Electron Transport Chain

Question 1

What is the concept of chemiosmosis?

Answer 1

NADH and FADH2 have just been produced in the TCA cycle (in the matrix of the Electrons from NADH + H+ and FADH2 are transferred in steps to molecular oxygen to produce water during the electron transport chain
Energy released by these redox reactions is conserved in the form of a PROTON GRADIENT that is used to drive synthesis of ATP (exergonic process)
This leads to a more negative charge within the matrix of the mitochondria and more positive outside

Question 2

Describe the mitochondria?

Answer 2

Outer membrane - contains pores to allow small molecules (10 kDa) to diffuse through 
Inner membrane (cristae) - very impermeable, needs proteins to help move other molecules 
The intermembrane space is acidic and positive compared to the matrix

Question 3

How are larger molecules moved into the mitochondria?

Answer 3

NADH - malate-aspartate shuttle

FADH2 - glycerophosphate shuttle

Question 4

What are the components involved in the electron transport chain?

Answer 4

Made up of redox carriers associated with proteins and cofactors in the inner membrane
It is made up of 4 complexes but 6 elements:
Complex I NADH dehydrogenase - oxidises NADH
Complex II Succinate dehydrogenase - oxidises FADH (succinate -> fumarate)
Ubiquinone
Complex III Q cytochrome c oxidoreductase
Complex IV Cytochrome oxidase - reduction of oxygen
Cytochrome C

Question 5

What are some types of redox carriers?

Answer 5

Haems - carry electrons, found in complexes I, II and IV
Iron sulphur centres - carry electrons, found in complexes I, II and III
Flavins - carry hydrogen, found in complexes I and II
Quinones - carry hydrogen and UQ links the complexes I&III and II&III

Question 6

Why can the electron transport chain take place?

Answer 6

Electron transport chain components are organised in the inner membrane such that their individual redox potentials become successively more positive
This allows electrons to be transferred along the chain
Oxygen, with a large positive redox potential, is the final acceptor of the electrons

Question 7

Describe the mechanism of the electron transport chain using NADH - complex I?

Answer 7

Complex I - NADH dehydrogenase
1. NADH transfers 2 electrons to complex I NADH dehydrogenase, accompanied by a H+
○ NADH is an electron donor as its redox potential is more negative
2. 2 electrons and 2 protons pass through complex I
3. The 2 electrons are donated to ubiquinone, the 2 protons accompany the e- and react to form QH2
○ This leaves regenerate NAD+, which can go back to the TCA cycle in the matrix
4. Energy released by the movement of electrons through complex I is used to pump 4 protons from the matrix into the intermembrane space

Question 8

What is the overall equation at complex I?

Answer 8

NADH + H+ + Q -> NAD+ + QH2

For each time of this reaction 4H+ from the matrix move across the inner membrane

Question 9

What are some structural features of complex I?

Answer 9

It contains multiple coenzymes - all within a 140-Å high peripheral arm
It contains 1 favin mononucleotide (FMN) - a redox active prosthetic group
8 iron-sulfur clusters (mammals)

Question 10

How does redox of electron transfer drive the proton pumping?

Answer 10

Changes in redox state of the protein - releases energy
This results in conformational changes of the protein
These changes alter the pK values of ionisable side chains - allowing protons to be taken up and released
H+ (protons) use a ‘proton wire’ - hydrogen-bonding to a protein and ‘jumping’ between molecules

Complex I has 4 proton wires each pumping 1 proton for every pair of electrons

Question 11

Describe the mechanism of the electron transport chain using NADH - complex III?

Answer 11

Complex III - cytochrome C oxidoreductase

1. Electrons are transferred from QH2 to complex III
2. 2 electrons and 2 protons pass through complex III
3. The 2 electrons are transferred to cytochrome C
4. Energy released by the movement of electrons through complex III is used to pump 4 protons from the matrix into the intermembrane space

2 of the protons came from the QH2 and 2 of the protons came from the matrix
CoQH2 undergoes a two-cycle reoxidation in which the semiquinone, CoQ⋅¯, is a stable intermediate

Question 12

What are some structural features of complex III?

