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Overview:
E1: Pyruvate Dehydrogenase cleaves the carboxyl group from pyruvate which releases CO2. It also transfers the remaining hydroxyethyl group to TPP (its cofactor).
E2: Dihydrolipoyl Transacetylase catalyzes the transfer of the acetyl group to Coenzyme A to yield Acetyl-CoA.
E3: Dihydrolipoyl Dehydrogenase re-oxidizes the lipoamide arm of E2 back to its functional state, producing NADH in the process.
TPP interacts with pyruvate and kicks off a CO2, resulting in TPP having a hydroxyethyl group, naming it now, hydroxyethyl TPP.
The ring structure you see from E2 functions as an "arm" and swings over to E1 and "grabs" the hydroxyethyl group off of TPP. It hops onto one of the sulfers in the ring. This causes there to be one reduced sulfur and one sulfur with an acetyl group on it.
Now, the "arm" swings again, and interacts with CoA forming Acetyl-CoA! From here, we now have to return the system back to normal.
Do do this, the fully reduced sulfur ring must be oxidized. (Both of the sulfurs have a hydrogen on them). The hydrogens get transfered onto FAD in E3 to make FADH2
Next, NAD+ oxidizes FADH2 and takes one H+ to generate NADH and H+. Now, everything is regenerated.
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