Chapter 20

Question 1

Overview of Oxidative Phosphorylation

Answer 1

• Oxidative phosphorylation captures the energy of high energy electrons to synthesize (create) ATP
• Flow of electrons from NADH and FADH2 to oxygen occurs in ETC (respiratory chain)
• Exergonic set of oxidation-reduction reactions generate a proton gradient. Proton gradient is used to power synthesis (creation) of ATP
• CAC and oxidative phosphorylation are called cellular respiration or respiration

Question 2

What is respiration?

Answer 2

Respiration is an ATP-generating process where an organic compound (ex. oxygen) serves as ultimate electron acceptor

• Electron donor can either be organic or inorganic compound
• Cellular respiration takes place in bacterial cells w/ or w/o use of oxygen as final electron acceptor
• Oxidation and ATP-synthesis are coupled by transmembrane proton fluxes
• Respiratory chain transfers electrons from NADH and FADH2 to oxygen, which generates a proton gradient. ATP-synthase (which catalyzes the formation of ATP) converts energy of proton gradient to ATP within mitochondrial matrix

Question 3

What is endosymbiosis, or the endosymbiotic theory?

Answer 3

Endosymbiosis (endosymbiotic theory) has been shown from evidence that all mitochondria are descendants of an independent organism (Rickettsia prowazekii) that was engulfed by another cell

Endosymbiotic Theory:
- Mitochondria produced energy for cells, and cells provided raw materials and proteins to the mitochondria

• The two adjacent cells were living in very close proximity to one another
• The Larger cell that was providing raw materials engulfed the smaller cell, which was the mitochondria that was producing energy

Question 4

What are the 5 pieces of evidence that support the endosymbiotic theory?

Answer 4

1. Mitochondria has its own circular DNA, similar to the ancestral prokaryotic cells that also have circular DNA

• Mitochondria have some DNA but not enough to be independent, and the DNA is maternally inhibited

2. Mitochondria have ribosomes that are 70S
3. Mitochondria are roughly the same size and shape of prokaryotic cells
4. Mitochondria have a double membrane
5. Mitochondria within cells can also undergo binary fission ~ a type of division that prokaryotic cells routinely undergo

• Even though prokaryotic cells lack a mitochondrion, prokaryotic cells are still able to perform cellular respiration across their cell membranes

Question 5

Describe the structure of the mitochondria

Answer 5

• Mitochondria are bound by a double membrane
• Intermembrane space is the space between outer and inner membrane
• Mitochondrial matrix = innermost region of mitochondria

OUTER MEMBRANE:
- Is permeable to most small ions and molecules due to channel proteins called mitochondrial porins
- Generally, resembles outer cell membranes

INNER MEMBRANE:
- More impermeable to most molecules
–> There will be a high content of specific transport proteins

• Has several folds and ridges (cristae) which allow for ample surface area for the ETC and synthesis of ATP
• Has high amounts of cardiolipin

Question 6

What is cardiolipin?

Answer 6

• Unique phospholipid localized and synthesized in the inner mitochondrial membrane
• Plays central role in many reactions and processes involved in mitochondrial function and dynamics

Question 7

True or False: Oxidative phosphorylation depends on electron transfer

Answer 7

True

Question 8

The electron transfer potential of an electron is measured as _______

a) redox potential
b) calcium potential
b) ATP-synthase

E0’ = reduction potential/redox potential/electron transfer potential = a measure of the molecules tendency to donate or accept electrons

Answer 8

redox potential

Question 9

Strong reducing agent (ex. NADH) readily _____ electrons = ______reduction potential

Answer 9

donates; negative

Question 10

Strong oxidizing agent (ex. O2) readily _____ electrons = _____ reduction potential

Answer 10

accepts; positive

Question 11

Which of the following statements are true with regards to electron movement?

a) Electrons move down a chain of protein complexes
b) The complexes of the ETC are located in the inner mitochondrial membrane
c) Electrons will go from high electron transfer potential to low
d) all of the above

Answer 11

d) all of the above

Question 12

List the proteins that contain electron carriers

Answer 12

• flavin mononucleotide (FMN)
• iron sulfur proteins (iron associated w/ Sulphur in proteins)
• cytochromes (iron incorporated into hemes in proteins)
• Co-enzyme Q10A (mobile electron carrier called CoQ10)

Question 13

True or False: Electrons from FADH2 enter via succinate Q-reductase

Answer 13

TRUE

Question 14

ETC Complexes: Overview

Answer 14

• Electrons flow from NADH to oxygen through large protein complexes located in inner mitochondrial membrane
• As electrons flow from complex to complex, protons are pumped out of the matrix and into the inner membrane space, generating a proton gradient

The complexes include the following:

1. Complex I: NADH-Q Oxidoreductase
2. Complex II: Succinate Q-Oxidoreductase
   - not a proton pump; gets electrons from FADH2
   - Since FADH2 is entering at complex II and not at complex I, the total # of protons pumped will be less = less ATP generated
3. Complex III: Q-cytochrome c oxidoreductase
4. Complex IV: cytochrome c oxidase

Question 15

ETC Complexes: Flavin Mononucleotide (FMN)

Answer 15

 FMN = electron carrier
 similar structure to FADH, but lacks the nucleotide adenine

Question 16

ETC Complexes: Cytochrome c

Answer 16

• Cytochrome c = protein w/ heme group
• Is soluble in an aqueous environment and can transfer electrons from complex III to complex IV
• The iron atom will transition from the ferrous form (Fe2+: reduced state) to the ferric form (Fe3+: oxidized state)

