Lecture 6: The Electron Transport Chain Flashcards Preview

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Flashcards in Lecture 6: The Electron Transport Chain Deck (11)
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1

What is the terminal enzyme in the ETC?

Complex 4, also known as Cytochrome oxidase

2

What is an important property of the inner mitochondrial membrane which allows a proton motive force to be built up?

The inner mitochondrial membrane is impermeable to protons. Protons must be pumped across the membrane.

3

What is the energy released by the ETC transduced into and stored as?

The electrochemical potential of a proton

4

What are the consequences of the movement of protons across the membrane?

- It generates a pH gradient with the pH being higher (more alkali) in the matrix.
- It generates a voltage gradient (membrane potential delta psi) with the inside negative and the outside positive
Together the two forces constitute an electrochemical proton gradient which exerts a proton motive force (mV).

5

How are the complexes of the ETC arranged in the inner mitochondrial membrane?

Complexes 1, 3 and 4 are in that order and are transmembrane. Complex 2 is attached to the matrix side of the inner mitochondrial membrane, situated between complexes 1 and 3.

6

Describe the passage of electrons along the ETC.

1) NADH, which is generated in the matrix, donates 2e- to FMN in Complex 1, becoming NAD+ + H+.
2) FMN then donates 1e- to FeS (still in complex 1)
3) The e- is passed along 9 FeS centres before being passed to Coenzyme Q, which is inside the membrane between complexes 1 and 3. This makes Coenzyme Q a semiquinone. CoQ has binding sites on complex 1 and 3.
4) The second e- follows and fully reduces Coenzyme Q.
5) Coenzyme Q then passes the e- one at a time to the FeS centre in Complex 3.
6) After passing through 3 FeS centres in complex 3, each e- is then passed to cytochrome c in turn. When cytochrome c has accepted one e- it moves from the cyt c1 site on complex 3 and diffuses along the outside of the inner mitochondrial membrane (inter-membrane space side) to complex 4. Cyt c passes the e- to CuA (Copper A) in Complex 4, then returns to complex 3 to accept the second e-.
7) The CuA then passes the e- on to Cytochrome a, which passes it to cytochrome a3-CuB.
8) The e- then pass out of complex 4 onto oxygen, reducing it to form water. H+ are also used.

7

Which complex is also called CoQ-cytc reductase?

Complex 3

8

What is complex 1 also called?

NADH dehydrogenase

9

What is complex 2 also called?

succinate-CoQ reductase

10

What happens when the e- are passed from cyt a3-CuB to O2?

The reduction of O2 to 2 H2O by complex 4 occurs in multiple steps - called the ROS prevention cycle. This enables reduction of oxygen to water without the production of dangerous Reactive Oxygen Species (ROS), which could damage the mitochondrion/cell.
The ROS prevention cycle is as follows:

1 and 2) The oxidised Fe(3+)a3-OH Cu(2+)B complex is reduced to Fe(2+)a3 Cu(1+)B by 2 consecutive one-electron transfers from cyt c via cyta-CuA complex. Iron and copper are reduced.
3) O2 binds to the reduced complex (binds to Fe(2+)a3).
4) Internal electron redistribution producing an oxy-ferrel complex: Fe(4+)=O(2-) HO-Cu(2+). Tyrosine 244 has donated an electron and a proton and assumed a radical state (Y-O.)
5) A third electron transfer from cyt c (plus 2 protons) reconverts tyrosine to phenolic state, yielding the F compound (ferryl) and releasing water.
6) A fourth electron transfer and proton acquisition yields the oxidised Fe(3+)a3-OH Cu(2+)B. The cycle is complete.

11

What happens in complex 2?

FADH2 donates electrons to complex 2, becoming FAD. Complex 2 passes these electrons to quinone in the membrane and they join the ETC. They miss out complex 1, so miss out the 4 protons which it translocates.