Steps in mechanism of pyruvate dehydrogenase

I am having some trouble with certain steps in the mechanism below (from Mike Lu on Proteopedia) detailing the mechanism of action of pyruvate dehydrogenase converting pyruvate to acetyl-CoA. This reaction happens under the physiological conditions present in the human body ($\mathrm{p}H = 7.4, 37^{\circ}C$, 1 atm, various intracellular ionic concentrations: http://www.ld99.com/reference/old/text/2878909-566.html).

• In the first step, how does the ylide of TPP form?
• Why is the deprotonation of the hydroxyl group to form a ketone in the 6th step favoured, especially given that the $\ce{-OH}$ group normally has a very high $\mathrm{p}K_\mathrm{a}$?

• TPP = thiamine pyrophosphate? Your first structure is not an anion, it is a neutral molecular species, depending on E1 being neutral, too. In this resonance form it is an ylide, there is another form without partial charges. It is a persistent carbene and an aromatic system. It would be good to know the conditions under which this forms. I cannot get my head around the fact that the anion of the alpha-keto acid would act as an electrophile. – Martin - マーチン Sep 2 '15 at 8:21
• I would assume it happens under the physiological conditions present in the human body. – James Harrison Sep 8 '15 at 5:37
• Unfortunately I am not educated enough in that area to know what these physiological conditions are. I can only try to understand what you have posted here and therefore I might be missing information that you can better define - or that are defined in some standard way, which you could probably link to. That would possibly enhance your question and some non biochemists might give it a chance, too. – Martin - マーチン Sep 8 '15 at 5:44