Question 17

ETC complexes: Coenzyme Q ubiquinone & Q-pool

Answer 17

• Coenzyme Q ~ also called ubiquinone b/c its a “ubiquitous” quinone
• Coenzyme Q10 ~ 10 stands for the # of isoprene units in the side chain
• Long hydrophobic isoprene tail keeps Q within the lipid bilayer of the membrane
• Co-Q10 serves as a shuttle for electrons (and protons) by moving electrons from complex I and complex II to complex III
• Electron transfer reactions are couple to proton binding and release
• When Co-enzyme Q (or just Q or ubiquinone) binds to protons, this will generate QH2
• Oxidized and reduced forms of Q are present in the inner mitochondrial membrane, and present within the Q pool
• Reduction states of ubiquinone (Q) into ubiquinol (QH2) occurs via a semiquinone intermediate (QH), which can be transfer via oxidation into a semiquinone radical ion (Q * -)

Question 18

Complex I: NADH-coenzyme Q Oxidoreductase

Answer 18

PROCESS:

High-potential electrons of NADH enter the respiratory chain at NADH-coenzyme Q Oxidoreductase, also called: Complex I, NADH-Q oxidoreductase, or NADH-dehydrogenase

Several prosthetic groups are associated with this complex: FMN and iron-sulfur clusters

1. Electrons flow from NADH to FMN, then through iron-sulfur clusters, and then to Q (ubiquinone) to form QH2 by Complex-I
2. QH2 leaves the enzyme for the Q-pool in hydrophobic interior of the inner mitochondrial membrane
3. 4 protons are simultaneously pumped from matrix into intermembrane space

NOTES:

• NADH-coenzyme Q oxidoreductase = very large complex with approx. 45-polypeptide chains organized into 14-core subunits. This complex is highly conserved amongst all organismal species
• Complex results in an L shaped proton pump: there’s a hydrophobic horizontal arm embedded into the membrane; and a vertical hydrophilic component that projects into the matrix

Question 19

Complex II: Succinate-Q Oxidoreductase

Answer 19

• Succinate Q-reductase = second complex in ETC
• This is an integral membrane protein within the inner mitochondrial membrane
• FADH2 is generated in CAC
• FADH2 is generated w/ reaction catalyzed by succinate dehydrogenase in oxidation of succinate to form fumarate
• The enzyme for the reaction for the CAC step is associated w/ the complex of the ETC
• Enzyme succinate dehydrogenase is incorporated into succinate Q-reductase complex
• Electrons from FADH2 are passed to ubiquinone, also known as Q, thereby reducing Q to QH2
• QH2 then enters the Q-pool
• NOTE: no protons are transported

Question 20

Complex III: Q-cytochrome c Oxidoreductase

Answer 20

Receives electrons from QH2 and delivers to cytochrome c
- QH2 passes 2-electons and 2-protons are pumped during this process
- But cytochrome c can only receive 1 electron at a time

Q-cycle: coupling electron transfer to proton pumping
- 2-half cycles:
- First half: QH2 enters and leaves as Q into Q-pool
1 e- passed directly to Cytochrome c
1e- passed to Q  makes a Q- (semiquinone radical anion)
- Second half: 2nd QH2 enters and leaves as Q into Q-pool
1 e- passed directly to Cytochrome c
1e- to Q-  makes a QH2 that goes back to the “Q pool”

Net reaction: 4 protons from matrix to intermembrane space
- reduces 2 Cyt c
- 2 additional protons used in the QH2 regeneration

Question 21

Complex IV: Cytochrome c oxidase

Answer 21

• Cytochrome C oxidase catalyzes reduction of molecular oxygen to water
• Cytochrome C oxidase accepts 4-electrons from 4-molecules of cytochrome C to catalyze reduction of oxygen to 2 molecules of water
• In the cytochrome C oxidase reaction, 8-protons are removed from matrix:

 4-protons called chemical protons are used to reduce oxygen
 in addition, 4-protons are pumped into the inner membrane space
 This outlines the purpose of oxygen in aerobic metabolism

Question 22

Total of ___ protons are pumped per NADH through the ETC

a) 12
b) 10
c) 20
d) 15

Answer 22

b) 10

Question 23

Total of ____ protons are pumped per FADH2 through the ETC

a) 10
b) 7
c) 5
d) 6

Answer 23

d) 6

Pumping of these protons creates the proton gradient and the flow of protons back into the matrix, which will drive ATP-synthesis

Question 24

What is a respirasome?

Answer 24

The depiction of the ETC is often as complexes in an assembly line in the inner mitochondrial membrane

Experimental evidence has shown that the complexes coalesce to create a super molecular complex termed a respirasome that is ~1.7 MDa in size.

Within this respirasome there are multiple copies of the individual complexes: 2-copies of complex-I, 2-copies of complex-II, and 2-copies of complex III.

Question 25

True or False: In aerobic metabolism, w/ the use of oxygen as the final electron acceptor, is ideal as it has high electron affinity, and full reduction of oxygen is ideal

Answer 25

True

Question 26

What are reactive oxygen species? How are they generated? List some examples?

Answer 26

Partial reduction of oxygen generates highly reactive oxygen derivatives called reactive oxygen species

Reactive oxygen species are in many pathological conditions and can damage cellular proteins structurally and functionally

Some species include:
1. superoxide ion
2. peroxide ion
3. hydroxyl radical

2-4% of oxygen molecules consumed by mitochondria are converted into superoxide ions

Superoxide dismutase and catalase help to protect against the damage that could potentially be caused by reactive oxygen specie