How Does the Reducing Equivalents Go from the Cytoplasm to the Mitochondrion?

• How does the reducing equivalents go from the cytoplasm to the mitochondrion? They don’t cross the membrane, but electrons do through two ways:

• Glycerol 3-phosphate shuttle:

1. In cytoplasm, DHAP is reduced to glycerol 3-phospahe by NADH. Enzyme: cytoplasmic Glycerol 3-phoshate dehydrogenase.
2. FAD is reduced to FADH2 in the enzyme called mitochondrion Glycerol 3-phoshate dehydrogenase. FAD is prosthetic group in the enzyme.
3. Ubiquinone (Q) is reduced to Ubiquinol (QH2)

• Malate-Aspartate shuttle:

1. In cytoplasm, OAA + NADH → Malate + NAD+. Enzyme: cytoplasmic Malate dehydrogenase.
2. Malate is transported into the Matrix
3. Malate + NAD+ → OAA + NADH. Enzyme: Mitochondrial Malate dehydrogenase.
4. Glutamate is transformed to a-Ketoglutarate , and OAA is transformed to Aspartate. Enzyme: Mitochondrion Aspartate amino transferase. 

Glu+ OAA → Asp + a-ketoglutarate.

1. Asp + a-ketoglutarate are transported to the cytoplasm.
2. Asp + a-ketoglutarate → Glu+ OAA. Enzyme: cytoplasmic Aspartate amino transferase.

• The more negative Eo, the greater the electron-transfer potential and the greater the tendency to donate electrons. The more positive Eo, the greater the greater the tendency to accept electrons.

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• Electron Transport Chain: Integral proteins

1. Complex I (NADH-Q oxidoreductase, NADH dehydrogenase):

1. Pumps protons? Yes
2. Electron source: NADH
3. Electron carriers: FMN – multiple Fe-S centers
4. Ultimately reduce: Ubiquinone

1. Complex II (Succinate-Q reductase, succinate dehydrogenase):

1. Pumps protons? No
2. Electron source: Succinate
3. Electron carriers: FAD – multiple Fe-S centers
4. Ultimately reduce: Ubiquinone

1. Complex III (Q-cytochrome C oxidoreductase):

1. Pumps protons? Yes
2. Electron source: Ubiquinol
3. Electron carriers: 3 cytochromes– Rieske Fe-S centers
4. Ultimately reduce: Cytochrome C

1. Complex IV (Cytochrome C oxidase):

1. Pumps protons? Yes
2. Electron source: cytochrome C
3. Electron carriers: CuA/cytochrome a- CuB/cytochrome a3
4. Ultimately reduce: O2

1. Ubiquinone (oxidized) / Ubiquinol (reduced)

1. Hydrophobic; travels within the inner membrane
2. Accepts electrons from G3P, complex I, and complex II.
3. Carries electrons to complex III.
4. There is semiquinone intermediate.

1. Cytochrome C

1. Hydrophilic; travels within the intermembrane space.
2. Accepts electrons complex III
3. Carries electrons to complex IV.

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• Complex III has 3 cytochromes that contain hemes: bH, bL, c1. Electrons are transported from one Fe to another until reaching rieske Fe-S center. They are coordinated by cysteines and histidines.
• In complex IV:

1. 2 cytochromes C transfers electrons to reduce CuB/heme a3
2. reduced CuB/heme a3 bind oxygen to form peroxide bridge.
3. 2 electrons and 2 protons cleaves the bridge.
4. 2 more protons release the water

• there are histidine and tyrosine adducts

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• Mitochondrial ATP synthasome:

• ATP synthase
• ATP/ADP antiporter
• Inorganic phosphate transporter

• ATP synthase has two regions:

1. F0: region in which protons flow.
2. F1: the site of ATP synthesis using rotational energy.

• Rotating components:

1. C rings: contains Apartate/ Aspartic acid
2. γ: central shaft; asymmetric; cause change in β subunits when rotating  
3. ε

• Stationary components:

• Stator:

1. a: cytoplasmic half channel/matrix half channel
2. b2
3. δ

• Hexameric ring:

1. α3: for structure
2. β3: site of ATP synthesis

• Conformations in β subunits:

1. Open: ADP and P are bound; ATP is released
2. Loose: ADP and P are bound
3. Tense: ATP is synthesized.

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