Oxidative phosphorylation (lecture 26)

Question 1

Why do we need oxygen?

Answer 1

To release the energy from the electron carriers NADH & FADH2

Question 2

What is oxidative phosphorylation?

Answer 2

The electron transport chain

Works by combining these molecules with oxygen to form water
The energy released is converted to ATP in a stepwise manner

Energy is used to pump protons out of the matrix into the intermembrane space
This generates a proton concentration gradient
The gradient is used to drive passage of protons back into the matric through ATPase
This is the electron motive force

Question 3

How are all the proteins in the electron transport chain similar?

Answer 3

All the proteins contain iron in their centre
• Can loose or gain electrons – has multiple oxidation states
• Makes it a perfect protein to be a part of the electron transport chain

The big ones are all transmembrane proteins
• Each time there is a change in oxidation state, there is a conformational change causing protons to be moved across the membrane
• Happens until molecular oxygen is reached & water is formed

Question 4

How are protons pumped across the membrane?

Answer 4

1) Electron comes in & binds to the first enzyme complex
2) Iron changes its oxidation state
3) Causes a change in ∆G
4) Conformational change
5) Protons pumped into the intermembrane space

Lots of protons are moved in the first step – bigger the change in ∆G, the more protons moved

Question 5

What is complex I?

Answer 5

NADH-Q oxidoreductase (46 polypeptide chains)

• Big transmembrane protein
• Transfer of 2 high-potential electrons from NADH to FMN
• Electrons from FMNH2 are transferred to a series of Fe-S clusters
• Electrons from Fe-S clusters are shuttled to coenzyme Q (ubiquinone) & reduces it
• 4 H+ pumped out of the matrix into the intermembrane space
• 2 chemical H+ removed from the matrix

Question 6

What is FMN?

Answer 6

flavinemononucleotide

Question 7

What is coenzyme Q?

Answer 7

Known as ubiquinone because it is an ubiquitous quinone in biological systems

Q carries the electrons from NADH & FADH2
• Is hydrophobic & diffuses rapidly within the inner mitochondrial membrane – makes it highly mobile
• Can accept hydrogen from the reductase complex

Question 8

What is complex III?

Answer 8

Q-cytochrome c oxidoreductase

• Electrons transferred from QH2 to oxidised cytochrome c (Cyt cox)
• The mechanism that couples electron transfer from QH2 to cytochrome c is known as the Q-cycle
• 2 chemical H+ removed from matrix
• 4 pumped H+ released to intermembrane space

Question 9

What is cytochrome C?

Answer 9

Cytochrome c is present in all organisms with mitochondrial respiratory chains & has a highly conserved structure

Small soluble protein containing a c-type haem

Can only carry 1 electron from Q-cytochrome c oxidoreductase to cytochrome c oxidase to cytochrome c oxidase
• Process needs to be done twice to transfer 2 electrons

Question 10

What is complex IV?

Answer 10

Cytochrome C oxidase

• 4e- from cytochrome c are transferred to O2
• 4H+ from matrix complete reduction of O2 to H2O
• 4 more H+ are pumped across the membrane
• 4H+ pumped from matrix to intermembrane space
• 4 chemical H+ removed from matrix
• Whole process doesn’t work if the this step is blocked

Question 11

What is ATPase?

Answer 11

Create the proton driving force

We have millions of ATPAses – a group of proteins that span the inner membrane & act as a channel

Channel physically rotates as protons move through it
• Rotational energy generates ATP from ADP & inorganic phosphate

Question 12

How does ATPase work?

Answer 12

1) protons bind to the ‘a’ subunit that takes them from the aqueous environment into the membrane
2) proton joins the ‘c’ unit by binding the the CA residue of the aspartate group
3) Causes aspartate to loose -tive charge and it moves into the membrane
4) This generates rotational energy
5) Once in a non-polar environment the protons jump off the aspartate residue
6) Rotation turns the gamma subunit
7) Energy phosphorylates ADP to ATP

Question 13

What is complex II?

Answer 13

Succinate-Q reductase complex

Takes hydrogens from FAD & brings it in further down the chain

Question 14

What can go wrong with oxidative phosphorylation?

Answer 14

1. Carbon monoxide – high affinity for Hb reduces amount of oxygen getting to the cells
2. Cyanide – binds to complex IV & blocks oxidative phosphorylation
3. Salicylate – forms pores in the inner membrane so protons pass through without generating ATP
4. Mitochondrial defects

Question 15

How does carbon monoxide effect oxidative phosphorylation?

Answer 15

High affinity for Hb reduces amount of oxygen getting to the cells

Question 16

How does cyanide effect oxidative phosphorylation?

Answer 16

Binds to complex IV & blocks oxidative phosphorylation

Question 17

How does salicylate effect oxidative phosphorylation?

Answer 17

Forms pores in the inner membrane so protons pass through without generating ATP

Question 18

Reactive oxygen species & free radicals leak from the electron transport chain
What defence mechanisms do the mitochondria have to deal with this?

Answer 18

Converts them to hydrogen peroxide

Unbound iron has to be kept at very low concentrations as it reacts with hydrogen peroxide

Question 19

What is the electron motive force?

Answer 19

The electron flow from NADH to FADH2 to O2

Question 20

What is the proton motive force?

Answer 20

The unequal distribution of protons creating a transmembrane electrical potential