Answer 12

It contains two b-type cytochromes, one cytochrome c1, and one [2Fe–2S] cluster in which one of the Fe atoms is coordinated by two His residues

Question 13

Describe the mechanism of the electron transport chain using NADH - complex IV?

Answer 13

Complex IV - Cytochrome oxidase
1. Electrons are transferred from cytochrome C to complex IV
2. These electrons are used to reduce molecular oxygen
○ This then reacts with protons form the matrix to form water
3. Energy released by the movement of electrons through complex IV is used to pump 2 protons from the matrix into the intermembrane space
½ O2 +2H+ +2e-→ H2O

Question 14

Describe cytochrome C?

Answer 14

It is a soluble electron carrier - soluble within the intermembrane space
It shuttles electrons between the portions of Complexes III and IV
It has several highly conserved Lys residues and a buried heme group

Question 15

What are some structural features of Complex IV?

Answer 15

410 kDa homodimer - the component protomers are composed of 13 subunits
The core has 3 large hydrophobic subunits
it has 4 redox centers: cytochrom a & a3 and copper atom (CuB) & a pair of copper atoms (CuA center)

Question 16

Describe the mechanism of the electron transport chain using FAHD2- complex II?

Answer 16

Complex II - Succinate dehydrogenase
This enzyme is also used in the TCA cycle: Succinate + FAD -> Fumarate + FADH2
FADH2 can’t donate electrons to complex I as its reduction potential isn’t negative enough

1. FADH2 transfers 2 electrons directly to ubiquinone, accompanied by 2 protons producing QH2
   ○ Complex II doesn’t pump protons as there isn’t enough energy produced by the transfer of these electrons to facilitate this movement

FADH2 + Q -> FAD + QH2
The following electron transport chain is the same as in NADH

Question 17

How can FADH2 be supplied?

Answer 17

FADH2 can also be supplied by several other FAD-requiring enzymes
E.g. Glycerol-3-phosphate dehydrogenase and acyl-CoA dehydrogenase

Question 18

What are some structural features of complex II?

Answer 18

It contains a linear chain of redox cofactors
Complex II is a mushroom-shaped homotrimer
It’s protomers each consist of two hydrophilic subunits, a flavoprotein (Fp) and an iron–sulfur subunit (Ip) and two hydrophobic membrane-anchor subunits, CybL and CybS

Fp binds the substrate and the FAD prosthetic group
Ip binds the 3 Fe-S clusters

Question 19

How many H+ ions are pumped across the membrane?

Answer 19

When electrons are donated by one molecule of NADH:
Complex I - 4 H+
Complex III - 4 H+
Complex IV - 2 H+
Total - 10 H+

When electrons are donated by FADH2
Complex III - 4 H+
Complex IV - 2 H+
Total - 6 H+

Question 20

How does the electron transport chain generate a membrane potential?

Answer 20

There is a chemical (ΔpH) and electrical difference (Δψ) between the matrix and the intermembrane space - due to a proton being an ion

Matrix - fewer H+ ions and higher pH
Outside the mitochondrion - more H+ ions and lower pH
Therefore endergonic
However, protons moving down the gradient is exergonic - providing energy for ATP synthesis

Question 21

What can the electrochemical proton gradient be called?

Answer 21

The proton motive force (pmf) or Δp
Therefore can be described as:
pmf = Δψ -2.3RT ΔpH/F

Question 22

What is the overall process of the electron transport chain with NADH?

Answer 22

Complex I
Ubiquinone
Complex III
Cytochrome C 
Complex IV

Question 23

What is the overall process of the electron transport chain with FADH2?

Answer 23

Complex II
Ubiquinone
Complex III
Cytochrome C 
Complex IV