L5: Oxidative Phosphorylation and Mitochondrial Function Flashcards
LO1: Identify the major components of the respiratory chain (6)
Overall=the enzymes and coenzymes involved in handling electrons
- NAD-linked dehydrogenases (complex I)
- FAD-linked dehydrogenases (flavoproteins)
- Iron-sulfur proteins (non-heme iron proteins associated with complexes I, II and III)
- Coenzyme Q (transfers electrons from FMN/FAD to cytochromes)
- Cytochromes (contain heme, which has an iron atom that undergoes cyclic oxidation/reduction reactions)
- Terminal acceptor (molecular oxygen; gets reduced to water)
LO2: How does the ETC covert chemical energy into ATP?
Overall: electrons are transferred from donors to acceptors in a series of redox reactions (ending with terminal acceptor oxygen), releasing energy used to form ATP
- Complex I pumps four hydrogen ions across membrane (from matrix to intermembrane space) to generate gradient
- Complex II (succinate dehydrogenase) receives FADH2 (bypassing complex I) which delivers electrons directly to the ETC
- Ubiquinone (Q, moves freely through membrane core) accepts electrons from complexes I and II and delivers them to complex III
- Complex III pumps protons through membrane and passes its electrons to cyochrome c (1/a time, also moves freely) for transport
- Complex IV allows reduction of oxygen, and reduced oxygen picks up two hydrogen ions from the surrounding medium to make water (removal of hydrogen ions contributes to ion gradient)
- Energy released from electron transport is used to pump protons (from matrix into space between inner and outer membranes) to create a proton gradient across IMM
- The flow of protons back through the F0 domain of ATP synthase releases gradient’s energy, which is translated into rotation of the stalk used by F1 of ATP synthase to synthesize ATP
LO3: Oxidative phosphorylation vs. substrate level phosphorylation
Oxidative phosphorylation refers to formation of ATP via ETC (chemiosmosis+cellular respiration), while substrate level phosphorylation refers to direct phosphorylation of ADP to form ATP; occurs in mitochondria
- OP requires ATP synthase and PMF
- SLP usually catalyzed by kinases and occurs in glycolysis pathway; occurs in cytoplasm
LO4: Why are cytosolic NADH reducing equivalents shuttled into mitochondria?
- NADH produced in cytosol by glycolysis cannot cross the IMM and doesn’t have a transport protein
- shuttle system required to get NADH from cytosol into mitochondria
LO4: Describe the malate-aspartate shuttle
- consists of two isozymes of malate dehydrogenase (MDH); net effect is to generate cytosolic NADH to mitochondrial NADH
- don’t lose energy in the process (cystolic and mitochondrial NADH=3 ATP equivalents)
- this shuttle is enriched in liver and cardiac muscle
LO4: Describe the glycerol-phosphate shuttle
- consists of two isozymes of alpha-glycerol phosphate dehydrogenase (GPDH); net effect is to generate mitochondrial FADH2 from cytosolic NADH
- lose energy in the process (cytosolic NADH=3 ATP equivalents, mitochondrial FADH2=2 ATP equivalents)
- this shuttle is enriched in brain and fast-contracting skeletal muscle
LO4: Describe the steps of the glycerol-phosphate shuttle
- Cytosolic form reduces DHAP to alpha-glycerol phosphate, using NADH’s reducing power
- Alpha-glycerol phosphate crosses the OMM
- Mitochondrial form oxidizes alpha-glycerol phosphate back into DHAP, using FAD as an electron acceptor and generating FADH2
- Electrons from FADH2 are passed directly to coenzyme Q in the ETC
LO5: Describe the steps of the malate-aspartate shuttle
- Cytosolic form reduces oxaloacetate to malate, using NADH’s reducing power
- Malate is transported across IMM
- Mitochondrial form oxidizes malate back into oxaloacetate, forming NADH
LO5: Why is the ATP yield different in reactions catalyzed by pyruvate dehydrogenase vs. succinate dehydrogenase?
- succinate dehydrogenase-complex II (pumps fewer protons)
- FADH2 enters ETC at complex II, so has less opportunity to increase redox potential - pyruvate dehydrogenase (in PDH complex) used in glycolysis to generate TCA cycle intermediates from the oxidation of pyruvate
- NADH enters ETC at complex I
LO6: Compare/contrast the toxicity of cyanide and dinitrophenol
Cyanide inhibits cytochrome c oxidase (complex IV) by binding to it, halting the electrons in the ETC and inhibiting ATP production (oxygen unable to be bound)
Dinitrophenol uncouples oxidation from phosphorylation by making the IMM abnormally permeable to protons, so energy dissipates as heat instead of being used to make ATP (was formerly used as diet aid to increase metabolic activity…led to many deaths and ADRs)
LO7: How is mitochondrial function related to heat generation in the newborn?
Newborns have higher levels of brown fat, a tissue-type whose mitochondria contain more thermogenin an uncoupling protein (UCP1)
- these UCPs allow some energy to be dissipated as heat instead of used to make ATP
- UCPs work by letting protons back across the gradient
LO8: Why can inherited defects in the oxidative phosphorylation pathway result in a wide range of clinical findings in various tissues?
- some ox-phos defects are located in the mitochondrial genome, which is not inherited via Mendelian genetics (replicative segregation, heteroplasmy)
- mitochondria are everywhere! ox-phos pathway has lots of steps in all different tissues!
- myopathy, stroke, bone marrow failure, seizures, visual impairment, paralysis of eye muscles, failure to thrive, developmental delay, autonomic dysfunction
LO9: How does the movement of protons through the F1FO ATP synthase complex provide energy for ATP synthesis?
ATP synthase: F1 domain has synthase activity, but can’t generate ATP alone (alone, it actually catalyzes the hydrolysis of ATP to ADP)
F0 domain extends through the IMM and creates a channel for H+ to flow through
-flow of protons back through the F0 channel of ATP synthase releases energy associated with the electrochemical gradient, and this energy is translated into rotation of the stalk proteins (which connect F0 to F1) to synthesize ATP
