Electron transport chain, membrane-bound

This is a crucial point because (as we will see) proton transport is coupled with ATP synthesis. Oxidation of one FADHg in the electron transport chain results in synthesis of approximately two molecules of ATP, compared with the approximately three ATPs produced by the oxidation of one NADH. Other enzymes can also supply electrons to UQ, including mitochondrial 5w-glyc-erophosphate dehydrogenase, an inner membrane-bound shuttle enzyme, and the fatty acyl-CoA dehydrogenases, three soluble matrix enzymes involved in fatty acid oxidation (Figure 21.7 also see Chapter 24). The path of electrons from succinate to UQ is shown in Figure 21.8. 

Recent work has shown that bacteria, in common with chloroplasts and mitochondria, are able, through the membrane-bound electron transport chain aerobically, or the membrane-bound adenosine triphosphate (ATP) anerobically, to maintain a gradient of electrical potential and pH such that the interior of the bacterial cell is negahve and alkaline. This potential gradient and the electrical equivalent of the pH difference (1 pH unit = 58 mV at 37°C) give a potential difference across the membrane of 100-180 mV, with the inside negative. The membrane is impermeable to protons, whose extmsion creates the potential described. 

A second group of electron carriers in mitochondrial membranes are the iron-sulfur [Fe-S] clusters which are also bound to proteins. Iron-sulfur proteins release Fe3+ or Fe2+ plus H2S when acidified. The "inorganic clusters" bound into the proteins have characteristic compositions such as Fe2S2 and Fe4S4. The sulfur atoms of the clusters can be regarded as sulfide ions bound to the iron ions. The iron atoms are also attached to other sulfur atoms from cysteine side chains from the proteins. The Fe-S proteins are often tightly associated with other components of the electron transport chain. For example, the flavoproteins Flavin 1, Flavin 2, and Flavin 3 shown in Fig. 10-5 all contain Fe-S clusters as does the Q-cytochrome b complex. All of these Fe-S clusters seem to be one-electron carriers. 

Cytochromes b, a, and o. Protoheme-containing cytochromes b are widely distributed.127,128 There are at least five of them in E. coli. Whether in bacteria, mitochondria, or chloroplasts, the cytochromes b function within electron transport chains, often gathering electrons from dehydrogenases and passing them on to c-type cytochromes or to iron-sulfur proteins. Most cytochromes b are bound to or embedded within membranes of bacteria, mitochondria, chloroplasts, or endoplasmic reticulum (ER). For example, cyto-... 

Functions of iron-sulfur enzymes. Numerous iron-sulfur clusters are present within the membrane-bound electron transport chains discussed in Chapter 18. Of special interest is the Fe2S2 cluster present in a protein isolated from the cytochrome be complex (complex III) of mitochondria. First purified by Rieske et al.,307 this protein is often called the Rieske iron-sulfur protein 308 Similar proteins are found in cytochrome be complexes of chloroplasts.125 300 309 310 In... 

Formate dehydrogenases from many bacteria contain molybdopterin and also often selenium (Table 15-4).664/665 A membrane-bound Mo-containing formate dehydrogenase is produced by E. coli grown anaerobically in the presence of nitrate. Under these circumstances it is coupled to nitrate reductase via an electron-transport chain in the membranes which permits oxidation of formate by nitrate (Eq. 18-26). This enzyme is also a multisubunit protein.665 666 Two other Mo- and Se- containing formate dehydrogenases are produced... 

The b cytochromes and cytochrome c, fit into this scheme between reducing substrates and cytochrome c. The idea thus developed that the respiratory apparatus includes a chain of cytochromes that operate in a defined sequence. The next question was whether the cytochromes are all bound together in a giant complex, or whether they diffuse independently in the membrane. Before we address this point, we need to consider three other types of electron carriers that participate in the electron-transport chain flavo-proteins, iron-sulfur proteins, and ubiquinone. 

Fig. 5.3. The major components involved in mitochondrial NADH oxidation in facultative anaerobic mitochondria. In anaerobically functioning mitochondria, NADH is oxidized either by soluble enzymes (left) or by membrane-bound complexes of the electron-transport chain (middle). Under aerobic conditions, a classic respiratory chain is used to oxidize NADH (right). Proton translocation is indicated by H with arrows. Ovals represent the electron transporters RQ, UQ and cytochrome c (cyt. c), and electron transport is indicated by dashed arrows. The vertical bar represents a scale for the standard redox potentials in millivolts. Fum fumarate, NADH-DH NADH dehydrogenase, NADH-ECR soluble NADH enoyl-CoA reductase, RQH2 rhodoquinol, Succ succinate, UQH2 ubiquinol...

