Lecture 6: The Electron Transport Chain Flashcards
What is the terminal enzyme in the ETC?
Complex 4, also known as Cytochrome oxidase
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.
What is the energy released by the ETC transduced into and stored as?
The electrochemical potential of a proton
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).
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.
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.
Which complex is also called CoQ-cytc reductase?
Complex 3
What is complex 1 also called?
NADH dehydrogenase
What is complex 2 also called?
succinate-CoQ reductase
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.
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.
