Electron transport shuttle mechanisms

E. There are two shuttle mechanisms, the malate-aspartate shutde and the glycerol 3-phosphate shuttle, that transport electrons to the inner mitochondrial matrix to be used in the electron transport chain. 

A. The electrons released in glycolysis and transported into the mitochondria by shuttle mechanisms (see Chapter 6) and those derived from the TCA cycle are transferred to oxygen and combined with protons to form HjO. 

The FADH2 enters the electron-transport chain at coenzyme Q, while the dihydroxyacetone phosphate can return to the cytoplasm. Although this shuttle is generally inefficient, in the sense that only two ATP molecules are produced per FADH2 molecule oxidized, compared with three for NADH oxidation, it provides a mechanism for regeneration of NAD+ in the cytosol. The presence of cytosolic NAD+ is essential for continued glycolysis (see Fig. 11-20). 

Plastoquinone is one of the most important components of the photosynthetic electron transport chain. It shuttles both electrons and protons across the photosynthetic membrane system of the thylakoid. In photosynthetic electron flow, plastoquinone is reduced at the acceptor side of photosystem II and reoxidized by the cytochrome bg/f-complex. Herbicides that interfere with photosynthesis have been shown to specifically and effectively block plastoquinone reduction. However, the mechanisms of action of these herbicides, i. e., how inhibition of plastoquinone reduction is brought about, has not been established. Recent developments haVe brought a substantial increase to our knowledge in this field and one objective of this article will be to summarize the recent progress. 

To resolve the dilemma, one needs an additional analysis of the conduction mechanisms. For instance, the amount of electrons transported per period has been estimated on the basis of the known source-drain current I 100 pA [Park 2000]. However, when taking into account the frequency Q 1.2 THz of the "bouncing-ball mode", it turned out that the CV>o transistor transmits only a small fraction q 10-3 e of the elementary charge e per vibrational period r = 27t/Q 10 12 s. This excess charge is too small to compete with and to affect the shuttling dynamics of the Cm SET. 

DHAP is also employed in the glycerophosphate shuttle, which functions as an electron-transporting mechanism in insect muscle (Figure 15.11a). 

Transport processes are especially important for substances, such as oxaloacetate, that cannot cross the mitochondrial membrane. The same is true for electrons. Shuttle mechanisms must exist to transport electrons as the reduced form of important compounds. Compounds that cannot cross the membrane must be converted to ones that can, and then must be converted back to their original form on the other side of the membrane. 

This transformation occurred because electrons were shown to be shuttled from one site to the next through multilayered polymeric films, and finally to species in solution, some distance from the surface of the electrode. This mechanism is similar to that for a redox enzyme linked to an electron transport chain. Substrate species can also enter the polymeric phase and exchange... 

NADH is used to reduce oxaloacetate to this compound which is part of a shuttle mechanism for transferring electrons to the electron transport chain 2. Oxaloacetate combines with acetyl CoA to form this compound in the citric acid cycle 3. Transamination of this amino acid forms oxaloacetate ... 

The complexity of the bio-electronic interface necessitates that material selection and electrode fabrication be understood to obtain an optimal result. In the design of an anode or a cathode, the materials should enhance stability of the immobilized biocatalyst and foster electron transfer (ET) between the biocatalyst and the conductive material. As a result, various materials and methodologies have been described in the literature to coordinate redox biocatalysts and electrodes [24,25]. In addition, several factors should be considered when selecting materials, including the electro-chemically accessible surface area (EASA), mechanical stability, and conductivity. The electrode architecture can be designed to enhance mass transport of fuel to the biocatalyst, and, in some instances, to include mediators to shuttle electrons between... 

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