Lecture 32 - The electron transport chain

Question 1

Oxidative phosphorylation

Answer 1

Oxidative phosphorylation is the coupled process of: electron transport through the electron transport chain AND the phosphorylation of ADP to ATP by ATP-synthase

They are coupled by a proton gradient
The ETC makes the proton gradient
ATP-synthase uses the proton gradient

Question 2

Proton gradient

Answer 2

Oxidative phosphorylation process is coupled by a proton gradient
The ETC makes the proton gradient - the inner membrane provides a barrier so that the proton gradient can be created
ATP-synthase uses the proton gradient

Question 3

Where is the ETC?

Answer 3

The ETC is in the inner membrane of mitochondria

All processes except for glycolysis (cytoplasm) occurs in the mitochondria

Question 4

Experimental evidence of the location of the ETC

Answer 4

Isolate mitochondria from cells via centrifuge then …
Treat with strong detergent, soubise all the membranes which results in the ETC not working
Treat with mild detergent, only removes outer membrane, ETC still works

Note - detergents disrupt membranes

Question 5

Overview of the ETC

Answer 5

Electrons are passed through a series of carriers
Electrons from NADH and FADH2 are fed into the electron transport chain (NADH and FADH2 are oxidised)
These electrons will ultimately reduce molecule oxygen to water (oxygen is the terminal electron acceptor)
Protons are pumped as the electrons are transported through the ETC

Question 6

The ETC is organised into a series of _______ between which ___________ transport ______

Answer 6

Complexes
Mobile carriers
Electrons

Question 7

ETC organisation

Answer 7

ETC organised into four complexes
Each complex contains multiple carriers
Two mobile carriers - UQ (CoQ) and cytochrome C (Cyt C)

Question 8

Movement of electrons through the electron transport chain involves carriers undergoing a series of redox reactions

Answer 8

Each carrier accepts electrons (s) (is reduced) in one redox reaction and then donates electron(s) (is oxidised) in another redox reaction

ETC is a series of redox reactions tasing on electrons to the next carrier and it is also going to regenerate the previous carrier so that the next lot of electrons can come through

Question 9

What happens as the electrons move through the carriers?

Answer 9

Energy is released

Electrons move to carriers with a higher reduction potential (oxygen has the highest reduction potential) - for each of the redox reactions in the ETC, we are going up in reduction potential and a negative delta G and therefore are releasing energy which is used to form the proton gradient
Energy is released (the delta G is negative)

Oxygen has the highest reduction potential hence why it is the terminal electron acceptor

Question 10

Energy released in the ETC is used to …

Answer 10

Used to translocate protons across the mitochondrial inner membrane

Intermembrance space has higher H+ concentration and therefore a lower pH
Matrix has a lower H+ concentration and therefore has a higher pH

Question 11

Electron flow through the ETC

Answer 11

NADH - complex I - UQ - Complex III - Cyt C - Complex IV - O2
OR
FADH2 - complex II - UQ - Complex III - Cyt C - Complex IV - O2

Question 12

Electron flow path with NADH

Answer 12

NADH - complex I - UQ - Complex III - Cyt C - Complex IV - O2

Question 13

Electron flow path with FADH2

Answer 13

FADH2 - complex II - UQ - Complex III - Cyt C - Complex IV - O2

Question 14

Inhibitors of electron flow through the ETC

Answer 14

Rotenone
Cyanide
Carbon monoxide

Question 15

Rotenone and the ETC

Answer 15

Inhibits the transfer of electron from complex I to CoQ/UQ

Question 16

Cyanide and the ETC

Answer 16

Binds to a carrier in complex IV

Question 17

Carbon monoxide and the ETC

Answer 17

Carbon monoxide binds where oxygen binds

Question 18

Problems with inhibiting the ETC…

Answer 18

Stop flow of electrons through the ETC
Buildup of reduced coenzymes (NADH and FADH2) - they are holding on to electrons and want to give it to something
No proton gradient formed
Reactive oxygen species produced

Question 19

Complex I

Answer 19

NADH is oxidised at Complex I
Two electrons released into the ETC - two electrons travel as a pair through complex one
4 protons are pumped for each NADH oxidised

Question 20

Complex II

Answer 20

FADH2 is oxidised at Complex II (use FADH2 cause it needs more power for this reaction, FAD is always associated with the enzyme and needs to be regenerated back (shared reaction with CAC))
SDH reaction is shared with the CAC
Two electrons released into the ETC
No protons are pumped

Question 21

UQ/CoQ

Answer 21

Complex I and Complex II both pass two electrons to UQ/CoQ
UQ can move within the inner mitochondrial membrane
CoQ releases one electron at a time to complex III

CoQ undergoes two- electron redox reactions (like NADH and FADH2) - but can accept/release one electron at a time so can have a kind of intermediate

Question 22

Complex III

Answer 22

Complex III releases one electron at a time to cytochrome C

Complex III pumps 4 protons across the inner membrane (for one coenzyme/2 electrons)

Question 23

Cytochrome C

Answer 23

Moves on outer surface of the inner mitochondrial membrane

Cytochrome C carries one electron at a time from complex III to complex IV

Question 24

Cytochrom C is a _______ containing protein

Answer 24

Heme

Cyt C carries one electron via reversible Fe2+/Fe3+ redox reactions

Question 25

Complex IV

Answer 25

Complex IV accepts one electron at a time from cut c
Reduces oxygen to water (terminal electron acceptor)
For 1 NADH/FADH2 (2 electrons) - 2 H+ pumped
For 1 NADH/FADH2 (2 electrons) - 1/2 O2 + 2H+ -> H2O
Biologically the last carrier in complex IV waits until it has 4 electrons: O2 + 4H+ -> 2H2O

Question 26

Energy accounting for the ETC

Answer 26

NADH : 4 + 4 + 2 = 10 protons pumped

FADH2: 4 + 2 = 6 protons pumped