Etc

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ETC

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systems for conserving the energy of electron transfer as chemical energy in the form of ATP
The electron transport chain is located in the cytoplasmic membrane of Bacteria, and the inner membrane of eukaryotic mitochondria

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ETC locations

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standard reduction potential

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is a quantitative measure of the ease with which a reduced species releases electrons

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Succinate dehydrogenase

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(also called Complex II) also delivers electrons to the
electron transport chain, but SDH is not a proton pump, so the number of protons
translocated is lower for each succinate than for each NADH

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In mitochondria and some bacteria there are two extra components:

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complex III
accepts electrons from ubiquinol, and uses them to reduce cytochrome c; Complex III is a proton pump, so increases the energetic efficiency of the mitochondrial electron transport chain

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Cytochrome c

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delivers electrons to cytochrome oxidase

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NADH dehydrogenase contains

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multiple redox centers, including a protein bound coenzyme Q

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Complex III

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oxidizes two molecules of UQH2 and reduces one molecule of UQ in the Q cycle

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Cytochrome oxidase

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uses electrons from cytochrome c to reduce oxygen to water

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metabolic shunts

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cytosolic NADH (eg from glycolysis) is transported into mitochondria for electron
transport by metabolic shunts

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Positive
reduction potentials mean

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that the compound on the left of the reaction is a good oxidizing agent

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Negative
reduction potentials mean

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that the compound on the right of the reaction is a good reducing agent

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Coenzyme Q (ubiquinone)

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is a mobile electron carrier. It has a long isoprenoid tail, making it hydrophobic. It is localized to the membrane, or can be buried in enzymes as a cofactor (eg in Complex I)
Reduced coenzyme Q (UQH2, ubiquinol) releases 2 electrons and 2 protons when converted to oxidized coenzyme Q (Q, ubiquinone). Since this occurs in two one electron reactions, coenzyme Q can receive electrons from Fe-S clusters

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complex II

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is succinate dehydrogenase from the TCA cycle. Electrons also enter the ETC from succinate

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Succinate dehydrogenase

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is a b type cytochrome, it contains a non-covalently bound iron protoporphyrin IX (heme). The iron can be in 2+ or 3+ oxidation states, so the heme participates in 1 electron redox reactions

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Q cycle

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cytochrome oxidase

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Complex IV is called cytochrome oxidase, because it oxidizes cytochrome c, then uses the electrons released to reduce oxygen to water
4cytc (Fe2+) + 4H+ + O2 4cytc (Fe3+) + 2H2O
This is the end of the electron transport chain, so oxygen is the terminal electron acceptor and Complex IV is the terminal oxido- reductase

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Cytochrome oxidase

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Three subunits (I, II and III) contain the redox centers, in mammals, there are 10 additional subunits that do not participate in electron transfer
Electron transfer involves two hemes and two copper centers
(which cycle between Cu+ and Cu2+)
Four electrons are required to reduce one oxygen to water
~4 protons are pumped for each oxygen reduced

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Cytochrome Oxidase

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http://web.virginia.edu/Heidi/chapter21/Images/21_17.jpg

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The P/O ratio

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The P/O ratio is the number of ATP molecules made per 2 electrons that enter electron transport. For NADH oxidation, it is thought that 10 protons are translocated per 2e-
For an ATPase with 10 c subunits, 10 protons drive one complete turn of the enzyme, which generates 3 molecules of ATP
So in this case, the H+/ATP ratio is 10/3 = 3.33 [that is one ATP made per 3.33 H+ translocated by electron transport]
If one ATP is made per 3.33 H+, the number made per 2e- is 10/3.33 = 3 But, one proton is consumed in the export of each ATP molecule Therefore, P/O = 10/(3.33 + 1) = 2.3 [+1 accounts for the cost of ATP transport] For an ATPase with 8 c subunits
H+/ATP ratio is 8/3 = 2.67 P/O = 10/(2.67 + 1) = 2.7 In bacteria, ATP is not exported, so P/O ratios are higher

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Uncoupler

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Increase the proton conductance of the membrane and cause a shirt-circuit, the proton gradient is collapsed