Electron transport chain potentials

Although electrons move from more negative to more positive reduction potentials in the electron transport chain, it should be emphasized that the electron carriers do not operate in a simple linear sequence. This will become evident when the individual components of the electron transport chain are discussed in the following paragraphs. 

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 ... 

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. 

The energy obtained by oxidation of the substrate with oxygen through the electron transport chain is thus accumulated as a difference in the electrochemical potential for H+ between the intracristal and matrix spaces. 

Redox potentials were also used to arrange the electron carriers in their correct order. This procedure was applied to the cytochromes by Coolidge (1932). There were however serious difficulties. Electrochemical theory applies to substances in solution the values obtained are significantly affected by pH and the concentrations of the different components. Of the members of the electron transport chain only the substrates NAD+, NADP+, and cytochrome c are soluble. The other components were difficult to extract from tissue particles without altering their properties. Further, it was hard to determine their concentration and to decide on appropriate values for pH and oxygen concentration. Nevertheless, mainly from work by Ball (1938), at the time in Warburg s laboratory, an approximate order of redox potentials was drawn up ... 

Marine organisms concentrate metals in their tissues and skeletal materials. Many of these trace metals are classified as micronutrients because they are required, albeit in small amounts, for essential metabolic functions. Some are listed in Table 11.4, illustrating the role of metals in the enzyme systems involved in glycolysis, the tricarboxylic acid cycle, the electron-transport chain, photosynthesis, and protein metabolism. These micronutrients are also referred to as essential metals and, as discussed later, have the potential to be biolimiting. 

Coenzyme QIO (21) is one of the essential enzymes in the mitochondrial electron transport chain, participating in the aerobic respiration cycle. The role of Co-QlO as a cardioprotective substance and an antioxidant are well studied. Recently, it was found that Co-QlO is also capable of attenuating the intracellular deposition of Ap in transgenic AD mouse models. Additionally, the same group reported that Co-QlO administration also led to reduction of preexisting plaque burden in the same model. Such properties are suggestive of a potential therapeutic role for Co-QlO in AD. 

Oxidizible substrates from glycolysis, fatty acid or protein catabolism enter the mitochondrion in the form of acetyl-CoA, or as other intermediaries of the Krebs cycle, which resides within the mitochondrial matrix. Reducing equivalents in the form of NADH and FADH pass electrons to complex I (NADH-ubiquinone oxidore-ductase) or complex II (succinate dehydrogenase) of the electron transport chain, respectively. Electrons pass from complex I and II to complex III (ubiquinol-cyto-chrome c oxidoreductase) and then to complex IV (cytochrome c oxidase) which accumulates four electrons and then tetravalently reduces O2 to water. Protons are pumped into the inner membrane space at complexes I, II and IV and then diffuse down their concentration gradient through complex V (FoFi-ATPase), where their potential energy is captured in the form of ATP. In this way, ATP formation is coupled to electron transport and the formation of water, a process termed oxidative phosphorylation (OXPHOS). 

Tyr 143, 36 293 Tyr 254, 36 291-293 NMR spectroscopy, 36 271-272 pH dependence, 36 274-275 primary stmcture, 36 261-263 prosthetic groups structure, 36 258 quaternary structure, 36 261-262 reduction potentials, 36 268-269 short electron transport chain, 36 258-259 site-directed mutagenesis, 36 289-290 substrate specificity, 36 272-274 Flavocytochrome C552 electrochemistry, 36 365-367, 369... 

The basic mechanism underlying the toxicity of salicylate is the uncoupling of oxidative phosphorylation. For oxidative phosphorylation to take place, there is a requirement of a charge difference between the intermembrane space and the matrix of the mitochondria (Fig. 7.60). This is achieved when electrons move down the chain of multienzyme complexes and electron carriers (the electron transport chain), causing protons to move from the mitochondrial matrix to the intermembrane space. Consequently, a pH difference builds up, which is converted into an electrical potential across the membrane of approximately 200 mV over 8 nm. 

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