Question

When flavoprotein transfers electrons directly to the final electron acceptor hydrogen peroxide is produced. What other consequences might result from electroncarriers in the electron transport chain being bypassed? This is in relation to bacteria, not mitochondria.

Answer 1

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Answer 2

The electron transport chain carries both protons and electrons, passing electrons from donors to acceptors, and transporting protons across a membrane.These processes use both soluble and protein-bound transfer molecules. In mitochondria, electrons are transferred within the intermembrane space by thewater-soluble electron transfer protein cytochrome c.[6] This carries only electrons, and these are transferred by the reduction and oxidation of an ironatom that the protein holds within a heme group in its structure. Cytochrome c is also found in some bacteria, where it is located within the periplasmicspace.[7]

Within the inner mitochondrial membrane, the lipid-soluble electron carrier coenzyme Q10 (Q) carries both electrons and protons by a redox cycle.[8] Thissmall benzoquinone molecule is very hydrophobic, so it diffuses freely within the membrane. When Q accepts two electrons and two protons, it becomesreduced to the ubiquinol form (QH2); when QH2 releases two electrons and two protons, it becomes oxidized back to the ubiquinone (Q) form. As a result, iftwo enzymes are arranged so that Q is reduced on one side of the membrane and QH2 oxidized on the other, ubiquinone will couple these reactions and shuttleprotons across the membrane.[9] Some bacterial electron transport chains use different quinones, such as menaquinone, in addition to ubiquinone.[10]

Within proteins, electrons are transferred between flavin cofactors,[3][11] iron–sulfur clusters, and cytochromes. There are several types of iron–sulfurcluster. The simplest kind found in the electron transfer chain consists of two iron atoms joined by two atoms of inorganic sulfur; these are called[2Fe–2S] clusters. The second kind, called [4Fe–4S], contains a cube of four iron atoms and four sulfur atoms. Each iron atom in these clusters iscoordinated by an additional amino acid, usually by the sulfur atom of cysteine. Metal ion cofactors undergo redox reactions without binding or releasingprotons, so in the electron transport chain they serve solely to transport electrons through proteins. Electrons move quite long distances through proteinsby hopping along chains of these cofactors.[12] This occurs by quantum tunnelling, which is rapid over distances of less than 1.4×10-9 m.


Many catabolic biochemical processes, such as glycolysis, the citric acid cycle, and beta oxidation, produce the reduced coenzyme NADH. This coenzymecontains electrons that have a high transfer potential; in other words, they will release a large amount of energy upon oxidation. However, the cell doesnot release this energy all at once, as this would be an uncontrollable reaction. Instead, the electrons are removed from NADH and passed to oxygen througha series of enzymes that each release a small amount of the energy. This set of enzymes, consisting of complexes I through IV, is called the electrontransport chain and is found in the inner membrane of the mitochondrion. Succinate is also oxidized by the electron transport chain, but feeds into thepathway at a different point.

In eukaryotes, the enzymes in this electron transport system use the energy released from the oxidation of NADH to pump protons across the inner membraneof the mitochondrion. This causes protons to build up in the intermembrane space, and generates an electrochemical gradient across the membrane. The energystored in this potential is then used by ATP synthase to produce ATP. Oxidative phosphorylation in the eukaryotic mitochondrion is the best-understoodexample of this process. The mitochondrion is present in almost all eukaryotes, with the exception of anaerobic protozoa such as Trichomonas vaginalis thatinstead reduce protons to hydrogen in a remnant mitochondrion called a hydrogenosome.



Alternative reductases and oxidases
Many eukaryotic organisms have electron transport chains that differ from the much-studied mammalian enzymes described above. For example, plants havealternative NADH oxidases, which oxidize NADH in the cytosol rather than in the mitochondrial matrix, and pass these electrons to the ubiquinone pool.[41]These enzymes do not transport protons, and, therefore, reduce ubiquinone without altering the electrochemical gradient across the innermembrane.[42]

Another example of a divergent electron transport chain is the alternative oxidase, which is found in plants, as well as some fungi, protists, and possiblysome animals.[43][44] This enzyme transfers electrons directly from ubiquinol to oxygen.[45]

The electron transport pathways produced by these alternative NADH and ubiquinone oxidases have lower ATP yields than the full pathway. The advantagesproduced by a shortened pathway are not entirely clear. However, the alternative oxidase is produced in response to stresses such as cold, reactive oxygenspecies, and infection by pathogens, as well as other factors that inhibit the full electron transport chain.[46][47] Alternative pathways might,therefore, enhance an organisms' resistance to injury, by reducing oxidative stress.[48]

Organization of complexes
The original model for how the respiratory chain complexes are organized was that they diffuse freely and independently in the mitochondrial membrane.[17]However, recent data suggest that the complexes might form higher-order structures called supercomplexes or "respirasomes."[49] In this model, the variouscomplexes exist as organized sets of interacting enzymes.[50] These associations might allow channeling of substrates between the various enzyme complexes,increasing the rate and efficiency of electron transfer.[51] Within such mammalian supercomplexes, some components would be present in higher amounts thanothers, with some data suggesting a ratio between complexes I/II/III/IV and the ATP synthase of approximately 1:1:3:7:4.[52] However, the debate over thissupercomplex hypothesis is not completely resolved, as some data do not appear to fit with this model.

Answer 3

When flavoprotein transfers electrons directly to the final electron acceptor hydrogen peroxide is produced. What other consequences might result fromelectron carriers in the electron transport chain being bypassed? This is in relation to bacteria, not mitochondria.

electron carriers in the electron transport chain being bypassed produces only two ATP