試題詳解

44. In the citric acid cycle, citrate is isomerized to isocitrate by aconitase. H₂O was first abstracted  from citrate, and then rehydrated with H– and HO–adding back in opposite positions to produce  isocitrate. This reaction catalyzed by aconitase is quite stereospecific. As citrate has a total of four chemically equivalent hydrogens (labeled as H₁~H₄ in the figure), only one of the four HsㅡC-HA hydrogens, the pro-R H atom of the pro-R arm of citrate, is abstracted during the reaction of the H₂O abstraction by aconitase.  Which one of the hydrogens in the citrate molecule shown in the figure is the pro-R H atom of the pro-R arm?

(A) H₁
(B) H₂
(C) H₃
(D) H₄
(E) None of them.