L14: Mitochondrial Electron Transport & ATP Generation Flashcards
Explain the chemiosmotic coupling mode for ATP production. What enzyme is responsible for the generation of ATP directly? What is the role of each of its components?
- Chemiosmotic coupling mode refers to the free-energy derived from the ETC driving the active transport/pumping of protons of the mitochondrial matrix and into the intermembrane space, establishing an electrochemical gradient across the inner membrane. This gradient means a potential energy is established and can be dissipated when the protons flow through the ATP synthase complex, releasing free-energy and allowing the enzyme to change conformation and couple Pi with ADP to form ATP. - The F0 unit forms the proton channel in the ATP synthase complex through which protons move. The F1 component catalyzes the formation of ATP.
Why do uncouplers generate so much heat?
- The free-energy stored in the electrochemical gradient created by the difference in proton concentration across the inner membrane must be released when protons move from high to low concentration. When uncouplers create an alternate pathway, this free-energy still is released, but not to a protein/enzyme, but as heat.
List endogenous uncoupling proteins. What are their functions?
- UCP1-4. These are found in brown fat, skeletal muscle, brain and other parts of the body and function to increase body temperature.
What inhibits ATP from moving from mitochondria to cytosol?
- atractyloside
Which complexes function as sites where enough free-energy is released to form ATP? Which complexes do not?
- The same complexes that function as proton pumps: complexes I, III and IV.
What is the function of cyanide? What are treatments?
- Cyanide inhibits cytochrome oxidase (complex IV) by binding Fe3+ and preventing reduction to Fe2+. It is fast acting and lethal. Causes lactic acidosis. Treatment: nitrites (oxidize Hb so cyanide binds to Hb instead of cytochrome oxidase) and Na thiosulfate (converts cyanide to non toxic SCN- form).
Provide examples of defects in oxidative phosphorylation.
- LHON: Leber’s Hereditary Optic Neuropathy is due to mutations in subunits of complex I and is characterized by sudden-onset blindness in young adults. - MERRF: Myoclonic Epilepsy and Ragged Red Fibers is due to abnormally shaped mitochondria with dimished cytochrome oxidase (complex IV) activity as the result of a point mutation in the mitochondrial gene for lysine tRNA. It is characterized by myoclonus and ataxia with generalized seizures. - MELAS: Mitochondrial Encephalopathy, Lactic acidosis and Stroke-like activity is due to abnormally shaped mitochondria with normal cytochrome oxidase activity that is caused by a point mutation in the gene for leucine tRNA. Early symptoms include muscle weakness, pain, recurrent headaches, loss of appetite, vomiting and seizures. Stroke-like episodes begin before age 40. Episodes involve transient hemiparesis (temporary muscle weakness on one side of body), altered consciousness, vision abnormalities, seizures and severe headaches resembling migraines. Episodes lead to brain damage and in some cases dementia.
How many ATPs are formed from the oxidation of NADH? Of succinate or FADH2?
- NADH generates 3 ATPs. Succinate via reduction of FAD to FADH2 generates 2 ATPs.
ETC. a.) What are the components? What are the enzyme complexes involved? Name them b.) What reaction does each component catalyzed? List reduced substrates and oxidized products. c.) What is the sequence of flow through these carriers? Draw and indicate the reactions of each of the complexes. Include movement of protons. What coenzymes and metals do each of the complexes require? d.) Which components are proton pumps? e.) There are other dehydrogenases besides complexes I through IV contained in the mitochondrion. What are they?
- a.) dehydrogenase enzymes (complexes), heme-iron proteins (aka cytochromes), iron-sulfur proteins, CoQ/ubiquinone Dehydrogenase enzymes are: Complex 1: NADH-CoQ reductase Complex 2: Succinate-CoQ reductase Complex 3: CoQH2-cytochrome c reductase Complex 4: Cytochrome c oxidase - b.) Complex 1: NADH + H+ + CoQ = NAD+ + CoQH2 Complex 2: Succinate + CoQ = Fumarate + CoQH2 Complex 3: CoQH2 + 2 cyt c (Fe3+) = CoQ + 2 cyt c (Fe2+) + 2H+ Complex 4: 4 cyt c (Fe2+) + 4H+ + o2 = 4 cyt c (Fe3+) + 2H2o - c.) see picture - d.) Protons are moved to the intermembrane space via complex I, II and IV - e.) Fatty acyl-CoA DH and glycerol-3-P DH. These both reduce CoQ and are not part of the std I-IV complexes
What inhibits cytochrome oxidase (complex IV)?
- cyanide and CO
Mitochondrion. a.) Describe its structure. b.) Define functionally each of its compartments. What occurs in each?
- a.) From outer to inner: outer membrane, intermembrane space, inner membrane and matrix - b.) Outer membrane: fairly permeable, permeable to pyruvate, contain specific translocases Inner membrane: impermeable, contains elements of ETC with coupled ATP synthase complex, specific translocases Intermembrane space: together with matrix, serves as sandwhich to inner membrane across where proton gradient is created to drive ATP synthase complex and convert ADP to ATP Matrix: contains enzymes of TCA cycle with exception of succinate dehydrogenase (aka complex II), which is part of the ETC, ie. bound to the inner membrane on the matrix aspect.; also contains the PDH complex and enzymes for beta-oxidation and some urea cycle enzymes. Also contains mitochondrial DNA
What is atractyloside? What does it do?
- It is a poisonous herb that is a specific inhibitor of the adenine nucleotide translocase protein that resides in the inner membrane of the mitochondria and functions to move ATP to the cytosol.
What is DNP? What is its effect? What symptoms does it cause?
- It is a pesticide and poison that uncouples ETC/ox-phos. It causes sweating, flushing, nausea, inc RR, tachycardia, fever, coma, death in 1-2 days. Treatment with ice baths, oxygen and fluid/electrolyte replacement.
How does pyruvate move from cytoplasm to mitochondrial matrix? Can it move across the mitochondrial membranes?
- Pyruvate crosses the outer membrane of the mitochondria, but cannot cross the inner membrane without a protein carrier. Pyruvate carrier is present in the inner membrane and functions to bring pyruvate into the matrix and take hydroxide ions out.
What is the function of CO? What are treatments?
- CO inhibits the cytochrome oxidase (complex IV). It kills people by reacting with iron in Hb and preventing oxygen loading in lungs. It stabilizes the T-form when 2 subunits of Hb are bound to CO and prevents any oxygen from loading (and if any are on, from releasing). Causes lactic acidosis. - Treatment with 100% o2 in hyperbaric chamber, which increases oxygen dissolved in plasma and displaces CO from Hb iron.
