THE ELECTRON TRANSPORT CHAIN Oxidative phosphorylation
Overview of Metabolism
Mitochondria Structure -Schematic
Mitochondria Structure -Photomicrograph
Overview of ETC Impermiable to ions Permiable via VDAC Matrix and cytoplasmic side
Overview of Oxidative Phosphorylation ETC
Overview of the ETC cytoplasm Cytoplasmic side + + + + + + + + Matrix side --------- Negative charges In contrast to electron pushing in organic chemistry where pairs of e- were transferred the ETC generally handles one e- at a time
Details of the Complexes We ll look at each component of the enzyme when we study it
Reduction Potentials at ph = 7 Complex IV Complex III Reduction Half-Reaction E o '(V) O 2 + 2H + + 2 e - H 2 O 0.816 Fe 3+ + e - Fe 2+ 0.771 Photosystem P700 0.430 NO 3- +2 H + +2 e - NO 2- +H 2 O 0.421 Cytochrome f( Fe 3+ )+ e - cytochrome f(fe 2+ ) 0.365 Cytochrome a 3 ( Fe 3+ )+ e - cytochrome a 3 (Fe 2+ ) 0.350 Cytochrome a(fe 3+ )+ e - cytochrome a(fe 2+ ) 0.290 Rieske Fe-S(Fe 3+ )+ e - Rieske Fe-S(Fe 2+ ) 0.280 Cytochrome c( Fe 3+ )+ e - cytochrome c(fe 2+ ) 0.254 Cytochrome c 1 ( Fe 3+ )+ e - cytochrome c 1 (Fe 2+ ) 0.220 UQH + H 1 + e - UQH 2 (UQ=coenzyme Q) 0.190 UQ + 2 H + + 2 e - UQH 2 0.060 Cytochrome b H (Fe 3+ ) + e - cytochrome b H (Fe 2+ ) 0.050 Fumarate + 2 H + + 2 e - succinate 0.031 UQ + H + + e - UQH 0.030 Cytochrome b 5 ( Fe 3+ )+ e - cytochrome b 5 (Fe 2+ ) 0.020 [FAD]+2 H + +2 e - [FADH 2 ] [FAD] = bound FAD 0.003-0.091* Cytochrome b L ( Fe 3+ )+ e - cytochrome b L (Fe 2+ ) -0.100 Oxaloacetate + 2 H + + 2 e - malate -0.166 Pyruvate + 2 H + + 2 e - lactate -0.185 Acetaldehyde + 2 H + + 2 e - ethanol -0.197 FMN + 2 H + + 2 e - FMNH 2-0.219 FAD + 2 H + + 2 e - FADH 2-0.219 Glutathione (oxidized) + 2 H + + 2 e - 2 glutathione (reduced) -0.230 Lipoic acid + 2 H + + 2 e - dihydrolipoic acid -0.290 1,3-Bisphosphoglycerate + 2 H + + 2 e - glyceraldehyde-3-phosphate+p i -0.290 NAD + + 2 H + + 2 e - NADH + H + -0.320 NADP + + 2 H + + 2 e - NADPH + H + -0.320 Lipoyl dehydrogenase [FAD ] +2 H + +2 e - lipoyl dehydrogenase [FADH 2 ] -0.340 -Ketoglutarate + CO 2 + 2 H + + 2 e - isocitrate -0.3802 H + + 2 e - H 2-0.421 Ferredoxin (spinach) ( Fe 3+ ) + e - ferredoxin (spinach) (Fe 2+ ) -0.430 Succinate + CO 2 + 2 H + + 2 e - -ketoglutarate + H 2 O -0.670
Electro-potential Gradient E o (v) -0.32 +0.03 +0.04 +0.23 Complexes I,II, III, IV Why does FADH 2 not join until after NADH + H + have already gone through Complex I? +0.82
Electron Transport Chain Energetics
Major Electron Carriers in ETC
Loss in Free Energy from High Energy Electrons to Water
Energy per proton pumped NAD + + 2H + + 2e - NADH + H + (1/2) O 2 + 2H + + 2e - H 2 O -0.32 v +0.82 v So NADH + H + + (1/2) O 2 NAD + + H 2 O +1.14 v G o = -220.1 kj/(mole NADH) or about 7.5 ATP (makes 2.5 ATP) Have 10 protons pumped or -220.1/10 = -21.8 kj/mole e - So each proton pumped carries with it -21.8 kj/mole H +
Free Radicals Up to now, we ve had heteroytic bond cleavage which resulted in transferring electrons in pairs Can also have hemolytic bond cleavage these produce free radicals
Free Radicals For example ozone depletion Generally, free radicals are short lived due to reactivity
Free Radicals and ETC All complexes in ETC transfer electrons one at a time. Therefore need stable free radicals to feed electrons to the ETC one electron at a time. An example is Flavin Mononucleotide (FMN) Indeed many of the flavin molecules like flavin adenine dinucleotide (FAD) have this property
