Topic 19: ETC Flashcards
Where do the redox reactions of oxidative phosphorylation occur?
In four large protein complexes located in the inner mitochondrial membrane
How do we define the tendency of a molecule to give it’s electrons?
It is measured by its reduction/redox potential
What are the protein complexes involved in shuttling electrons to O2
Electrons are transferred from NADH to O2 through a chain of three large protein complexes called NADH-Q oxidoreductase, Q-cytochrome c oxidoreductase and cytochrome c oxidase. These complexes appear to be associated in a supramolecular complex, which facilitates the rapid transfer of substrate and prevents the release of reaction intermediates. A fourth large protein complex, called succinate-Q reductase, contains the succinate dehydrogenase that generates FADH2 in the citric acid cycle. Electrons from this FADH2 enter the electron-transport chain at Q-cytochrome c oxidoreductase
Describe the movement of electrons from NADH to the first complex?
The electrons of NADH enter the chain at complex I, also called NADH-Q oxidoreductase. The electrons flow from NADH to FMN, and then through a series of seven iron-sulfur clusters to Q. The redox reactions occur in the extramembranous part of NADH-Q oxidorectuase. The flow of two electrons from NADH to coenzyme Q through NADH-Q oxido-reductase leads to the pumping of four hydrogen ions out of the matrix of the mitochondrion. Q2- subsequently takes up two protons from the matrix as it is reduced to QH2. The removal of these protons from the matrix contributes to the formation of the proton-motive force. The QH2 subsequently leaves the enzyme for the Q pool, allowing another reaction cycle to occur. The free energy change for this reaction is -81 kJ/mol due to the preferred reductive potentials, which provides the energy needed to move protons into the intermembrane space.
Describe the second complex
Recall that the FADH2 is formed in the citric acid cycle in the oxidation of succinate to fumarate by succinate dehydrogenase. Complex II is called succinate-Q reductase, an integral membrane protein of the inner mitochondrial matrix. The electron carriers in this complex are FAD, iron-sulfur proteins and Q. FADH2 does not leave the complex. Rather, its electrons are transferred to Fe-S centers and then to Q for entry into the ETC. This complex does NOT pump protons, unlike complexes I, III and IV. Consequently, less ATP is formed from the oxidation of FADH2 than from NADH.
This process is also much less exergonic than NADH-Q oxidoreductase, at -13 kJ/mol
Describe the third complex
The second of three proton pumps in the respiratory chain is Q-cytochrome c oxidoreductase, complex III. The function of this complex is to catalyze the transfer of electrons from ubiquinol from the NADH-Q oxidoreductase complex and the succinate-Q reductase complex to reduce cytochrome C (Cyt C), a water soluble protein. Concomitantly, the complex pumps proteins out of the mitochondrial matrix. The flow of a pair of electrons through this complex leda to the effective net transport of 2 H+ to the intermembrane space, half the yield obtained with NADH-Q oxidoreductase due to a smaller thermodynamic driving force
The free energy change is -34 kj/mol
Cyt C can only accept one electron for some reason so two molecules of cyt C are used to accept the two electrons of QH2
Describe the fourth complex
The last of three proton-pumping assemblies of the respiratory chain is cytochrome c oxidase (complex IV). Cytochrome c oxidase catalyzes the transfer of electrons from the reduced form of cytochrome c to molecular oxygen, the final electron acceptor
The requirement of oxygen for this reaction is what makes “aerobic” organisms aerobic. Obtaining oxygen for this reaction is the primary reason human beings must breathe. Four electrons are funneled to O2 to completely reduce it to two molecules of H2, and concurrently, protons are pumped from the matrix to the cytoplasmic side of the inner mitochondrial membrane. This reaction is very favorable, with a standard free-energy change of -110 kj/mol, and an overall energy change of -231.8 kJ/mol for the entire process. As much of this free energy as possible must be captured in the form of a proton gradient for subsequent use in ATP synthesis
