Protons Pumped Per NADH/FADH2 for ETC

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Protons Pumped Per NADH/FADH2 for ETC

Post by mikapaprika » Wed Mar 25, 2020 9:53 pm


I just watched the CR for Oxidative Phosphorylation and I am having a hard time understanding why NADH pumps a total of 10 protons in the ETC (the video segments are cut off so there was an answer, no explanation :cry: ). I got 10 for NADH, but how many protons are pumped per FADH2?

If someone could explain for each electron carrier where the protons are pumped from which complex, contributing to the total protons pumped, I would really appreciate it. I tried looking it up and reading in the book, but I cannot seem to find a good answer. I am also getting confused because some explanations explain protons per electron, and some explain per Q/cyt. c, etc.

Thank you!!! :o
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Re: Protons Pumped Per NADH/FADH2 for ETC

Post by NS_Tutor_Mathias » Thu Mar 26, 2020 12:32 am

Ah, welcome to like... my favorite topic ever. It's super cool because we don't normally get to dork out and just keeping asking "BUT WHY? HOW?" and have it be part of our studying. Here we can!

So let's break ATP synthesis down to it's most foundational concept:
A proton (H+) gradient is established, then protons flow down that gradient (from high concentration to low). The energy of that flow (literally, the kinetic energy!) is used to turn a machine that smushes ADP together with an inorganic phosphate. That machine has the fancier name of ATP synthase.

Make sure that this is completely solid in your mind, because every part of the function of the ETC hinges on this idea.

The function of the ETC is to take reduced electron carriers from the mitochondrial matrix and oxidize them, using the electric potential energy of that process to pump protons out into the intermembrane space. So the ETC is creating the gradient. Easy!

This also means that as a consequence, every single proton pumped results in almost exactly the same net production of ATP. Now:
Complex I pumps 4 H+
Complex II pumps 0 H+
Complex III pumps 4 H+
Complex IV pumps 2 H+

NADH donates electrons to Complex I, passing through Q, Complex III and Complex IV. It causes the pumping of 10 H+ total.
FADH2 donates electrons to Complex II, passing through Q, Complex III and Complex IV. It causes the pumping of 6 H+ total. (Nota bene: FADH2 is actually part of complex II, and complex II is just called the succinate dehydrogenase complex - the enzyme you may be familiar with from the Krebs cycle)

As a result, if we say that it takes about 4 protons moving down their gradient (through that little smushing-machine, ATP synthase) to make 1 ATP, then each NADH would generate 2.5 ATP and each FADH2 would generate 1.5 ATP. You may see different numbers (3.33 is a popular and fairly correct one) for protons per ATP, but with this information you can easily adapt any answer you give to new values of protons per ATP. For the most part the 4 protons per ATP figure is used on the MCAT unless otherwise specified.

I've attached a little diagram that both illustrates this and shows the connection to electrochemistry here. Notice the more positive reduction potential of each successive substrate (because Ecathode - Eanode has to give a positive value for the transfer of electrons to be spontaneous!).

Also, here is a closeup video on the structure and function of ATP synthesis. I'm not really exaggerating when I say it is a machine for smushing ADP + Pi.
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