Protein control 6: Complex II, III, IV

Question 1

Theoretical limit of ATP per FADH2 molecule?

Answer 1

5 ATP (under standard conditions)

Question 2

Full names of complex II?

Answer 2

Coenzyme Q (ubiquinone) reductase or

succinate dehydrogenase

Question 3

2 Main roles of complex II?

Answer 3

As part of the citric acid / krebs cycle it converts succinate into fumarate (hence succinate dehydrogenase)

As part of the ETC it uses electrons from succinate via FADH2 to reduce ubiquinone to ubiquinol (hence coenzyme Q reductase)

Question 4

Equation for the reaction occurring in complex II?

Answer 4

UQ + FADH2 -> UQH2 + FAD

no protons are created, transferred or consumed, electrons transferred from FADH2 to UQ via iron-sulphur clusters

Question 5

Full names of complex III?

Answer 5

Cytochrome C reductase or

cytochrome bc1 complex

Question 6

What are the 3 key subunits of complex III? Individual important properties of each?

Answer 6

Cytochrome b - carries 2 hemes, 1 low (bL), 1 high (bH) potential

cytochrome c1

Rieske iron-sulphur protein - contains a single Fe2S2 cluster

Question 7

Mechanism of complex III electron transfer?

Answer 7

Complex III transfers electrons from ubiquinol (2e-) to cytochrome c (accepts 1e-)

Electron transfer is coupled with the translocation of protons across the membrane

The electron transfer reactions from QH2 to cyt c through the cyt bc1 complex constitute the protonmotive Q-cycle

Question 8

Stages of the Q (quinone) cycle?

Answer 8

Stage 1 - oxidise UQH2 -> UQ, 2H+ released, 1 electron to heme bL and heme c1
-> electrons transferred
heme bL to bH
heme c1 to Cyt c
-> electrons transferred again
heme bH to UQ forming semiquinone radical
cyt c leaves

Stage 2

• electrons undergo same pathway, but result is semiquinone radical is completely reduced
• > UQ2- takes up 2H+
• > UQH2 leaves, UQ takes place

Question 9

Complex IV full name?

Answer 9

Cytochrome c oxidase

Question 10

Function of complex IV?

Answer 10

Accepts electrons from reduced cytochrome c

• > transports them to molecular OXYGEN
• for reduction to WATER

translocates 4H+ per 4 electrons,
consumes 4H+ from the matrix in generating 2 water from 1 oxygen