Oxidative Phosphorylation

Question 1

Where does oxidative phosphorylation take place?

Answer 1

Mitochondria (IMM)

Question 2

What are key architectural features of mitochondrial membrane?

Answer 2

Outer mito mem (OMM)
-porins
Intermembrane space
-composition similar to cytoplasm 
IMM 
-proteins, cardiolipins, OxPhos
-impermeable
-cristea for SA
Metric 
-Kreb's 
-beta oxidation 
-mtDNA 
-pH ~8

Question 3

What are key elements of chemiosmotic theory?

Answer 3

• electron transport drives proton pumping
• oxygen serves as final electron acceptor to produce water
• pumping of protons out of matrix to IMS creates electrochemical gradient - electrical gradient (mem pot.) + pH gradient = proton motive force
• protons flow down the gradient into matrix to drive ATP synthesis that is dependent on ADP availability

Question 4

What is negative reduction potential?

Answer 4

Reduced species has a tendency to donate e-
Strong reducing agent has (-) red. pot. (NADH)

Reduction potential is the tendency of the oxidized species of a redox couple to accept e- (become reduced)

Question 5

Why does ETC transfer electrons by default?

Answer 5

Electrons flow down an energy gradient, from low to high reduction potential

(neg redox potential = reduced species of redox couple has high tendency to donate e-)

Question 6

Why is O2 used as final e- acceptor?

Answer 6

Strong oxidizing agent, wants to accept e-

Liberates free energy used to produce ATP when reduced to H2O (respiration)

Question 7

What are six key components of ETC?

Answer 7

Complex I: NADH Co-Q Oxidoreductase

Complex II: succinate dehydrogenase

CoQ: Oxidized form ubiquinone, reduced form ubiquinol

Complex III: Co-Q cytochrome c oxidoreductase

Cytochrome c

Complex IV: Cytochrome c oxidase

Question 8

Where do e- enter and how does entry point affect efficiency (P/O ratio)?

Answer 8

Entry at complex I allows 4 more H+ to be pumped (total of 10) - 10/4 = 2.5 ATP for electrons entering as NADH

Entry at complex II does not have that proton pumping (total of 6) - 6/4 = 1.5 ATP for electrons entering at complex II

Question 9

Which complexes contribute to proton pumping?

Answer 9

Proton pumping occurs at complexes I, III, IV.

4,4,2

Question 10

How is e- transport coupled to ATP synthesis?

Answer 10

Through proton gradient
Proton influx drives rotation of ATP synthase, permits ATP synthesis

ADP binding required (demand driven) 3H+/ATP

(1 more H+ to transfer out of mit.)

Proton pumping cannot occur without electron flow, and electron flow cannot occur without proton pumping.

Question 11

What is the difference between state 3 and state 4 respiration?

Answer 11

State III is limited by ADP

• demand driven system
• proton leak + rate of ATP use (ADP) set the pull

State IV

• high membrane potential favors electron leak (ROS)
• low demand (high ATP, low ADP)
• compounds want to donate electrons to something

Question 12

What factors control rates of respiration?

Answer 12

ATP/ADP ratio

ADP is necessary for ATP synthesis - demand driver

Question 13

What conditions favor mitochondrial ROS production and why?

Answer 13

High ATP, low ADP
High membrane potential
State IV
Compounds want to donate e- to something, will donate e- elsewhere is ETC is reduced

Question 14

What is meant by respiratory uncoupling? How does this occur?

Answer 14

Proton leaking to destroy chemiosmotic gradient
Uncoupling protein (UCP): channel to allow influx of protons to matrix (dissipates proton gradient)
Brown adipose tissue (lots of mit)
Uncoupling of e- transport and OxPhos - produces heat instead of ATP (non shivering thermogenesis)
Results in increased substrate catabolism and lower ROS

Question 15

What are the pros/cons of targeting mitochondrial biogenesis as an anti-obesity drug strategy?

Answer 15

Would induce mitochondrial biogenesis - probably increase exercise capacity
But, just making more mitochondrial doesn’t give weight loss