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Reminder:
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.
The PDH complex is regulated by covalent modification→ Phosphorylation!
When the complex is phosphorylated→ INACTIVE
When it is dephosphorylated → Active
So... what happens?
PDH Kinase phosphorylates the E1 complex, turning it off. This kinase is activated by: ATP, Acetyl CoA, and NADH. This is because if there is an excess of ATP, Acetyl CoA, and NADH, it measn that the system is backed up. Aka, we do not need to be making energy. Therefore, PDH kinase will phosphorylate the complex and TURN IT OFF. This results in a halt in the system, as pyruvate is not being converted to acetyl-coA to begin the TCA cycle.
Alternatively, PDH Phosphatase dephosphorylates the E1 complex, turning it on. It is activated by: Ca2+, ADP. A build up of these things tells the cell it needs to make energy. Excess ADP means that ATP is being used up and converted to ADP (adenosine diphosphate), so the cell should make more. On top of that, Ca2+ build up indicates muscle contraction as this process results in an influx of Ca2+.
Additionally, E2 is inhibited by acetyl-CoA and E3 is inhibited by NADH. Intuitively, this makes sense, as E2 generates acetyl-CoA while E3 generates NADH. If there is excess of each product, more is not needed and the production will halt.
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