Electron-transport assemblies

Observations in chloroplasts played a key role in the development of the chemiosmotic theory of oxidative phosphorylation, which we discussed in chapter 14. Andre Jagendorf and his colleagues discovered that if chloroplasts are illuminated in the absence of ADP, they developed the capacity to form ATP when ADP was added later, after the light was turned off. The amount of ATP synthesized was much greater than the number of electron-transport assemblies in the thylakoid membranes, so the energy to drive the phosphorylation could not have been stored in an energized... 

FADH2 and NADH + H+ are oxidized in the electron-transport assemblies of the mitochondria ... 

Submitochondrial particles (SMPs) are produced by disruption of the mitochondria by mechanical, osmotic, or sonic shock treatment. The fragmentation results in the release of water-soluble components and inner membrane fragments that re-form into vesicles (Figure 14-2). The components are then separated by differential centrifugation. The membranes of SMPs have the characteristic inner membrane spheres on their outside. SMPs are capable of electron transport and oxidative phosphorylation (i.e., the synthesis of ATP from ADP and phosphate). Removal of the inner membrane spheres by further mechanical treatment, urea, or trypsin results in the dissociation of the electron transport assembly from ATP synthesis. The ability to synthesize ATP resides in the overall structure, which includes the spheres (called FO, the stalks, and a membrane protein subunit (called Fq). The Fo-subunit spans the inner membrane and thus is retained in the vesicles. 

Microporous inorganic solids, such as zeolites, clays, and layered oxide semiconductors offer several advantages as organizing media for molecular electron transport assemblies. Because these materials are microcrystalline, their internal pore spaces have well-defined size and shape. This property can be exploited to cause self-assembly, by virtue of size exclusion effects, ion exchange equilibria, and specific adsorption, of photosensitizers, electron donors, and electron acceptors at the solid/solution interface. 

Under aerobic conditions, a greater energy yield may be derived from the electrons contained within the NADH molecule by their participation in the process of oxidative phosphorylation (Section 10.4). The location of the electron-transport assemblies within the inner mitochondrial membrane necessitates the penetration of the membrane by NADH (Section 9.5). The inner mitochondrial membrane is, however, impermeable to NADH molecules. This obstacle is circumvented by the use of shuttle systems (Section 10.5) which do not transport the NADH molecules across the membrane but transfer the electrons as components of another substance which can transverse the membrane. Two shuttle systems exist for this purpose the glycerol phosphate shuttle and the malate-aspartate shuttle. Their relative activities are tissue dependent, e.g. the glycerol phosphate shuttle predominates in the cells of mammalian skeletal muscle and brain whilst the malate-aspartate shuttle is... 

In the complete oxidation of fuel molecules relatively little ATP is produced directly by substrate-level phosphorylation (Section 12.5). Irrespective of the metabolic fuel (carbohydrates, fatty acids or amino acids), most of the ATP is derived from the electrons released on the reoxidation of coenzymes, NADH or FADHj. During dehydrogenase-catalysed reactions, electrons are removed from substrates and transferred to coenzymic acceptors which in turn deliver the electrons to an organization of numerous proteins, called an electron-transport assembly. These assemblies are located in the inner membrane of mitochondria, in chloroplast thylakoids (Section 9.5) or in the plasma membrane of bacteria. Electrons are passed along the assembly to molecular oxygen, the final acceptor, which is reduced in the presence of protons to water. During their transfer from component to component, a portion of their energy is released and may be conserved by utilization in the phos-... 

The electron-transport assembly consists of two coupled systems an electron-transport chain and a system for ADP phosphorylation. By the treatment of the inner mitochondrial membrane with detergents, the hydrophobic protein-protein and protein-lipid interactions may be disrupted and the assembly proteins released from the membrane. 

TABLE 13.1 The composition of the protein complexes of the electron-transport assemblies of bovine heart mitochondria... 

Another approach to molecular assembly involves siloxane chemistry [61]. In this method, the electrically or optically active oligomers are terminated with tii-chlorosilane. Layers are built up by successive cycles of dip, rinse, and cure to form hole transport, emissive, and electron transport layers of the desired thicknesses. Similar methods have also been used to deposit just a molecular monolayer on the electrode surface, in order to modify its injection properties. 

C. Electron Transport Properties of Nanocrystals Either Isolated or Self-Assembled in 2D and 3D Superlattices... 

Emberly EG, Kirczenow G (2001) Models of electron transport through organic molecular monolayers self-assembled on nanoscale metallic contacts. Phys Rev B 64(23) 235412... 

Jiang P, Morales GM, You W, Yu LP (2004) Synthesis of diode molecules and their sequential assembly to control electron transport. Angew Chem Int Ed 43 4471 1475... 

Holmlin RE, Haag R, Chabinyc ML, Ismagilov RF, Cohen AE, Terfort A, Rampi MA, Whitesides GM (2001) Electron transport through thin organic films in metal-insulator-metal junctions based on self-assembled monolayers. J Am Chem Soc 123 5075-5085... 

Duati M, Grave C, Tcbeborateva N, Wu J, Mullen K, Shaporenko A, Zhamikov M, Kriebel JK, Whitesides GM, Rampi MA (2006) Electron transport across hexa-peri-hexabenzo-coronene units in a metal-self-assembled monolayer-metal junction. Adv Mater 18 329-333... 

Lee T, Wang W, Klemic JF, Zhang J, Su J, Reed MA (2004) Comparison of electronic transport characterization methods for alkanethiol self-assembled monolayers. J Phys Chem B 108 8742-8787... 

Function and Assembly of Electron-Transport Complexes in Desulfovibrio vulgaris Hildenborough... 

At the cellular level, rotenone inhibits cellular respiration by blocking electron transport between flavoprotein and ubiquinone. It also inhibits spindle microtubule assembly. ... 

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