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UT BIOL 3030 - Cellular Energetics

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Lecture 11: Cellular Energetics- First Step of Harvesting Energy from Glucose: Glycolysiso ATP hydrolysis provides energy for cells to do worko Aerobic oxidation  4 stage process to convert energy released by glucose into ATP terminal phosphoanhydride bondo Anaerobic respiration  cells metabolize pyruvate to lactic acid or ethanol and CO2 to convert NADH back to NAD+ required for glycolysis- Glycolytic Pathwayo Occurs in cytoplasm; harvests only a fraction of energy available from glucoseo DOES NOT REQUIRE O2o 6-C glucose converted to two 3-C pyruvates 2 ATP consumed and produced One reaction yields NADH by reduction of NAD+ Steps 1, 3, and 10 are irreversible b/c large -delta G values- Glycolysis Rate: Adjusted to Meet a Cell’s Needs for ATPo Glucose catabolism is continuously regulated by allosteric mechanismso Three allosteric enzymes that play key roles in regulating pathway: Phosphofructokinase-1 (step 3)- Activated allosterically by AMP and fructose 2,6-bisphosphate- Elevated when the cell’s energy stores are low- Activates glycolysis- Inhibited allosterically by ATP and citrate when there are high concentrations Phosphofructokinase-2- Enzyme that forms fructose 2,6-bisphosphate from fructose-6-phosphate- Feed-forward activation  high abundance of fructose-6-phosphate accelerates its formation of fructose-2,6-bisphosphate- High glucose  insulin released promotes enzyme activity  stimulates glycolysis- Low glucose  glucagon released promotes phosphofructokinase 2 phosphatase  inhibits glycolysis Other Regulatory Steps- Hexokinase  inhibited by its product, glucose 6 phosphate- Pyruvate kinase  inhibited by ATP  too much ATP slows down glycolysis- Glyceraldehyde-6-phosphate dehydrogenase  inhibited by NADH- Anaerobic versus Aerobic Metabolismo Pyruvates’ fate depends on presence//absence of O2o Aerobic respiration Pyruvate goes into mitochondria and turns into acetic acid and CO2 Acetic acid turns into acetyl-CoA NADH is producedo Anaerobic metabolism (fermentation) Purpose is to regenerate NAD+, which is required for glycolysis, to keep making ATP In muscles, reduced pyruvate becomes lactic acid In yeast, pyruvate is reduced by NADH to become ethanol- Structure and Function of Mitochondriao Two distinct membranes, inner and outer, and two distinct sub-compartmentso Uses aerobic oxidation of carbon-containing molecules to generate ATPo Number of mitochondria is regulated based on ATP needs- In the Mitochondrion: Pyruvate Oxidationo Stage 2: 3-C pyruvate is oxidized in mitochondrion matrix One molecule of CO2 produced One NADH produced Pyruvate is converted into acetyl-CoAo Most energy released is stored in NADH and FADH2  released in stage 3, the ETC- Citric Acid Cycleo Takes place in matrix of the mitochondriao Two main goals Get rid of carbons in form of CO2 (4C6C5C4C) Steal electrons that were holding those carbons and give to NAD+ and FADo What is produced/harvested in this cycle CO2, NADH, H+, GTP (a bonus!), FADH2 6 NADH and 2 FADH2 produced, these are the electron carriers!- Electron-Transport Chain: The Proton Motive Forceo Flow of electrons from NADH/FADH2 through ETC complexes drives H+ across inner mitochondrial membrane Reduction potentials of e- carriers favor downhill e- flow to form H2O- Mitochondrial ETCo E- flow through four complexes; their movement mediated by Coenzyme Q Cytochrome C- Electron and Proton Transport  Complexes I and IIo Electron pathway allows 6 protons to be translocated per pair of e- that flow from NADHto O2- Electron-Carrying Prosthetic Groupso Each complex contains several prosthetic groups that participate in the process of moving electrons from donor molecules to acceptor molecules- Oxidized and Reduced forms of e- carrierso CoQ: only ETC e- carrier not irreversibly bound to protein as prosthetic group Mobile in hydrophobic core Transports between protons and electrons 2-step reduction- Changes in Reduction Potential and Free Energy in ETCo ETC e- carrier reduction potentials favor e- flow from NADH to O2o Standard E of ETC e- increases from NADH to O2o E- pass through complexes from those with lower reduction potential to those with higher reduction potentialso Energy released as e- flow through complexes is sufficient to power pumping of H+ across membrane  proton-motive force- Harnessing the Proton-Motive Force to Synthesize ATPo Chemiosmotic hypothesis: Proton-motive force is the immediate source of energy for ATP synthesiso ATP synthase catalyzes ATP synthesis as protons flow across the membrane down their electrochemical proton gradient, which rotates the F1 y subunit- Proton translocation across the membraneo Structure ATP synthase F0 complex  3 membrane proteins: a, b, and c ATP synthase F1 complex  3 alpha-beta subunits forming a hexamer connectedto rod-shaped g unit inserted into the F0 c ring F0 a and b subunits and F1 d subunits and alpha-beta 3 hexamer  rigid structure anchored in the membraneo Proton path: through 2 proton half channels near the interfaces of subunits an and with the c subunit ring: Half-channel 1: Protons move one at a time to c subunit Half-channel 2: Protons move from c subunit into cytosolic medium Proton flow  drives rotation of c ring and attached F1 e and g subunits  rotates as a unit G subunit rotation  causes conformation changes in F1 beta subunits, leading to ATP synthesis - Photosynthesiso Produces energy-rich sugarso End products are O2 and starch/sucroseo Light-capturing and ATP-generating photosynthesis reactions occur in thylakoid membranes of chloroplasto 4 stages Light absorption, generation of high-energy e-, and O2 formation from H2O E- transport leading to reduction of NADP+ to NADPH and proton motive force Synthesis of ATP Conversion of CO2 into carbohydrates- Structure of leaf and chloroplasto Chloroplast Outer membrane  contains porins permeable to low MW metabolites Intermembrane space  continuous with lumen of crista Inner membrane  permeability barrier; contains transport proteins for metabolite import/export Stroma  contains enzymes that catalyze CO2 fixation and starch synthesis  also contains chloroplast DNA Thylakoid membrane  flattened vesicles enclosed in luminal space  also contains chlorophylls (light-absorbing pigments) Granum  stack of adjacent


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