle (Figure 11). Cytosolic dihydroxyacetone phosphate is reduced by NADFl to s.n-glycerol 3-phosphate, catalyzed by s,n-glycerol 3-phosphate dehydrogenase, and this is then oxidized by s,n-glycerol 3-phosphate ubiquinone oxidoreductase to dihydroxyacetone phosphate, which is a flavoprotein on the outer surface of the inner membrane. By this route electrons enter the respiratory chain.from cytosolic NADH at the level of complex III. Less well defined is the possibility that cytosolic NADH is oxidized by cytochrome bs reductase in the outer mitochondrial membrane and that electrons are transferred via cytochrome b5 in the endoplasmic reticulum to the respiratory chain at the level of cytochrome c (Fischer et al., 1985). 

Normally, these reactive species are destroyed by protective enzymes, such as superoxide dismutase in mitochondria and cytosol and catalase in peroxisomes, but if a tissue has been anoxic the respiratory chain is very reduced and reoxygenation allows dangerous amounts to be formed. Muscle also contains significant quantities of the dipeptide, camosine ((J-alanylhistidine) (10—25 mM). The functions of camosine are obscure although it has been suggested to be an effective antioxidant (Pavlov et al., 1993). 

Mitochondrial DNA is transcribed as a polycistronic RNA which is subsequently cleaved to generate the various mature mRNA, tRNA, and rRNA (Clayton, 1984). The 13 proteins encoded by mtDNA are all components of the respiratory chain and are seven subunits of complex I, one subunit of complex III, three subunits of complex IV, and two subunits of complex V. 

It is conventional to discuss the stoichiometry for proton extrusion as HV2e ratios, although there are two-, one-, and four-electron reductions at different stages in the respiratory chain. Most textbooks still assert that the flow of two electrons... 

Brand, M.D. Murphy, M.P. (1987). Control of electron flux through the respiratory chain in mitochondria and cells. Biol. Rev. 62, 141-193. 

Sumegi, B., Porpaczy, Z., Alkonyi, 1. (1991). Kinetic advantage of the interaction between the fatty acid P-oxidation enzymes and the complexes of the respiratory chain. Biochim. Biophys. Acta 1081, 121-128. 

Metabolic Myopathies Glycogen Storage Disease Disorders of Lipid Metabolism Respiratory Chain Disorders Mitochondrial DNA Abnormalities Myotonias, Periodic Paralyses, and Malignant Hyperpyrexia Myotonias... 

Mitochondria are unique organelles in that they contain their own DNA (mtDNA), which, in addition to ribosomal RN A (rRNA) and transfer RN A (tRNA)-coding sequences, also encodes 13 polypeptides which are components of complexes I, III, IV, and V (Anderson et al., 1981). This fact has important implications for both the genetics and the etiology of the respiratory chain disorders. Since mtDNA is maternally-inherited, a defect of a respiratory complex due to a mtDNA deletion would be expected to show a pattern of maternal transmission. However the situation is complicated by the fact that the majority of the polypeptide subunits of complexes I, III, IV, and V, and all subunits of complex II, are encoded by nuclear DNA. A defect in a nuclear-coded subunit of one of the respiratory complexes would be expected to show classic Mendelian inheritance. A further complication exists in that it is now established that some respiratory chain disorders result from defects of communication between nuclear and mitochondrial genomes (Zeviani et al., 1989). Since many mitochondrial proteins are synthesized in the cytosol and require a sophisticated system of posttranslational processing for transport and assembly, it is apparent that a diversity of genetic errors is to be expected. 

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