FMN and CoQ can accept or discharge either One or Pair of Electrons
Overview of Complex I + + + + + + + + FMN accepts 2e - at a time but releases e - one at a time - - - - - - - - Electron carrier in wall of inner Membrane is QH 2 (UQH 2 )
Created Equal All NADHs Aren t
Transfer Along Chain Fe-S complexes
Complex I 1 st reaction E 0, v G o kj FMN + 2 H + + 2 e - FMNH 2-0.219 NADH + H + NAD + + 2e - + 2H + 0.32 0.101-19.4
Complex I 2 nd -6 th Reactions FMNH 2 transfers e - (one at a time) to iron complexes in protein, oxidizing iron from Fe +2 to Fe +3 4Fe-4S 2[Fe 3+ ]+ 2e - 2Fe-2S FMNH 2 FMN + 2 H + + 2 e - E o G o kj +0.219 v 2[Fe 2+ ] +0.1 to 0.4 v +0.5-96.5
Complex I 3 rd Reaction Ubiquinone (Ubiquitous Quinone, Q) Fat soluble charge carrier membrane resident Quinone 2[Fe 2+ ] 2[Fe 3+ ]+ 2e - -0.1 to -0.4 Q + 2 H + + 2 e - QH 2 0.060 one =ketone ol = alcohol
Complex I Overall reaction NADH + Q +6H + matrix side NAD + + QH 2 + 4H + cytoplasm side NADH + H + NAD + + 2e - + 2H + 0.328 CoQ + 2e - +2H + CoQH 2 0.06 0.36 v G = -69.5 kj/mole NADH Current thinking is that complex I protein binds H + on the matrix side and undergoes conformation changes to release them in the inner membrane space
Complex II (Succinate Dehydrogenase) Both Complex I and II generate QH 2
Complex II Introduced first in Citric Acid cycle No protons transported by complex II But energetic electrons transferred onto complex III and IV to pump protons FAD reduced by e- pair; FADH 2 oxidized one e- at a time
Complex - II We came across this enzyme, succinate dehydrogenase before in the Citric Acid cycle, where as you recall, it was embedded in the mitochondrial membrane And it generated FADH 2
Complex - II The succinate dehydrogenase compound is part of Complex II called the succinate-q-reductase complex FADH 2 s electrons are transferred to Fe-S centers and then onto Q to make QH 2 and oxidized back to FAD This step comes in after the NADH oxidation step since it is lower in free energy Overall reaction (potentials from Table 19.1 Succinate Fumarate + 2e - + 2H + -0.031 CoQ + 2e - + 2H + CoQH 2 0.06 +0.029 G = -5.6 kj/mole
Complex III Path shows electron flow
Complex III In complex III, we first meet cytochromes Group of red and brown heme proteins Spectra undergo color changes when undergoing oxidation and reduction First observed by an Englishman (Keilin) using a microscope on the flight muscles of insects when the immobilized insect tried to free itself Classified as a, b, or c depending on spectral characteristics
Complex III Also known as cytochrome c oxidreductase Where do we get the names cytochrome: from early work where these were identified by their absorbance color Red oxidized Green - reduced
Complex III Oxidizes QH 2 coming from complexes I and II to Q; transfer to cyt c Pumps 2 protons Rieske Fe-S differs from 2Fe-2S (seen before) since one Fe bound to two histidine groups The Heme groups are similar to Hemoglobin
Rieske Fe-S
Heme Group A cytochrome is an electron-transferring protein that contains the Heme group Histidine
All complexes process electrons one at a time Up to now, had FMN or CoQ interface between two electron carriers and discharging one electron at a time. So how can we accommodate a two-electron carrier like QH 2 without a flavin intermediary to transduce from 2 e - to 1 e - at a time? The complex III operates in 2 stages as shown on the next slide called the Q-cycle Complex III produces a single electron carrier: cytochrome c The overall equation is: 2QH 2 +2Cyt c ox + 2H + matrix 2Q + 2Cyt c red + 4H + inner Note; 2 H + pumped from matrix, 2H + comes from loss of 2H+ from QH 2
