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TCA Cycle

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The Electron Transport System (ETS) and Oxidative Phosphorylation (OxPhos)

We have seen that glycolysis, the linking step, and TCA generatea large number of reduced cofactors, mostly NADH. This will later be seen to be true for β-oxidation of lipids as well.

The paths we discussed now pass those electrons to an eager receptor, oxygen, and the released energy is converted to the more common ATP coinage.

Standard state free energies show:

C6H12O6 + 6 O2 6 CO2 + 6 H2O ∆Go’ = -2823 kJ/mol

And we want to see how much of that we can get back.

Recall, anaerobic glycolysis only gave us 2 ATPs/glucose

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Figure 22-1 The sites of electron transfer that form NADH and FADH2 in glycolysis and the citric acid cycle.

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2 GTP

2 ATP

C6H12O6 + 6 H2O

6 CO2 + 10 NADH

+ 2 FADH2 + 2 ATP + 2 GTP

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Rough ER

Porins

O2, CO2H2O

Figure 22-2a Mitochondria. (a) An electron micrograph of an animal mitochondrion. (b) Cutaway diagram.

Matrix: PDC / TCA / β-oxid.

Inner Membrane: ET / OxPhos

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Mitochondria are selectively permeable to a range of chemicals. The outer membrane passes most <10K, but the inner membrane is more selective; it has a range of transporters and carriers.

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A cytosolic transport system, the glycerophosphate“shuttle”.

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Converting cytoplasmic NADP to ATP

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Figure 22-7 The malate–aspartate shuttle.

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Transport of cytosolicNADH

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Overview of the Electron Transport System

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Figure 22-13 Determination of the stoichiometry of coupled oxidation and phosphorylation (the P/O ratio) with different electron donors.

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This classic experiment uses an oxygraph to measure lose of O2. At time 0, 90 umoles ADP and excess substrate are added. It runs until substrate is exhausted and we observer 15 umoles O2, ie 30 umolesO are consumed. P/O ration is thus 90/30 = 3.

C1 is blocked by rotenone and ADP + succinate added. 90 umoles ATP are produced from 22.5 umoles O2, so P/O =2.

Etc. In each case, ADP is limiting, not the organic substrate.

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Researchers spent years searching for direct phosphorylation, creation of ATP, at each of the 3 major complexes. That is, they THOUGHT the ETS created ATP in the way glycolysis made it, by enzymatic transfer of phosphate to ADP.

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Figure 22-14 The mitochondrial electron-transport chain.

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43 proteins 900 kDa

11 proteins

250 kDa

13 proteins

160 kDa

The purpose of an ETS is to take electrons from an electron “rich” reduced compound, and hand them to an oxidizing agent, O2, tapping off the difference in energy.

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Figure 22-9 The mitochondrial electron-transport chain.

There is a sufficient voltage drop at each complex to generate ATP.

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A worked example

In the first stage of the ETS, electrons from NADH are passed through complex 1 to CoQ. NADH is oxidized and CoQ reduced. The overall reaction, and subsequent voltage drop is:

NADH + H+ NAD+ + 2e- +2H+ E0’ = 0.315 VCoQ + 2e- + 2H+ CoQH2 E0’ = 0.045 V

_______________________________________NADH + H+ + CoQ NAD+ + CoQH2 E0’ = 0.36 VIn terms of more conventional free energy measures:

∆G0’ = -nFE0’ = -2 x 96 KJ/molV x 0.36 V = -69.5 KJ/mol

Or: ∆G0’ = -nFE0’ = -2 x 23 Kcal/molV x 0.36 V = -16.6 Kcal/mol

Note: written as oxidation.

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Energetics of the ETS

We want to take electrons from NADH and “drop” them in energy to oxygen.

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ETS energy retrieval

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Electron transport involves metals and other redox susceptible chemicals.

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Figure 22-17

(a) FMN

(b) CoQ

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Figure 22-15a Structures of the common iron–sulfur clusters. (a) [Fe–S] cluster. (b) [2Fe–2S] cluster (c) [4Fe–4S] cluster

ferredoxin

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Figure 22-22a Porphyrin rings in cytochromes.

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Figure 22-19a X-Ray structure of E. coli quinol–fumarate reductase (QFR) in complex with its inhibitor oxaloacetic acid (OAA). (a) Ribbon diagram. (b) QFR’s redox cofactors (Homolog of Complex II – Succinate CoQ reductase)

The complex contains a flavoprotein (blue), an iron-cluster protein (red) and a membrane spanning proteins (green and purple)

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Complex III

11 Subunits

Figure 22-23a X-ray structures of cytochrome bc1

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Figure 22-23b X-ray structures of cytochrome bc1. (b) The yeast enzyme in complex with cytochrome cand the inhibitor stigmatellinviewed with a ~90° rotation about its 2-fold axis.

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11 Subunits

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Complex III is known in some detail

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The “Q cycle” of complex III reveals the details of the proton pump.

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Figure 22-25c X-Ray structure of fully oxidized bovine heart cytochrome c oxidase. (c) A protomer viewed similarly to Part a showing the positions of the complex’s redox centers.

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Complex IV

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Figure 22-26 The redox centers in the X-Ray structure of bovine heart cytochrome c oxidase.

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Complex IV

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Figure 22-28 Proposed reaction sequence for the reduction of O2 by the cytochrome a3–CuB binuclear complex of cytochrome c oxidase.

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The exact mechanism of O2 reduction to water is uncertain. The trick is to provide 4 electrons sequentially and maintain stable intermediates.

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Oxidative phosphorylation

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Coupling of Electron Transport with ATP Synthesis

Chemiosmotic Hypothesis

Proton Gradient

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Walker won the 1997 Nobel Prize in Chemistry for this work.

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Figure 22-38a X-Ray structure of F1–ATPase from bovine heart mitochondria. (a) A ribbon diagram. (b) Cross section through the electron density map of the protein.

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α3β3 γ1δ1ε1F

F

1

0

a1b2c9-11

Architecture of the ATP Synthase

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Mechanism of ATP Synthesis.

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Energetics of ATP Synthesis

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Figure 22-44b Rotation of the c-ring in E. coli F1F0–ATPase. (a) The experimental system used to observe the rotation.(b) The rotation of a 3.6-µm-long actin filament in the presence of 5 mM MgATP as seen in successive video images taken through a fluorescence microscope.

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Figure 22-42 Energy-dependent binding change mechanism for ATP synthesis by proton-translocating ATP synthase.

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Membrane transporters complement the synthesis of ATP in the mitochondria

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Figure 22-46 Uncoupling of oxidative phosphorylation.

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DNP

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Figure 22-47 Mechanism of hormonally induced uncoupling of oxidative phosphorylation in brown fat mitochondria.