Aconitase reactions catalyzed, stereospecificity

Aconitase (citrate(isocitrate)hydro-lyase) catalyzes the isomerization of citrate and isocitrate via cM-aconitate, as shown in Figure 2. The reaction is stereospecific, as shown, with the hydration reactions occurring in a trans orientation across the double bond. Also as shown, the H... 

Two consecutive reactions of the citric acid cycle (Fig. 10-6), the dehydration of citrate to form czs-aconi-tate and the rehydration in a different way to form isocitrate (Eq. 13-17), are catalyzed by aconitase (aconi-tate hydratase). Both reactions are completely stereospecific. In the first (Eq. 13-17, step a), the pro-R proton from C-4 (stereochemical numbering) of citrate is removed and in step c isocitrate is formed. Proton addition is to the re face in both cases. 

Two of the enzymes of the tricarboxylic acid cycle, aconitase and fumarase, catalyze reactions in which water is added reversibly to an unsaturated polycarboxylic acid. Both enzymes exhibit rigid stereospecificity fumarase forms only L-malate from fumarate and forms only fumarate (trans) and not maleate (czs-ethylenedicarboxylic acid), and aconitase reacts with only cis-, not imns-aconitate, and with D-, not L-isocitrate. Citrate is a symmetrical molecule, with no optical isomers, but it will be shown that steric factors also enter into the reaction of this substrate with aconitase. The enzymes of the tricarboxylic acid cycle, in contrast to the glycolytic enzymes, are associated with intracellular granules known as mitochondria. Studies of the individual enzymes have depended to a large extent on the separation of soluble activities from these particles. Aconitase and fumarase are released from the particles very rapidly under mild conditions often in the preparation of cell-free homogenates these activities are largely solubilized, and special care must be taken to demonstrate their origin in mitochondria. 

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