Oxidative Phosphorylation

Transcription

1 Oxidative Phosphorylation

2 NADH from Glycolysis must be transported into the mitochondrion to be oxidized by the respiratory electron transport chain. Only the electrons from NADH are transported, these are used to form either NADH or FADH 2. This is accomplished by one of two shuttle mechanisms.

3 Glycerophosphate Shuttle (skeletal muscle, brain) NADH, H + 3-Phosphoglycerol Dehydrogenase NAD + Cytosol H 2 C - OH C = O CH 2 OPO 3 2- DHAP H 2 C - OH HO - C - H CH 2 OPO Phosphoglycerol ETC FADH 2 Flavoprotein D hase FAD Inner mitochondrial membrane

4 Malate-Aspartate Shuttle (Liver, Kidney, Heart) Malate NAD+ Malate D hase NADH OAA Aspartate Aminotransferase Aspartate a-kg Glutamate Cytosol Malate:α-Kg carrier Inner Mito. Membrane Glutamate: Aspartate carrier a-kg Glutamate Malate NAD+ Malate D hase NADH OAA Aspartate Aminotransferase Matrix Aspartate

5 Oxidative Phosphorylation: NADH and QH 2 are oxidized by the respiratory electron transport chain (ETC). ETC is set of membrane-embedded protein complexes that act as electron carriers, passing electrons from NADH and QH 2 to molecular oxygen. As electrons move through the complexes, protons are transported across the inner mito. membrane from the matrix to the intermembrane space. The energy stored in this ion gradient is used to synthesize ATP from ADP and Pi by a membrane bound ATPase. Chemiosmotic Theory : Formulated by Peter Mitchell in the 1960 s. Nobel Prize for this work awarded in 1978.

6 Succinate 2 FADH 2 NADH 2 Complex I NAD + (-0.315V) Ε o = 0.360V; ( G o = kj/mol) Complex II CoQ (+0.45V) ADP + Pi Rotenone; Amytal ATP Demerol Fumarate Complex III Ε o = 0.190V; ( G o = kj/mol) Cytochrome C (+0.235V) Complex IV Ε o = 0.580V; ( G o = -112 kj/mol) 2e 2H + + 1/2 O - 2 H 2 0 ( V) ADP + Pi Antimycin A ATP ADP + Pi CN - ; CO ATP

7 Meaning of the standard reduction potential: When compound loses an e- (serves as a reductant), the structure left behind becomes capable of accepting an e- (serves as an oxidant). cytochrome b (Fe++) + cytochrome c (Fe+++) cytochrome b (Fe+++) + cytochrome c (Fe++) Reductant X Oxidant Y Oxidant X Reductant Y Reductant X and Oxidant X are termed a REDOX PAIR! Electrons flow from the redox pair with the more negative E 0 to the more positive E 0.

8 The Reduction of Q is a 2 Electron Reduction! 1. Q + 1 Q.- 2. Q H + QH 2

9 Electron Transport Through Complex I (NADH Dehydrogenase) 2H + intermembrane space FMN FMNH 2 2 one transfers FeS 2 one e- transfers QH 2 Q NADH NAD + H + 2H + matrix

10 Electron Transport Through Complex II (Succinate Dehydrogenase) intermembrane space FAD 2 2 one transfers FeS b one e- transfers QH 2 Q Succinate Fumarate 2H + matrix

11 Complex III (Cytochrome bc Complex) 1 Q cycl first step: 2H + intermembrane space Q b 566 Q. QH 2 Fe-S cyt. c 1 b 562 Q Q. matrix

12 Complex III (Cytochrome bc Complex) 1 Q cycl second step: 2H + intermembrane space Q b 566 Q. QH 2 Fe-S cyt. c 1 b 562 Q. QH 2 2H + matrix

13 Complete Q Cycle: intermembrane space 2 x 2H + cyt. c 2 Q 2 x 1 2 x 1 b x 1 2 x 1 Q. 2 QH 2 Fe-S cyt. c 1 b 562 Q Q. QH 2 2H + matrix

14 Electron Transport Through Complex IV (Cytochrome Oxidase) cyt. c 2H + intermembrane space 2 one transfers Cu-a Cyt. a Cu-b 2 one Cyt. a3 transfers matrix 1/2 O 2 H 2 O 2H + 2H +

15 Complex V (ATP Synthase; F F ATPase) o 1 A Rotating Molecular Motor Consumes the energy in the proton gradient to synthesize ATP from ADP. Couples the phosphorylation of ADP to the oxidation of substrates in the mitochondrion (hence oxidative phosphorylation). Mechanism of ATP synthesis is now known, as well as the x-ray crystal structure. Paul Boyer and John Walker won the Nobel Prize in 1997 for this work.

16 F1 contains the catalytic subunits; structure is α3,β3,γ3,δ,ε Fo forms a channel in the membrane that allows the passage of protons; structure is a1;b2;c9-12 Fo is sensitive to oligomycin and DCCD (DCCD reacts with a single glutamate residue on the c subunit to block the channel.

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18 Binding Change Mechanism (Boyer): F1 has three interacting and conformationally distinct active sites. Protons bind to aspartate residues in the c subunit rotor and cause it to rotate. This rotations causes the γ subunit to turn relative to the three β subunit nucleotide sites of F1, changing the conformation of each in sequence, so that ADP is first bound, then phosphorylated, then released.

19 Electron Transport Inhibitors: Rotenone; Amytal and Demerol inhibit Complex I Antimycin A inhibits Complex III Cyanide; Azide; Carbon Monoxide inhibit Complex IV Electron transport can still proceed in the presence of inhibitors if an electron donor is added that bypasses the site of inhibition.

20 Uncouplers: Lipid-soluble weak acids that carry protons from the intermembrane space back into the matrix No ATP is produced, but electron transport can proceed. Uncoupled electron transport generates HEAT. This can be useful to both plants and animals. Thermogenin (a protein) is a natural uncoupler found in brown adipose tissue.

21 The Alternative Oxidase (AO): Occurs in plants; bypasses Complexes III and IV (less ATP made) QH 2 AO 2H+ 1/2 O 2 ; 2 e- H 2 O Runs when [ATP] is high. Turned on by wounding; flowering; chemicals. May act as a protective mechanism to alleviate effects of reactive oxygen species that form when the normal chain is backed up

22 Control of Ox. Phos.: Control is tied to the cellular energy demand. This is sensed largely by [ATP]. As ATP is consumed in the cytosol, ADP is transported into the matrix (in antiport with ATP) by the adenine nucleotide translocase. Electron transport, strictly coupled to [ADP] accelerates.

23 Since ATP carries a charge of -4 and ADP carries a charge of -3, transporting an ATP from the matrix to the cytosol results in addition of one negative charge on the cytoplasmic side of the inner mitochondrial membrane. This is equivalent to moving a proton back into the matrix from the cytosol. It takes 3 protons moving through Fo to produce 1 ATP. Taking into account moving this ATP to the cytosol, we can conclude that it takes a total of 4 protons/atp synthesized.

24 How many protons are translocated per 2 e- transferred? Complex I: 4H + Complex III: 4H + Complex IV: 2H + 1 ATP X 4 H + 10 H H+ + 10H + : 2 2 (NADH 1/2 O 2 )

25 1 ATP X 4 H + 10 H H+ + 2 (NADH 1/2 O 2 ) 1 ATP X 4 H + 10 H H O P O Ratio!