Electron Transport & Oxidative Phosphorylation

Question 1

What is the singular event in cellular respiration that requires oxygen?

Answer 1

Oxidative phosphorylation.

Question 2

After finishing the citric acid cycle, where does most of the energy from glucose, fat and proteins reside?

Answer 2

It resides in the high-energy electrons of NADH and FADH2.

Question 3

Where does oxidative phosphorylation take place?

Answer 3

The electron carriers and enzymes that carry out OXPHOS are localized to the plasma membrane of prokaryotes and the inner mitochondrial membrane in eukaryotes.

Question 4

Why is the composition of the intermembrane space of the mitochondria equivalent to the cytosol?

Answer 4

The outer membrane of the mitochondria is high permeable, allowing almost everything in the cytosol through the outer membrane of the mitochondria.

Question 5

What features of the mitochondria are evidence that they were once independent prokaryotic organisms?

Answer 5

Like prokaryotes, mitochondria have a double membrane, a circular DNA genome, and prokaryote-like translation machinery (ribosomes/tRNAs)

Question 6

Why are the B-sheets of porin channels stable?

Answer 6

Peptide bonds are H-bonded across strands of B-sheets rolled into a barrel structure. This structure is stable because the polar covalent bonds along the polypeptide backbone are stabilized through hydrogen bond interactions along the length of the B-sheets. As long as there are hydrophobic amino acids from the transmembrane segments, the B-barrel will be stable.

Question 7

How many transmembrane segments do you need to traverse the outer mitochondrial membrane, and how many amino acids are in each?

Answer 7

You need 8-22 transmembrane segments of 8-10 amino acids each.

Question 8

How can a microscopic image of the mitochondria be taken?

Answer 8

An EM technique called acid etching can do this. It takes a thin section of what needs to be viewed and puts an acid on it.

Question 9

Define electron transport chain.

Answer 9

The electron transport chain is a large collection of proteins in the inner membrane organized into massive multi-protein complexes. It can be likened to a “bucket brigade” which incrementally harvests the energy of electron transfer and passes the charge from one link in the chain to the next to generate ATP.

Question 10

How does electron affinity change from the first to third protein complex of the electron transport chain?

Answer 10

From the first to the third, the electron affinity increases.

Question 11

Describe the NADH dehydrogenase complex of the electron transport chain.

Answer 11

This complex (made of over 40 polypeptides) is the first in the electron transport chain. It is composed of proteins from both the nuclear and mitochondrial DNA. The function of this complex is to receive two electrons from a high-energy electron carrier (NADH).

Question 12

What is the role of ubiquinone (coenzyme Q) in the electron transport chain?

Answer 12

Ubiquinone takes electrons from the first complex to the second complex. (from the NADH dehydrogenase complex to the cytochrome b-c1 complex)

Question 13

What is the second protein complex in the electron transport chain?

Answer 13

The cytochrome b-c1 complex is the second. It is a dimer with at least 11 polypeptides.

Question 14

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

Answer 14

Cytochrome c transfers electrons from cytochrome b-c1 to the cytochrome oxidase complex.

Question 15

What is the third protein complex in the electron transport chain?

Answer 15

The cytochrome oxidase complex is the third. It is a dimer with 13 polypeptides.

Question 16

How and where are free oxygen radicals generated?

Answer 16

Free oxygen radicals are generated when cyotchrome oxidase finally transfers its electrons to oxygen by holding tightly to O2 until fully reduced to 2H2O (this requires 4 electrons).

Question 17

What powers the electron transport chain?

Answer 17

As an electron pair is transported through the protein complex, 4 protons (H+) are transported from the matrix to the intermembrane space. The movement of these protons generates energy which accomplishes the work of the pump.

Question 18

How many protons are transported from the matrix to the intermembrane space in each of the three protein complexes of the electron transport chain?

Answer 18

For the NADH dehydrogenase and the cytochrome b-c1 complexes, 4 protons are transported. For the cytochrome oxidase complex, the same process occurs, but it transports only 2 protons.

Question 19

In total, how many protons are transported across the inner membrane to establish an electrochemical proton gradient?

Answer 19

10 protons (H+).

Question 20

How does redox potential change as the electron transport chain progresses?

Answer 20

As the electron transport chain moves toward the final oxygen molecule, the complexes gain redox potential. The overall reaction has a change in redox potential of about 1.2 V (this is a significant amount).

Question 21

How does the free energy per electron change as the electron transport chain progresses?

Answer 21

The free energy per electron decreases. With each of the three complexes in the chain, there is a large free energy drop associated with the electron transport there. This change is about -52.6 kcal/mol. When free energy drops, work can be done.

Question 22

Why does FADH2 bypass the NADH dehydrogenase complex of the normal electron transport chain?

Answer 22

FADH2 is a lower-energy carrier. The electrons are not in a high enough energy state to pass through the NADH dehydrogenase complex. Therefore, it enters the chain at the succinate dehydrogenase complex.

Question 23

When FADH2 acts as the electron carrier for the electron transport chain (vs NADH), how many total protons cross the inner membrane?

Answer 23

6 protons (H+) pass across the inner membrane. This is in comparison with the 10 protons that cross the normal electron transport chain.

Question 24

How is a proton gradient used to power ATP synthesis?

Answer 24

Proton pumping coupled to electron transport generates a steep (10-fold) electrochemical gradient. The pH of the matrix is 8, vs a pH of 7 in the intermembrane space. The gradient generated is called the proton motive force. It combines the pH gradient and the voltage gradient to power ATP synthesis.

Question 25

For the F0F1 ATP synthase, what does the F0 and F1 parts refer to?

Answer 25

F0 refers to the section within the membrane and in the intermembrane space. F1 refers to the section that sticks into the matrix. This is the rotational catalyst.

Question 26

What powers the rotation of the rotor and stalk of the F0F1 ATP synthase?

Answer 26

Protons flowing through the transmembrane channel (formed by subunit a and c ring) power the rotation of the rotor and stalk. The rotation of the y-subunit in stalk drives conformational change in a/B catalytic heads, driving ATP synthesis.

Question 27

How many synthase complexes are found in a typical liver mitochondrion?

Answer 27

About 15,000 ATP synthase complexes.

Question 28

What does electron cryomicroscopy EM image analysis, when combined with x-ray crystallography, reveal?

Answer 28

It takes the average of >1000 dimers to create an image of the structure of a protein. The images obtained by both techniques are superimposed on one another for a better image.

Question 29

How many protons flow through the F0F1 ATP synthase turbine per ATP generated?

Answer 29

3 protons flow through. The movement of protons through each state of the turbine powers conformational changes.

Question 30

How is the ATP synthase reversible?

Answer 30

The regular proton-powered ATP pump can be reversed to an ATP-powered proton pump. There is normally a proton gradient established across the inner membrane. However, if you equalize that gradient, you can cause the ATP synthase pump to work in the opposite direction. Then, it will consume ATP rather than make it.

Question 31

What does the DNP diet drug do?

Answer 31

This drug is an ionophore that inserts into the inner membrane of the mitochondria and reverses the function of the proton-powered ATP synthase and removes the proton gradient.