So now, before this ENL intermediate can actually act as a nucleophile, we have to convert this oxyloacetate into a good electrophile. Because remember, anytime we have a nucleophilic attack, we have an electrophile that is actually being attacked. Now, in this form here, the oxyloacetate is not a good enough electrophile. And so what happens is now we have histidine 320 that donates its H ion onto this oxygen of the carbonyl group of the oxalo acetate. And this basically weakens the pi bond, it forms a carbocation intermediate and that converts this poor electrophile into a much better electrophile. And so now, because this is a very good electrophile, this pi bond of this ENL can act as a nucleophile and attack the carbon nucleophilically to form that connection between this acetyl group and this citrate molecule.