Question

(Fill in the following blanks with numbers.) Thus, at the QP site ____ UQH₂(s) are oxidized to UQ.  At the QN site ____ UQ(s) are reduced to UQH₂. _____ molecules of cyt c are reduced. This results in the net transport of _____ additional protons across the membrane.

Answer 1

At the QP site (intermembrane side) 2 UBQH2 are oxidized to UQ. At the QN site 1 molecule of UQ are reduced to UQH2 2 molecules of cyt c are reduced. This results in net transport of 2 additional protons across the membrane.

This passage is related to Q cycle part of electron transport chain. Here we should first note that Q can exist in 3 different forms- Reduced quinol, oxidized quinone and semiquinone.

The cycle starts with 1 UBQH2= it has 2 proton and 2 electron to transfer - it will transfer 1 electron is donated to Cytochrome c and second to UQ and sonverting it into UQ- (Semiquinone) and two hydrogen molecule is exported to intermembrane space.

Now scenario is UQH2= Q- and one cytochrome c with one eletron + 2 H+ (IMS)

Now both UQ and cytochrome C are done and leave membrane. While semiquinone is still hanging there.

Now another set of QH2 and cytochrome come and join the party. Same thing is repeated 2 protons are ejected in intermembrane space. cytochrome c gets another electron but semiquinone accepts another electron and gets converted to quinone and becomes doubly negative charged.(UQ2-)

Now UQ2- takes two protons from matrix and gets converted to UBQH2.

So net transport of proton is 2 because 2 molecule of QH2 donated 4 proton in intermembrane space but at the same time 2 protons from matrix was taken byUQ 2- to get converted to UBQH2.

Always remember that cytochrome unlike UQ can take only one electron at a time.