Lecture 7: The Q cycle and proton translocation

Question 1

What is chemiosmosis?

Answer 1

Excited electrons are passed along a chain of electron carriers in the inner mitochondrial membrane, being lowered in energy levels. The energy released is used to pump protons across the membrane into the inter-membrane space. An electrochemical proton gradient is built up.

Question 2

Give 4 pieces of experimental evidence to support chemiosmotic coupling.

Answer 2

1) membranes can establish proton gradients
2) an intact inner membrane is required for oxidative phosphorylation
3) uncouplers act by dissipating the proton gradient
4) generation of a proton gradient permits ATP synthesis without electron transport

Question 3

Name the two mechanisms proposed for proton translocation and give an example of a protein which uses each of these.

Answer 3

The redox loop mechanism (complex 3) and the proton pump mechanism (complex 4).

Question 4

Describe the redox loop mechanism.

Answer 4

This mechanism requires that the redox active centres of the ETC be arranged so that reduction would involve a redox centre simultaneously accepting an electron and a proton from the matrix side of the membrane. Re-oxidation of this redox centre by the next centre would involve release of the proton.

This mechanism requires that the first redox carrier contain more hydrogen atoms in its reduced state than its oxidised state and that the second redox carrier have no difference in its reduced and oxidised states.

Question 5

Describe the proton pump mechanism.

Answer 5

The proton pump mechanism postulates that the transfer in electrons results in conformational changes to the protein complexes. The translocation of protons occurs as a result of the influence of the conformational changes on the pKs of amino acid chains and their exposure to the matrix or inter-membrane space side of the inner mitochondrial membrane.

Question 6

What accounts for the extra protons which are translocated?

Answer 6

The Q cycle

Question 7

Where does the Q cycle take place?

Answer 7

Complex 3

Question 8

Describe the steps of the Q cycle.

Answer 8

QH2 from complex 1 arrives at the matrix side of the inner mitochondrial membrane (IMM) and then diffuses across to the other side of the membrane, where it releases 2 protons into the inter-membrane space and donates an e- to ISP. QH2 is now Q.- (semiquinone). ISP then reduces (passes e- to) cyt c1.
Then semiquinone reduces cyt bL, becoming fully oxidised quinone. Quinone is at that point in the Q0 binding site near the inter-membrane space side of the IMM. It then diffuses through the membrane to Qi binding site, which is near the matrix side of the IMM.
Meanwhile, the e- which semiquinone passed to cyt bL has been passed to cyt bH. Then the e- is passed to quinone in the Qi binding site, making it semiquinone again. This concludes cycle 1.
Cycle 2 is the same as cycle 1 up until the last step. the quinone stays in the Qi binding site and the semiquinone from cycle 1 accepts the e- from cyt bH. Together with 2 protons from the matrix, they combine to regenerate quinol, QH2.
This is a modified redox loop mechanism.
The e- which are passed to cyt c continue down the ETC.