Mechanism
Phosphopentose utilizes an acid/base type of catalytic mechanism. The reaction proceeds in such a way that trans-2,3-enediol phosphate is the intermediate. The two aspartic acids mentioned above act as proton donors and acceptors. Asp37 and Asp175 are both hydrogen bonded to the iron cation in the active site. When Asp37 is deprotonated, it attacks a proton on the third carbon of D-ribulose 5-phosphate, which forms the intermediate. In a concerted step, as Asp37 grabs a proton, the carbonyl bond on the substrate grabs a second proton from Asp175 to form a hydroxyl group. The iron complex helps stabilize any additional charges. It is C3 of D-ribulose 5-phosphate which undergoes this epimerization, forming D-xylulose 5-phosphate. The mechanism is clearly demonstrated in the figure.
Read more about this topic: Phosphopentose Epimerase
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