14: Energy Conversion: Mitochondria and Chloroplasts

Question 1

Which regions of the mitochondrial membrane contain ATP synthase and the protein complexes of the respiratory chain?

Answer 1

The cristae - specialized invaginations of the inner memb.

Question 2

Name some of the roles of mitochondria in cellular metabolism

Answer 2

Buffer the redox potential of the cell (provides electron transfer: NAD+ -> NADH)
- Important in the metabolic adaptation of cells to different nutritional conditions.

Central role in memb. biosynthesis:

• Produces cardiolipin (two-headed phospholipid) for own inner memb.
• Produces phosphatidyletanolamine, phosphatidylglycerol, phosphatidic acid

Important calcium buffers, taking up calcium from the ER and sarcoplasmic reticulum

Question 3

Give a brief description of the processes in the respiratory chain

Answer 3

1. 2e + H+ are removed from NADH->NAD+
2. 2e passed to an electron carrier in the inner memb.
   
   • e have large negative redox potential - high energy level - that gradually drops as they pass along the chain.
3. Three respiratory enz. complexes: each has a higher affinity for electrons than its predecessor.
4. Molecular oxygen is the final e-acceptor (highest affinity)

Net result: pumping of H+ out of the matrix across the inner memb. driven by the electrically favorable flow of electrons.
=> pH-gradient (highest in matrix) and voltage gradient => memb. potential, neg. in cristae, pos. in crista space

ΔpH + ΔV = electrochemical gradient (~180 mV)
=> proton-motive force, e back into matrix.

Question 4

What is the path of e-flow in the electron transport chain?

Answer 4

NADH → NADH dehydrogenase complex → ubiquinone (coenzyme Q) → cytochrome c reductase → cytochrome c → cytochrome c oxidase complex → molecular oxygen (O2)

Question 5

What is a redox potential?

Answer 5

Affinity for electrons.
Defined as E_0, measured for a std. state where all of the reactants are at a concentration of 1M (E’_0)
Weak affinity (low potential) are useful as carriers/donors.
Redox pair: oxidized/reduced forms of a compound.

Electrons move spontaneously from a redox pair like NADH/NAD+ with a lower redox potential (lower affinity for electrons) to a redox pair like O2/H2O with a higher redox potential.

Question 6

What are the electron-transfer cofactors of the respiratory chain?

Answer 6

Transition ions (Fe, Cu, Ni, Mn) bound to proteins in the complexes.
Cytochromes contain a bound heme group, in which an iron atom is tightly held. 1e at a time.
Iron-sulfur clusters. 1e at a time.

Simplest cofactor: quinone (ubiquinone/coenzyme Q), not always protein-bound. Small hydrophobic molecule that is freely mobile in the lipid bilayer. Donate 1 or 2e. Picks up a proton from water with each electron when reduced.

Mitochondrial e-transport chain:

• 6 cytochrome hemes
• 8 iron-sulfur clusters
• 3 copper atoms
• 1 flavin mononucleotide
• ubiquinone

Iron-sulfur dominates at the start (low e affinity).
Heme cytochromes dominate at the 2nd half.

Question 7

What is the contribution of NADH dehydrogenase to the proton gradient?

Answer 7

NADH -> 2e is transferred in the matrix arm via monoflavin nucleotide and iron-sulfur clusters.
2e reduces quinone (Q->QH2), giving energy to the transfer of 4H+ into the crista space.

Question 8

How are protons transferred from NADH dehydrogenase to cytochrome c reductase and further?

Answer 8

Via quinone and the Q cycle.
Cyt C reductase has a binding site for a 2nd QH.
1e transferred through cyt c red. to the carrier protein cytochrome c.
1e is recycled back into the quinone pool

=> for every e that is transferred from NADH dehydrogenase to cytochrome c, 2 protons are pumped out of the matrix.

Question 9

What is the role of cytochrome c oxidase in the electron-transport chain?

Answer 9

Binds O2 at bimetallic center clamped between a heme-linked Fe atom and a Cu atom until it has picked up 2e.
Prevents the formation of radicals.
Release of 2H2O
Enables 90% of O2 uptake in most cells.

Question 10

What is the advantage of the supercomplex in the crista membrane? What lipid holds the components together?

Answer 10

Help the mobile electron carriers ubiquinone (crista memb.) and cytochrome c (crista space) transfer electrons with high efficiency.

Cardiolipin

Question 11

How can ATP synthase produce ATP when it is an energetically unfavorable process?

Answer 11

Allosteric shape changes in ATP synthase couple ATP synthesis (energetically unfavorable) directly to the energetically favorable flow of protons across its memb.

Question 12

What is the ATP yield from the complete oxidation of one glucose molecule?

Answer 12

30 ATP (theoretically)