Electron transport chain, membrane-bound enzymes

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. 

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... 

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...

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

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. 11. Proposed function of electrochemical and Na potentials in energy conservation coupled to acetate fermentation to CH4 and CO2. The Na /H antiporter is involved in the generation of A/iH from A/iNa. CH3CO-S-C0A, acetyl-coenzyme A [CO], CO bound to carbon monoxide dehydrogenase CH3-H4MPT, methyl-tetrahydromethanopterin CH3-S-C0M, methyl-coenzyme M. The hatched boxes indicate membrane-bound electron transport chains or membrane-bound methyl-transferase catalyzing either IT or Na translocation (see Figs. 5, 6 and 12). It is assumed that enzyme-bound [CO] is energetically equal to free CO. ATP is synthesized via membrane-bound H -translocating ATP synthase. The stoichiometries of translocation were taken from refs. [107,234] n, X, y and z are unknown stoichiometric factors.

The membrane-bound PPase does not only hydrolyze PPj for the maintenance of a pool of high energy, it can also form PPj at the expense of the energy liberated in the electron transport chain [13,14]. This enzyme has been found in both purple nonsulfur [13-17] and sulfur photosynthetic bacteria [18], and in mitochondria of lower [19] and higher [20] heterotrophic organisms, and also in chloroplasts from algae and higher plants [21]. 

Azoles inhibit 14-a-sterol demethylase, a microsomal CYP that is essential for ergosterol biosynthesis (Figure 48-1). This results in the accumulation of 14-a-methylsterols that disrupt the packing of acyl chains of phospholipids and impair the functions of membrane-bound enzymes such as ATPase and those of the electron transport system, resulting in inhibited fungal growth. 

Fig. 20.6. Succinate dehydrogenase contains covalently bound FAD. As a consequence, succinate dehydrogenase and similar flavopro-teins reside in the inner mitochondrial membrane where they can directly transfer elechons into the electron transport chain. The elechons are hansferred from the covalently bound FAD to an Fe-S complex on the enzyme, and then to coenzyme Q in the electron hansport chain (see Chapter 21). Thus, FAD does not have to dissociate from the enzyme to transfer its electrons. All the other enzymes of the TCA cycle are found in the mitochondrial mahix.

Like the FAD in all flavoproteins, FAD(2H) bound to the acyl CoA dehydrogenases is oxidized back to FAD without dissociating from the protein (Fig. 23.8). Electron transfer flavoproteins (RTF) in the mitochondrial matrix accept electrons from the enzyme-bound FAD(2H) and transfer these electrons to ETF-QO (electron transfer flavoprotein -CoQ oxidoreductase) in the inner mitochondrial membrane. ETF-QO, also a flavoprotein, transfers the electrons to CoQ in the electron transport chain. Oxidative phosphorylation thus generates approximately 1.5 ATP for each FAD(2H) produced in the (3-oxidation spiral. 

The key point here is not the active site, which has a low tolerance for mutations, but the molecules with which the proteins in question are associated. Cytochromes are membrane-bound and must associate with other members of the electron transport chain most mutations are likely to interfere with the close fit, and thus they are not preserved (because they are lethal). Globins, although soluble, still form some associations, so more mutations can be tolerated, with some limits. Hydrolytic enzymes are soluble and not likely to associate with other polypeptides except substrates. They can tolerate a higher proportion of mutations. 

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). 

FIGURE 9.1. Sequence of components of the mitochondrial electron transport chain. NADH-Q reductase spans the mitochondrial membrane with the Q site within the membrane and the NADH site on the matrix side of the membrane. Succinate-Q reductase has a Q site within the membrane and a succinate site on the matrix side of the membrane. Cytochrome c reductase is a membrane-bound enzyme with cytochrome c on the cystolic side of the membrane and cytochrome b in the membrane. Cytochrome c is soluble and found on the cystolic side of the membrane, while cytochrome oxidase translocates protons or electrons across the membrane. (Adapted from Ref. 3.)... 

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