Oxidative Phosphorylation - Electron Transport Chain

Question 1

describe hoe oxidative phosphorylation is a coupled process

Answer 1

electron transport through the electron transport chain (ETC) 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

overview the ETC

Answer 2

• 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 molecular oxygen to water (oxygen is the terminal electron acceptor)
• protons are pumped as the electrons are transported through the ETC
• builds a proton gradient

Question 3

describe the location of the ETC

Answer 3

• the ETC is in the mitochondria (requires oxygen)
• a proton gradient requires a barrier (stop proton diffusion)
• membranes are a barrier to protons
• reduced coenzymes in the matrix from the CAC, beta-oxidation and pyruvate dehydrogenase
• proton conc is highest in intermembrane space

experiment to prove its on inner mitochondria membrane:
isolate mitochondria from cells
- treat with strong detergent, solubilises all membranes (ETC does not work, so ETC must be in a membrane)
- that with mild detergent, only removes outer membrane (ETC still works, therefore ETC is in the inner mitochondrial membrane)

Question 4

describe the complexes of the ETC (names, mobile carriers and how electrons move through)

Answer 4

the ETC is organised into a series of complexes, between which mobile carriers transport electrons
- ETC organised into four complexes: Complex I to Complex IV
- each complex contains multiple carriers
- two mobile carriers: ubiquinone (UQ) and cytochrome c (cyt c)
- movement of e- through the ETC involves carriers undergoing a series of redox reactions
- each carrier accepts electron(s) (is reduced) in one redox reaction and then donates electron(s) (is oxidised) in another redox reaction

**ubiquinone (UQ) = COQ (coenzyme Q)

Question 5

describe how energy is released as electrons move through the carriers and what it is used for

Answer 5

As electrons move to carriers with a higher reduction potential (oxygen has the highest reduction potential), energy is released (the delta G at biological conditions is negative)
- elections are always passed from a complex with a lower reduction potential to one with a higher reduction potential, energy is released each time they move

This energy is used to translocate protons across the mitochondrial inner membrane and into the inter-membrane space (creating the proton gradient)

Question 6

what are the two pathways electrons can flow through the electron transport chain?

Answer 6

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

NO electron movement between Complex I and II, they both feed independently into Complex III
FADH2 is different because its that SLD carrier from the citric acid cycle

Question 7

describe how inhibitors of electrons flow affect the ETC

Answer 7

Rotenone inhibits electron transfer from Complex I to Co-Q
Cyanide binds to carrier in Complex IV
Carbon monoxide binds where O2 binds (can’t accept the electrons so the complex behind it gets stuck/builds up and can’t be oxidised)
- stop flow of electrons through ATC
- no proton gradient made
- build-up of reduced coenzymes (NADH and FADH2) so no oxidising power for other pathways (CAC, beta-ocidation and glycolysis will be affected)

Question 8

describe election flow through the ETC: Complex I

Answer 8

• NADH is oxidised at Complex I
• Two e- released into the ETC
• Four protons are pumped for each NADH oxidised

Question 9

describe election flow through the ETC: Complex II

Answer 9

• FADH2 is oxidised at Complex II
• SDH reaction is hared with the CAC
• Two electrons released into ETC
• NO protons are pumped

Question 10

describe election flow through the ETC: UQ (and its structure that allows it to do this)

Answer 10

• Complex I and Complex II both pass two electrons to UQ/CoQ
• UQ can move within the inner mitochondrial membrane
• A coenzyme (organic but not from a vitamin)
• Required by last carrier in Complex I and first carrier in complex III
• exists in two forms
• Carries hydrogen atoms (reducing equivalents)
• Co-Q undergoes two-electron redox reactions (like NADH and FADH2), but can accept or release one election at a time
• stores the electrons on the two double bonded O’s making them -OH

Question 11

describe election flow through the ETC: CoQ

Answer 11

• Complex I and Complex II both pass two electrons to CoQ
• Co-Q releases one electron at a time to Complex III (Q-cycle)

Question 12

describe election flow through the ETC: Complex III

Answer 12

• Complex III releases one electron at a time to Cytochrome C
• Complex III pumps four protons across the inner membrane (for one coenzyme/two electrons)

Question 13

describe election flow through the ETC: Cytochrome C

Answer 13

• Moves on outer surface of the inner mitochondrial membrane
• Cytochrome C carries one electron at a time from Complex III to Complex IV

It is a haem containing protein
- Cytochrome C carries one electron via Fe2+/Fe3+ redox reactions

Question 14

describe election flow through the ETC: Complex IV

Answer 14

• Complex IV accepts one electron at a time from cytochrome C
• reduces oxygen to water (terminal electron accepter)
• for 1 NADH/FADH2 (2 electrons): 2H+ pumped
• For 1 NADH/FADH2 (2 electrons): 1/2O2 + 2H+ -> H2O
• biologically the last carrier in Complex IV waits until it has four electrons (oxidation of two coenzymes): O2 + 4H+ -> 2H2O

Question 15

Describe the energy accounting for the ETC

Answer 15

NADH: 4(Cl) + 4(CIII) + 2(CIV) = 10 protons pumped
FADH2: 4(CIII) + 2(CIV) = 6 protons pumped