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Proton Pumps in the Electron Transport Chain

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  • 19-12-2010 5:29pm
    #1
    huitzilopochtli
    Closed Accounts Posts: 39 ✭


    Hi,

    I'm trying to work out exactly how many protons are pumped by the various complexes in the ETC. In lectures we were told 4, 2 and 4 (by complexes I, III and IV respectively) but in Biochemistry (Stryer, sixth edition) the equations given for complexes III and IV don't seem to make sense. Those of you familiar with the book I hope will see what I mean. Those of you who haven't seen it, here are the equations given:

    Complex III
    QH2 + 2Cyt c(ox) --> Q + 2Cyt c(red) + 4H+(cytoplasm)

    According to the following page on the Q cycle this actually proceeds through:
    QH2 + Cyt c(ox) + Q --> Q + Cyt c(red) + 2H+(cytoplasm) + Q-
    QH2 + Cyt c(ox) + Q- + 2H+(matrix) --> Q + Cyt c(red) + 2H+(cytoplasm) + QH2

    Complex IV
    4Cyt c(red) + 8H+(matrix) + O2 --> 4Cyt c(ox) + 2H2O + 4H+(cytoplasm)

    Note (cytoplasm) here means the cytoplasmic side of the inner mitochondrial membrane.

    Now what I don't get is how does complex III pump 2 protons if the equation above says 4 (and really looks like 6 because Q- takes two more from the matrix)? And I realise that 8 protons are "pumped" because of water in complex IV (so 4 per molecule of NADH that added to make 2Cyt c) but how can we count those "pumped" by being added to oxygen if we didn't count those taken up by Q-?

    Thanks for looking,
    H


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