Electron transport systems

Monooxygenase systems consist of several components, the most prominent of which is cytochrome P450. Cytochromes are a group of proteins containing a b- 

Stabilize the inner mitochondrial membrane enzyme arrangement 

Mechanisms for proton removal from mitochondria include the following  

There is no direct loss of protons they are lost from mitochondria by being converted to HzO. 

The spatial arrangement of electron transport enzymes permits an easy exit of protons as they are released by various oxidative processes. 

The electron transport system involves a variety of redox reactions, so it is useful to review some electrochemical relationships. First, the free energy of a reaction is related to the reduction potential for electron transfer by the equation  

The Nernst equation, which describes the reduction potential for an electrochemical reaction, 

Note the standard reduction potentials and resultant standard free energies NADH 0.315 V O2 +0.815 V. So for the reaction  

815 - (- 0.315) = 1.130 V, which, using the relationship between free energy and potential gives -218 kj/mol. The free energy of hydrolysis of ATP is -30.5 kJ/mol, so if three ATP are made/pair of electrons flowing through ETS 91.5 kj are captured out of 218 kj available, or 42%. This is of course under Standard Conditions. However, under physiological conditions this may actually be closer to 70%. 

The irmer mitochondrial membrane is protein rich. If carefully broken down, it is very rich in five protein complexes I -IV are large protein complexes involved in electron transport, while V is the ATP sythatase driven by proton gradients. 

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Scheme 10.3 Electron-transport systems associated with cytochrome P450 monooxygenases. Arrows indicate electron transfer.

Polymerase responsible for microtubule formation Electron transport system of photosystem 1... 

Peroxisomes are found in many tissues, including liver. They are rich in oxidases and in catalase, Thus, the enzymes that produce H2O2 are grouped with the enzyme that destroys it. However, mitochondrial and microsomal electron transport systems as well as xanthine oxidase must be considered as additional sources of H2O2. 

Attention has been directed to the dechlorination of polychlorinated benzenes by strains that use them as an energy source by dehalorespiration. Investigations using Dahalococcoides sp. strain CBDBl have shown its ability to dechlorinate congeners with three or more chlorine substituents (Holscher et al. 2003). Although there are minor pathways, the major one for hexachlorobenzene was successive reductive dechlorination to pentachlorobenzene, 1,2,4,5-tetrachlorobenzene, 1,2,4-trichlorobenzene, and 1,4-dichlorobenzene (Jayachandran et al. 2003). The electron transport system has been examined by the use of specific inhibitors. lonophores had no effect on dechlorination, whereas the ATP-synthase inhibitor A,A -dicyclohexylcarbodiimide (DCCD) was strongly inhibitory (Jayachandran et al. 2004). 

Other enzyme systems may also be directly or indirectly involved in the generation of ROS in the lung, including those of the eicosanoid pathway, the mitochondrial electron transport system, and aldehyde, glucose and xanthine oxidases (Parks and Granger, 1986). These systems may also be relevant to lung damage. For example, the oedematous pulmonary injury that results from cessation of blood flow for a period followed by reinstatement of... 

Methemoglobinemia arises from poisoning with MHb-forming substances and from the hereditary deficiency of an enzyme system which either provides reduced pyridine nucleotides for MHb reduction or is involved itself in the MHb reduction mechanism (e.g., electron transport system). (See Section II of the Addendum, page 280.)... 

The spatial separation between the components of the electron transport chain and the site of ATP synthesis was incompatible with simple interpretations of the chemical coupling hypothesis. In 1964, Paul Boyer suggested that conformational changes in components in the electron transport system consequent to electron transfer might be coupled to ATP formation, the conformational coupling hypothesis. No evidence for direct association has been forthcoming but conformational changes in the subunits of the FI particle are now included in the current mechanism for oxidative phosphorylation. 

To explain how H+ transfer occurred across the membrane Mitchell suggested the protons were translocated by redox loops with different reducing equivalents in their two arms. The first loop would be associated with flavoprotein/non-heme iron interaction and the second, more controversially, with CoQ. Redox loops required an ordered arrangement of the components of the electron transport system across the inner mitochondrial membrane, which was substantiated from immunochemical studies with submitochondrial particles. Cytochrome c, for example, was located at the intermembranal face of the inner membrane and cytochrome oxidase was transmembranal. The alternative to redox loops, proton pumping, is now known to be a property of cytochrome oxidase. 

DR. MARSHALL NEWTON (Brookhaven National Laboratory) I d like to ask a question about Hopfield s numbers. The alpha parameter from his 1974 paper [Hopfield, J. J. Proc. Natl. Acad. Sci., USA 1974, 7 1, 3640] was based not on the direct metal-metal interaction but rather was based on carbon-carbon overlap because it was two carbons which were closest together in his electron transport system. In contrast, Dr. Sutin gave some different numbers based on metal orbitals. Depending on whether one is interested in carbon-carbon overlap between two organic rings, or in direct metal-metal overlap, one might or might not opt for the Hopfield parameters. However, at the level of fuzziness which we have, it may not make any difference, I realize. 

The desaturation process is particularly interesting as it provides an example of a microsomal (as opposed to mitochondrial) electron transport system. The enzymes responsible, fatty acyl-CoA desaturases, are examples of mixed function oxidases... 

Chan, T.M., Gillett, J.W. and Terriere, L.C. Interaction between microsomal electron transport systems of trout and male rat in cyclodiene epoxidation. Comp. Biochem. Physiol. (1967), 20, 731-7 +2. 

Fig. 1. Simplified diagram of the section of the electron transport system coupled to cyt c...

Scheme 3 Partial reactions of the electron transport systems F42oH2 heterodisulfide oxidoreductase and H2 heterodisulfide oxidoreductase...

As the key enzymes of the two electron transport systems with the electron carriers 9 and rac-10 almost exhibit the same specific activity, we may assume that the phenazine system is wholly responsible for the redox process and that the sole function of the terpenoid side chain in 10 is to anchor the electron carrier in the membrane. 

What should be emphasized is that the redox potentials measured for 10 and 22 allow for both the reduction of 10 to dihydro-10 via F420H2 and H2, and the oxidation of dihydro-10 to 10 by 22. This finding, supported by electrochemical experiments, also strongly corroborates the hypothesis that 10 plays a prominent role as an electron carrier in the electron transport system of methanogens. 

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