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Understanding Energy Synthesis: ATP Production from Reduced Fuels in Animal Cells

In animals, the synthesis of ATP is primarily driven by energy derived from reduced fuels such as carbohydrates, lipids, and amino acids. The reduction of NAD+ to NADH and FAD to FADH2 facilitates the process of oxidative phosphorylation, which generates ATP from the energy stored in these electron carriers. Key proteins involved in electron transport and oxidative phosphorylation reside in the inner mitochondrial membrane. This lecture also covers the intricate mechanics of ATP synthesis, the electron transport chain, and the role of mitochondrial inhibitors and uncouplers in respiration and ATP production.

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Understanding Energy Synthesis: ATP Production from Reduced Fuels in Animal Cells

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  1. Biochemistry Lecture 12

  2. Energy from Reduced Fuels is Used to Synthesize ATP in Animals • Carbohydrates, lipids, and amino acids are the main reduced fuels for the cell • Electrons from reduced fuels used to reduce NAD+ to NADH or FAD to FADH2. • In oxidative phosphorylation, energy from NADH and FADH2 are used to make ATP

  3. C H O H 2 O O H O H O H O H • Proteins that mediate e- transport & oxidative phosphorylation are integrally bound to the inner membrane • Liver mitochondria  few cristae( respiration) • Heart mito.  many cristae ( respiration) • TCA cycle enzymes & metabolites are located in the matrix. + 6H2O 6CO2 + 24H+ + 24e- 12H2O 24H+ + 24e-+ 6O2

  4. Oxidation of NADH by O2 is Highly Exergonic NADH  NAD+ + H+ + 2e- E° = +0.32 ½ O2 + 2e + 2H+  H2O E° = +0.82 ½ O2 + NADH + H+ H2O + NAD+ E° = +1.14 V  therefore: ∆G° = -nFE° = -2 x 23 kcal/mol/V x 1.14 V = -53 kcal/mol

  5. II

  6. Antimycin a Rotenone/ amytal CN II FAD FADH2

  7. The Oxidation of NADH or FADH2 by O2 is Tightly Coupled to the Phosphorylation of ADP How many moles of ATP are synthesized from the reduction of 1 mole O2 ? 2 NADH nADP + nPi 1 O2 nATP • Measure the amount of O2 consumed (reduced to 2H2O) for any given amount of ADP added. • Experimental Conditions • - same as the inhibitor expt (no ADP initially, excess PO4) • - isolated mito’s in buffer containing excess phosphate • - addition of ADP + an electron donor starts electron transport

  8. “Artificial Respiration”: Experiments that led to Understanding the sequence of Electron Transport Proteins

  9. O2 Consumption as a function of ADP P’n ADP=90 umoles (90 micromoles) 18 umoles 30 umoles 45 umoles Conditions: Isolated mitochondria in buffer containing excess PO4. Reaction is initiated by addition of ADP and e- donor.

  10. Interpretation of Results a) b-OH-butyrate Conversion of 90 umol ADP (or PO4)  ATP requires 18 umol O2 (36 umol O) P/O = 90/36 = 2.5 b) Succinate Conversion of 90 umol ADP (or PO4)  ATP requires 30 umol O2 (60 umol O) P/O = 90/60 = 1.5 c) TMPD/Ascorbate P/O = 90/90 = 1

  11. II

  12. II

  13. Q Complex II

  14. • Oxygen is a bi-radical • Can accept e-s only 1 at a time • ETC starts with e- pairs…. • O2 •

  15. II

  16. Complex IV (cytochrome oxidase)

  17. 4 2 4 II

  18. Evidence that supports the chemiosmotic hypothesis: 1. e- transport correlates with generation of a proton gradient 2. An artificial pH gradient leads to ATP synthesis in intact mitochondria 3. Complex I,III, and IV are proton pumps 4. A closed compartment is essential 5. Proton carriers (across IMM) “uncouple” oxidation from P’n.

  19. ATP Synthase

  20. Coupling H+ translocation through ATP Synthase with ATP synthesis Free energy of H+ translocation “forces” an internal cam shaft to rotate, which changes the conformation of each subunit during one complete turn.

  21. H+ IMS MATRIX Inhibitors of Oxidative Phosphorylation • Inhibitors of Complexes I, III, & IV. • Oligomycin – antibiotic which binds to ATP synthase and blocks H+ translocation. • Uncouplers: a) Dinitrophenol (DNP).

  22. M+ M+ H+ M+ = K+ >> Na+ M+ = K+ >> Na+ b) Ionophores i) Valinomycin – carries charge but not H+’s. - Dissipates electrical gradient. ii) Nigericin – carries protons but not charge. - Dissipates chemical gradient. (due to H+)

  23. H+ H+ + H+ + glycerol FFA’S HSL + TAG Norepinephrine c) Thermogenin – active component of brown fat. - acts as a H+ channel in the IMM of brown fat mitochon. P/O << 1. Regulation of Thermogenin Conductance **Uncoupling (and heat generation) occur only if plenty of FFA substrate is available. If not, ATP synthesis prevails.

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