Electron Transport Chain & NADH Shuttle Systems

Glycerol-3-phosphate Shuttle

• skeletal muscle

• enzymes:

  • cytoplasmic Glycerol-3-phosphate dehydrogenase

  • mitochondrial Glycerol-3-phosphate dehydrogenase

Mechanism for Glycerol-3-phosphate Shuttle

• NADH+H+ is oxidized to NAD+ while DHAP is reduced to Glycerol-3-phosphate by Cytoplasmic Glycerol-3-phosphate dehyrogenase

• Glycerol-3-phosphate is oxidized to DHA while FAD is reduced to FADH2 by Mitochondrial Glycerol-3-phosphate dehyrogenase

• FADH2 transfers its electrons to coenzyme Q (ubiquinone) → QH2

  • QH2 (ubiquinol) enters ETC

Malate-Aspartate Shuttle

• heart and liver cells

• enzymes:

  • cytoplasmic malate dehydrogenase

  • mitochondrial malate dehydrogenase

  • cytoplasmic aspartate aminotransferase

  • mitochondrial aspartate aminotransferase

  • malate-a-ketoglutarate antiporter (membrane bound)

  • glutamate-aspartate antiporter (membrane bound)

Malate-Aspartate Shuttle Mechanism

• NADH+H+ is oxidized to NAD+ while oxaloacetate is reduced to malate by cytoplasmic malate dehydrogenase

• malate traverses mitochondrial membrane via malate-a-ketoglutarate antiporter and a-ketoglutarate goes out

• malate is oxidized back to oxaloacetate as NAD+ is reduced to NADH+H+ by mitochondrial malate dehydrogenase

• a-ketoglutarate must be present in matrix and oxaloacetate must be present in cytoplasm to make the shuttle function

How to keep Malate-Aspartate Shuttle working?

• mitochondrial aspartate aminotransferase transfers an amine group from glutamate to oxaloacetate to produce aspartate and a-ketoglutarate

• aspartate is shuttled to cytoplasm via glutamate-aspartate antiporter (glutamate goes in)

• cytoplasmic aspartate aminotransferase transfers an amine group from aspartate and a-ketoglutarate to produce glutamate and oxaloacetate

Negative Reduction Potential

oxidized form of a substance has a lower affinity for electrons than H2 (ex. NADH is more likely to donate electrons)

Positive Reduction Potential

oxidized form of a substance has a higher affinity for electrons H2 (ex, O2 is more likely to accept electrons)

What is the driving force of electron-transfer potential?

electrons always flow to components with more positive reduction potentials

Ubiquinone

• Q

• fully oxidized form

Semiquinone, Radical

• Q*-

• addition of one electron

Semiquinone

• QH*

• addition of one electron and one proton

Ubiquinol

• QH2

• fully reduced form

Complex I (NADH Dehydrogenase)

1. NADH reduces FMN to produced FMNH2

2. FMNH2 donates ep to iron-sulfur cluster proteins (one at a time)

3. Iron-sulfur proteins donate e- to Q (one at a time)

4. QH2 leaves the complex (travels to complex III)

   1. flow of 2 e- from NADH to Q pumps out 4 protons

Complex II (Succinate Dehyrogenase)

1. Succinate reduces FAD to FADH2

2. FADH2 donates e- to iron-sulfur cluster proteins (one at a time)

3. Iron-sulfur proteins donate e- to Q (one at a time)

4. QH2 leaves the complex (travels to complex III)

   1. flow of 2 e- from FADH2 does not pump out any protons

Complex III (Cytochrome c reductase)

QH2 is oxidized and donates 2 e- to complex

1. one e- goes to iron-sulfur cluster. e- is transported to Cyt c1 and then Cyt c which pumps out 2 protons

2. one e- is sent to Cyt bH which is transported to Cyt bL and then back to Q to generate Q*-

3. remaining Q becomes a part of “Q pool”

4. a second QH2 enters and repeats 1-3, except e- sent to Cyt bH is transported to Q*- and is reduced back to QH2, 2 protons are pumped out

Complex IV (Cytochrome c oxidase)

1. two Cyt c each donate an e- each to 1 CuA peptide

2. 1 CuA reduces 2 Cyt a

3. 2 Cyt a reduces 2 Cyt a3

   1. 1 Cyt a3 reatains e-

   2. 1 Cyt a3 donates its e- to CuB peptide

4. Cyt a3 and CuB bind and reduce O2 to form peroxide bridge

5. repeat 1-4, except peroxide bridge gets cleaved to form ion-oxygen adjuncts from uptake of 2 protons

6. uptake of 2 more protons converts the ion-oxygen adjuncts to 2 H2O

Overall ETC

Complex I: 4 protons

Complex II: 0 protons

Complex III: 4 protons

Complex IV: 2 protons