Complex III UQ universal quinone given as Q in our book
Complex III Q Cycle
Proton/Electron Balance for Complex III Fed Exiting H + e - H + e - 2 QH 2 4 4 Q Protons from matrix 2 2 Cytochrome c 2 QH 2 2 2 Protons 4 Totals 6 4 6 4 So net is one QH 2 in (total 2e-) with 2 cytochrome c out (total 2 e-)
First half of cycle QH 2 enters Q p site and releases 2H + & e - (see paths below) giving QH - The e - goes to Cyt c which departs QH - releases 1 e - giving Q and e - goes to Cyt b L b H This electron then adds to Q at Q n site giving Q - which remains bound We need to further reduce Q - so need second cycle Rieske protein
Second Half of Cycle As in 1 st cycle QH 2 enters at Q p and releases 2H + & e - and e - goes to Cyt c In this 2 nd half of cycle, the second e - released is transferred (via b L &b H ) to Q - and with 2H + from matrix reduces it to QH 2 QH 2 is released and joins Q pool Cyt c is released with its 1e -
Complex III Overall reaction: QH 2 + 2H + matrix +2Cyt c ox + 2e - 4H + inner + 2 Cyt c red + Q 2H + are pumped from QH 2 (higher energy H + ) 2H + are pumped from the matrix (lower energy H + ) It permits a 2e - carrier interact with b L, b H, Rieske complex, and cytochrome c 1 all of which are 1e - carriers Cytochrome c 1 is mobile (water soluble) 1e - carrier Releases about -36. 6 kj/(mole NADH)of free energy
Cytochrome c Mobile electron carrier (like Q) with a heme group Carries 1e - from Complex III onto Complex IV Unlike Q it is a water-soluble protein
Complex IV
Complex IV Cu +1 and Cu +2 used here Cu A Cu B. O O.
Complex IV 1 e- carriers 4 4
Complex IV Overall equation 4Cyt c red + 8 H + matrix + O 2 4Cyt c ox + 2H 2 O + 4H + inner ΔG o = -231.8 kj Need to capture as much as possible of this loss in free energy in protons pumped to cytoplasm side of membrane
Complex IV -Mechanism Red- reduced. O O.
Possible Mechanism Fe +2, Cu +1 are reduced states Fe +3 : O O : Cu +2 2e - + 2H + Fe +3 : O H H O : Cu +2 12e - + 2 e - Fe +3 : O H H O : Cu +2 + 2H + H Fe +3 Cu +2 + 2 H O
Complex IV overall reaction Overall equation for oxygen reduction 4 Cyt c red + 4H + matrix + O 2 4 Cyt c ox + 2H 2 0 ΔG o = -87.2 kj/mole But we have a total of ΔG o = -112.0 kj in this reaction Use the remainder of the energy to pump four more H + to cytoplasmic side of membrane Thus overall equation is 4Cyt c red + 8 H + matrix + O 2 4Cyt c ox + 2H 2 O + 4H + cyto
Summary of Redox Among Complexes E o, v G o kj/mol 0.36-69.5 0.085-16.4 0.19-36.6 0.58-112.0 1.21-234.5
Overall Picture Water soluble Fat soluble
Bigger Picture
Oxygen Radicals The transfer of 4 e - and 4 H + to oxygen result in the following electrochemical reaction O 2 + 4e - + 4H + 2H 2 0 0.82 v However the partial reduction of O 2 leads to very reactive radical species: O 2- and O 2 2- The enzyme does not release these radicals However, they are inevitably formed so Superoxide dismutase: 2O 2- + 2H + O 2 + H 2 O 2 Catalase: 2H 2 O 2 O 2 + 2H 2 O
Diseases Originating in ETC Atherogenesis Emphysema; bronchitis Parkinson disease Duchenne muscular dystrophy Cervical cancer Alcoholic liver disease Diabetes Acute renal failure Down syndrome Retrolental fibroplasia Cerebrovascular disorders Ischemia; reperfusion injury
Toxins and ETC