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The mechanism for dihydrolipoic acid oxidation in the pyruvate hydrogenase complex is not clearly demonstrated in any Lehninger textbook, other than the fact that FAD becomes FADH2. I am wondering what intermediates the FAD/FADH2 undergoes, my textbook just does skips over the details.

In the diagram below, the mechanisms are mostly all clear, except for the arrow I've highlighted with orange.

enter image description here

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Interesting question. Let's look at the overall reaction first:

enter image description here

This reaction is catalysed by the dihydrolipoamide dehydrogenase in 6 steps and as you see from the schematic below (ref.1) the mechanism involves also four aminoacids near the active site (Cys48, Cys43, Hys451, and Glu456):

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References:
1.https://www.sciencedirect.com/science/article/pii/B9780080453828001374

First image attribuition: By Jkallini - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=24958884

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    $\begingroup$ The redox steps are hydride transfers, i.e. hydrogen with two electrons. A description of the structural aspects is here. $\endgroup$
    – Karsten
    Mar 2, 2023 at 16:55

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