Mitochondrial respiratory chain

Question 1

Complex I

Answer 1

NADH dehydrogenase

First and largest protein complex in the ETC that acts as a proton pump.

Initially electrons from NADH is passed to FMN to reduce it to FMNH2, as well as to iron-sulfur clusters.

Exergonic transfer:
Receive a hydride ion and a proton from NADH (2 e-).
Transfers it to ubiquinone.

Endergonic transfer:
Energy from electron transfer couples the transfer of 4 protons from the matrix to intermembrane space.

Question 2

Equation of the reaction NADH dehydrogenase catalyses

Answer 2

NADH + H+ + Q= NAD+ + QH2

Question 3

Complex II

Answer 3

Succinate dehydrogenase - the only protein of the ETC that does not act as a proton pump.

Also used in the TCA cycle- converts succinate to fumarate.

Complex II receives electrons from FADH2 and passes it to ubiquinone.

Question 4

Paths of electrons entering the electron-transfer chain

entering Q

Answer 4

1. NADH passes electrons to complex I which ultimately passes it to Coenzyme Q.
2. FADH2 passes electrons to complex II, which passes it to ubiquinone.
3. G-3-P passes electrons to G3P dehydrogenase—> ubiquinone
4. Acyl-CoA dehydrogenase transfer electrons from beta oxidation to ETF:Q oxidoreductase—> ubiquinone.

Question 5

Complex III

Answer 5

Ubiquinone: cytochrome c oxidoreductase

Second proton pump in the ETC- pumps 4 H+

Receives electrons from QH2 and transfers to cytochrome c.

Question 6

Complex IV

Answer 6

Cytochrome oxidase- final proton pump, pumps 2 H+

Carries electrons from cytochrome c to molecular oxygen to produce water:

1/2O2 + 2H+ = H2O

Question 7

Adenine nucleotide translocase

Answer 7

Antiporter in the inner mitochondrial membrane that transports:

• ADP3- to the matrix
• ATP4- to the intermembrane space

Due to protons being pumped into the intermembrane space, the electrochemical gradient favours this movement.

Question 8

Atractyloside

Answer 8

A glycoside that specifically inhibits adenine nucleotide translocase.

Acts as a poison.

Question 9

Phosphate transolcase

Answer 9

Symporter in the inner mitochondrial membrane.

Transports both H+ and H2PO4- from the intermembrane space to the matrix.

The H2PO4- acts as a source for inorganic phosphate to make ATP.

This movement is supported by the electrochemical gradient created by the proton pumps in the ETC.

Question 10

ATP synthase

Answer 10

An F-type ATPase with two domains:

F0- proton channel
F1- ATP synthase

This protein drives the synthesis of ATP

Question 11

F0 domain

Answer 11

An oligomycin sensitive proton channel domain of the ATP synthase.

3 subunits, which a complex of 13-15 further subunits.
Main subunits—> a, b, c

Subunits C1-10 are in a circle and this is the unit that initially rotates.

Question 12

F1 domain

- Subunits, and their functions

Answer 12

The ATP synthase domain in the f-type ATPase.

5 sub units:

-Alpha-3

-Beta-3: catalytic sites for ATP synthesis
gamma

-Delta- interacts with the 2 b units of F0

-Epsilon
All form a complex of 9 further subunits

Alpha and beta units are arranged alternately and are held by gamma and epsilon subunits.

Question 13

Rotational catalysis of ATP-synthase

Answer 13

Beta subunits of F1 domain take in turns making ATP by binding ADP and Pi.

1. The c-ring of the F0 domain rotates which causes the gamma and and epsilon units to rotate.
2. The beta-alpha units are stationary (and ab units of F0).
3. Every rotation of the gamma unit changes the catalytic property of the beta-subunit. Gamma unit can only associate with one alpha-beta unit at a time.
4. Starts at loose confirmation: ADP and Pi can bind.
5. Tight confirmation: active site of beta unit binds to form ATP tightly.
6. Beta-empty/ open confirmation:
   Beta active site has low affinity for ATP and ATP is released.

Question 14

Energy changes in ATP synthase

Answer 14

Protons flow down its electrochemical gradient across ATP-synthase.

This is exergonic, and releases energy for ATP synthesis in the F1 domain

Question 15

Conservation of electron transfer energy

Answer 15

When water is made at complex IV:
NADH + H+ +1/2O2= H2O + NAD+

This reaction is highly exergonic and releases energy for protons to be pumped.

The electrochemical energy generated shows that the energy from electron transfer was temporarily conserved.

Question 16

Energy yield of glycolysis from one glucose molecule

Answer 16

2 NADH= 3/5 ATP depending on what shuttle was used

2 ATP from making pyruvate.

Question 17

Energy yield of pyruvate oxidation from one glucose molecule

Answer 17

2 NADH = 5 ATP molecules

Question 18

Energy yield of the citric acid cycle from one glucose molecule

Answer 18

6 NADH = 15 ATP

2 FADH2= 3 ATP

2 ATP

Question 19

Uncoupling reagents

Answer 19

Substances that transport H+ back into the mitochondrial matrix from the intermembrane space, without passing through ATP-synthase.

This dissipates the proton gradient and blocks the coupling energy between energy flow and ATP synthesis.

This causes energy to be release as heat instead of ATP.

Question 20

Examples of uncoupling reagents

Answer 20

DNP

UCP1/ thermogenin in brown adipose tissue.

Question 21

UCP-1

Answer 21

This protein is a channel found in the mitochondrial inner membrane of brown adipose tissue.

It serves as an uncoupling reagent for ATP synthesis by allowing H+ to bypass ATP-synthase into the matrix.

This causes a lot of heat generation in brown fat (seen in babies, to help regulate heat).

This could also play a role in diabetes and obesity.

Question 22

DNP

Answer 22

A weak acid that acts as an endogenous uncoupling reagent.

When it crosses the inner mitochondrial membrane, it carries H+ and causes it to bypass ATP-synthase.

Used to be used in slimming pills but found out to have toxic effects like:
Liver damage

Respiratory acidosis

Hyperthermia