Lectures 18-23
Complex I of ETC
Complex I (NADH:ubiquinone oxidoreductase) transfers electrons (in pairs) from NADH to reduce Q to create QH2 (Q is the final electron acceptor). NADH is oxidized (since it loses 2 electrons) to release energy and to pump protons into the intermembrane space (the oxidation of NADH powers complex I to pump the protons), and it does the pumping). The electrons are transported to complex III
Complex II of ETC
Complex II (succinate dehydrogenase-->found in CAC, it’ll create FADH2) transfers electrons from succinate to reduce FAD to create FADH2, and then to reduce Q to create QH2. Protons are not pumped into the intermembrane space. QH2 goes to complex III.
Complex III of ETC
Complex III (cytochrome c reductase) transfers electrons (one at a time) from QH2 to cytochrome c. QH2 is oxidized to release energy and pump protons into the intermembrane space (complex III pumps the protons and Q can go back into the Q pool).
Complex IV of ETC
Complex IV transfers electrons from cyt c to O2 to create H2O. Cyt C is oxidized to release energy and to pump protons into the intermembrane space (complex IV pumps the protons).
What is the ultimate electron acceptor in mitochondrial electron transport?
O2 (becomes H2O)
Understand how an inhibitor impacts electron transport. Which molecules are oxidized or reduced?
Everything to the left of the inhibitor is reduced while everything to the right of the inhibitor is oxidized. Once the complexes are reduced, they won’t accept more electrons and they won’t continue to pump more protons-->there won’t be an electrochemical build up for the production of ATP.
Individuals with a thiamine-deficient diet have relatively high levels of pyruvate in their blood. Explain this in biochemical terms.
TPP is a cofactor involved in the PDH complex. Thiamine deficient diet means that TPP is lacking, which will end up inhibiting pyruvate dehydrogenase, so pyruvate will not be broken down and will build up.
Predict the state of oxidation of ubiquinone (Q) and cytochromes c, Complex III, and Complex IV:
Abundant O2, but no NADH
Reduced: nothing. Everything is oxidized bc there is no electron source (NADH is the electron source, it provides the electrons to complex I, which goes to Q, which then continues through the ETC)
Predict the state of oxidation of ubiquinone (Q) and cytochromes c, Complex III, and Complex IV:
Abundant NADH and O2
Reduced: all can be reduced and oxidized eventually but not at the same time. Partially oxidized-->depending on where the electrons are in the ETC. And O2 becomes H2O.
Predict the state of oxidation of ubiquinone (Q) and cytochromes c, Complex III, and Complex IV:
Abundant NADH and O2, but cyanide added. (Cyanide blocks the transfer of electrons from Complex IV to O2).
Reduced: complex IV, III, cyt c, Q.
O2 is the only thing not reduced
Predict the state of oxidation of ubiquinone (Q) and cytochromes c, Complex III, and Complex IV:
Abundant NADH, but no O2
Reduced: everything (complex IV can’t become oxidized b/c it can’t transport its electrons to O2)
Name the 2 shuttle systems (you don’t have to write them out in detail), and explain why electrons using one shuttle result in only ~1.5 ATP per cytosolic NADH, while the other shuttle system results in ~2.5 ATP per cytosolic NADH.
Malate-aspartate shuttle-->gives its electrons to complex I-->this shuttle results in more protons being pumped, so will produce more ATP (2.5 ATP per NADH). Malate carries the electrons into the matrix via the malate-alpha-ketoglutarate transporter. In the matrix, the electrons are passed to NAD+ to form NADH, which can carry the electrons to complex I.
Glycerol 3-phosphate shuttle-->fewer complexes pumping protons, so fewer protons that can travel back down and synthesize ATP (so 1.5 ATP per FADH2). This shuttle is using FAD to carry the electrons to complex III (skipping complex I). The enzyme of the shuttle (glycerol 3-phosphate dehydrogenase) delivers the electrons from NADH to FAD to FADH2 to Q.
Suppose you discovered a mutant yeast whose glycolytic pathway was shorter because of the presence of a new enzyme catalyzing the reaction: glyceraldehyde 3-phosphate--> 3-phosphoglycerate
Would shortening the glycolytic pathway in this way benefit the cell? Explain.
The regular reaction is G3P + NAD --> 1,3-BPG + NADH
1,3-BPG + ADP --> 3PG + ATP
This mutant enzyme is bypassing the ATP synthesis step. Glycolysis requires the input of 2 ATP and generates 4, with a net production of 2 ATP. So, this mutant is preventing the synthesis of 2 of the ATP, so the net production would be 0 ATP. Therefore, this would not benefit the cell.
Explain the general idea of chemiosmotic coupling of electron transport to ATP synthesis – what provides the coupling mechanism of energy yielded by oxidation reactions to synthesis of ATP?
Electrons flow from electron donors (NADH) through membrane bound carriers to a final electron acceptor with a large reduction potential (O2). The free energy from this exergonic electron flow is coupled to the endergonic (active transport) transport of protons across a proton-impermeable membrane. The transmembrane flow of protons back down their concentration gradient through specific protein channels provides the free energy for synthesis of ATP.