Editing Pyruvate dehydrogenase complex (section)

==Mechanism==
{{Citations needed|date=December 2023}}
{| class="wikitable"
! Enzymes !! Abbrev. !! [[cofactor (biochemistry)|Cofactor]]s !! # subunits prokaryotes !! # subunits eukaryotes
|- 
| [[Pyruvate dehydrogenase]]<br />({{EC number|1.2.4.1}}) || E1 || [[Thiamine diphosphate|TPP&nbsp;(thiamine&nbsp;pyrophosphate)]], Mg<sup>2+</sup>|| 32 || 30
|- 
| [[Dihydrolipoyl transacetylase]]<br />({{EC number|2.3.1.12}}) || E2 || alpha-[[lipoic acid]] (lipoate)<br /> || 24 || 60
|- 
| [[Dihydrolipoyl dehydrogenase]]<br />({{EC number|1.8.1.4}}) || E3 || [[flavin adenine dinucleotide|FAD]]<br /> || 16 || 12
|}
[[File:PDH schema.png|thumb|upright=2|class=skin-invert-image|PDC Mechanism with pyruvate (R=H)]]

=== Pyruvate dehydrogenase (E1) ===
{{Main|Pyruvate dehydrogenase}}
Initially, [[pyruvate]] and [[thiamine|thiamine pyrophosphate]] (TPP or [[thiamine|vitamin B<sub>1</sub>]]) are bound by [[pyruvate dehydrogenase]] subunits.<ref name=":0" /> The [[thiazole|thiazolium]] ring of TPP is in a [[zwitterion]]ic form, and the [[anion]]ic C2 carbon performs a nucleophilic attack on the C2 (ketone) carbonyl of pyruvate. The resulting intermediate undergoes [[decarboxylation]] to produce an acyl anion equivalent (see [[cyanohydrin]] or aldehyde-dithiane [[umpolung]] chemistry, as well as [[benzoin condensation]]). This anion attacks S1 of an oxidized lipoate species that is attached to a [[lysine]] residue. In a ring-opening S<sub>N</sub>2-like mechanism, S2 is displaced as a sulfide or sulfhydryl moiety. Subsequent collapse of the tetrahedral intermediate ejects thiazole, releasing the TPP cofactor and generating a thioacetate on S1 of lipoate. The E1-catalyzed process is the rate-limiting step of the whole pyruvate dehydrogenase complex.

=== Dihydrolipoyl transacetylase (E2) ===
At this point, the lipoate-thioester functionality is translocated into the [[Dihydrolipoyl transacetylase]] (E2) active site,<ref name=":0" /> where a transacylation reaction transfers the acetyl from the "swinging arm" of lipoyl to the thiol of [[coenzyme A]]. This produces [[acetyl-CoA]], which is released from the enzyme complex and subsequently enters the [[citric acid cycle]]. E2 can also be known as lipoamide reductase-transacetylase.

=== Dihydrolipoyl dehydrogenase (E3) ===
The [[lipoic acid|dihydrolipoate]], covalently bound to a lysine residue of the complex, is then transferred to the Dihydrolipoyl dehydrogenase (E3) active site,<ref name=":0" /> where it undergoes a [[Flavin group|flavin]]-mediated oxidation, similar in chemistry to e.g. thioredoxin reductase. First, [[flavin adenine dinucleotide|FAD]] oxidizes dihydrolipoate back to its lipoate (disulfide) resting state, producing FADH<sub>2</sub>. Then, the substrate [[nicotinamide adenine dinucleotide|NAD<sup>+</sup>]] oxidizes FADH<sub>2</sub> back to its FAD resting state, producing NADH and H<sup>+</sup>.