Hexachlorophane Inhibition of membrane-bound electron transport chain... 

Membrane-bound Electron Transport Chain Enzymes and Proteins... 

Spectroscopic and crystallographic studies have identified four Fe S clusters in the membrane-bound photosynthetic electron transport chain of plant and cyanobacterial chloro-plasts. One is the Rieske-type [2Fe-2S] + + center in the cyt b(,f complex, which catalyzes electron transfer from plasto-quinol to plastocyanin with concomitant proton translocation, and is functionally analogous to the cyt bc complex, with cyt / in place of cyt The remainder are low-potential [4Fe 4S] + + centers in Photosystem I which constitute the terminal part of the electron transfer chain that is initiated by the primary donor chlorophyll. One is a very low-potential [4Fe S] + + center, Fx (Em =-705 mV), that bridges two similar subunits (PsaA and PsaB) and is coordinated by two cysteines from each subunit in a C-Xg-C arrangement. This cluster transfers electrons to the 2Fe-Fd acceptor via an electron transfer chain composed of Fa, a [4Fe S] + + cluster with Em = -530 mV, and Fb, a [4Fe S] + + clusters with Em = -580 mV. Fa and Fb are in a low-molecular weight subunit (PsaC, 9 kDa) that shows strong sequence and structural homology with bacterial 8Fe-Fds. The center-to-center distance between Fx and Fa and between Fa and Fb are 14.9 A and 12.3 A, respectively, well... 

C554 is proposed to bind, and may thus be the electron exit heme. Cytochrome C554 also has two coplanar diheme pairs, which may indicate that it can also accept two electrons simultaneously. This cytochrome then transfers electrons to the membrane-bound tetraheme cytochrome Cm552 (see Section 4), which is a good candidate to reduce the membrane ubiquinone pool, from where electrons are partitioned between the ammonia monooxygenase reaction, the aerobic respiratory chain, and reverse electron transport. ... 

Figure 18.37. Glycerol 3-Phosphate Shuttle. Electrons from NADH can enter the mitochondrial electron transport chain by being used to reduce dihydroxyacetone phosphate to glycerol 3-phosphate. Glycerol 3-phosphate is reoxidized by electron transfer to an FAD prosthetic group in a membrane-bound glycerol 3-phosphate dehydrogenase. Subsequent electron transfer to Q to form QH2 allows these electrons to enter the electron-transport chain.

The findings clearly indicate that the last step of the CO2 reduction pathway, the reduction of CoM-S-S-HTP by H2, is a coupling site for ATP synthesis. It is concluded that CoM-S-S-HTP is the terminal electron acceptor ( = —200 mV) of a membrane-bound electron transport chain, with molecular H2 ( ° = -414mV) being the electron donor in hydrogenotrophic methanogens [115]. The physiological electron donor for CoM-S-S-HTP reduction is not known (Fig. 5). 

Fig. 5. Proposed mechanism of ATP synthesis coupled to methyl-coenzyme M (CH3-S-C0M) reduction to CH4 The reduction of the heterodisulfide (CoM-S-S-HTP) as a site for primary translocation. ATP is synthesized via membrane-bound -translocating ATP synthase. CoM-S-S-HTP, heterodisulfide of coenzyme M (H-S-CoM) and 7-mercaptoheptanoylthreonine phosphate (H-S-HTP) numbers in circles, membrane-associated enzymes (1) CH3-S-C0M reductase (2) dehydrogenase (3) heterodisulfide reductase 2[H] can be either H2, reduced coenzymeF420 F420H2) or carbon monoxide the hatched box indicates an electron transport chain catalyzing primary translocation the stoichiometry of translocation (2H /2e , determined in everted vesicles) was taken from ref. [117] z is the unknown If /ATP stoichiometry A/iH, transmembrane electrochemical...

Fig. 7. Proposed function of electrochemical H and Na" potentials in energy conservation coupled to CH4 formation from CO2/H2. The Na+/H antiporter is involved in the generation of from A/iNa ". CHO-MFR, formyl-methanofuran CH2=H4MPT, methylene-tetrahydromethanopterin CH3-H4MPT, methyl-tetrahydromethanopterin CH3-S-C0M, methyl-coenzyme M. The hatched boxes indicate membrane-bound electron transport chains or membrane-bound methyltransferase catalyzing either Na or translocation (see Figs. 5, 6 and 12). ATP is synthesized via membrane-bound H -translocating ATP synthase. The stoichiometries of Na" and translocation were taken from refs. [105,107,167]. x, y and z are unknown stoichiometric factors.

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