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 hydrogens, the pro-R H atom of the pro-R arm of citrate, is abstracted during the reaction of theH₂O 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.