Dehydration-rehydration reaction

Aconitase [citrate(isocitrate) hydro-lyase, EC 4.2.1.3] is the second enzyme of the citric acid cycle, which plays a central role in metabolism for all aerobic organisms. This enzyme catalyzes a dehydration-rehydration reaction interconverting citrate and 2R,3S-isocitrate via the allylic intermediate ds-aconitate. 

Together, the biochemical, spectroscopic, and structural data suggest a mechanism for aconitase in which the unique iron site of the [4Fe S] cluster serves as a Lewis acid in catalysis, binding and polarizing substrate to facilitate the dehydration/rehydration reactions. The use of an iron-sulfur cluster in a nonredox role, as well as its function in binding substrates, was novel and unexpected, and was the first indication of the remarkable diversity of these clusters in functions beyond electron transfer. 

The conversion of citrate to isocitrate in the citric acid cycle actually occurs by a dehydration-rehydration reaction with aconitate as an isolatable intermediate. A single enzyme, aconitase, catalyzes the conversion of citrate to aconitate and aconitate to isocitrate. An equilibrium mixture of citrate, aconitate, and isocitrate contains about 90, 4, and 6 percent of the three acids, respectively. 

The hemi-hydrate will slowly rehydrate hardening and setting . Many millions of tonnes of gypsum are used worldwide as a consequence of this dehydration - rehydration reaction and are used in bnilding boards, moulds, casts, plasters, etc. 

Worthy of note in this reaction is that citrate displays prochirality (see Section 3.4.7). The methylene carbons may be considered prochiral, in that enzymic elimination of a proton is likely to be entirely stereospecific. In addition, the apparently equivalent side-chains on the central carbon are also prochiral and going to be positioned quite differently on the enzyme. This means that only one of these side-chains is involved in the dehydration-rehydration... 

Aconitase is an iron-sulfur protein, or nonheme iron protein. It contains four iron atoms that are not incorporated as part of a heme group. The four iron atoms are complexed to four inorganic sulfides and three cysteine sulfur atoms, leaving one iron atom available to bind citrate and then isocitrate through their carboxylate and hydroxyl groups (Figure 17,12). This iron center, in conjunction with other groups on the enzyme, facilitates the dehydration and rehydration reactions. We will consider the role of these iron-sulfur clusters in the electron-transfer reactions of oxidative phosphorylation subsequently (Section 18.3.1). 

The dehydration and rehydration reactions of calcium sulfate dihydrate (gypsiun) are of considerable technological importance and have been the subject of many studies. On heating, CaS04.2H20 may yield the hemihydrate or the anhydrous salt and both the product formed and the kinetics of the reaction are markedly dependent upon the temperature and the water vapour pressure. At low temperatures (i.e. < 383 K) the process fits the Avrami-Erofeev equation (n = 2) [75]. The apparent activation energy for nucleation varies between 250 and 140 kJ mol in 4.6 and 17.0 Torr water v our pressure, respectively. Reactions yielding the anhydrous salt (< 10 Torr) and the hemihydrate ( (HjO) >17 Torr) proceeded by an interface mechanism, for which the values of E, were 80 to 90 kJ mol. At temperatures > 383 K the reaction was controlled by diffusion with E, = 40 to 50 kJ mol. ... 

The first one of these converts sucrose into valuable noncarbohydrate compounds by degradation of the sucrose skeleton itself. Thereby, hydrolysis of sucrose (the usual first step) is subsequently followed by further degradation under elevated temperature and pressure conditions in the presence of certain catalysts. Chemical reactions or reaction cascades, e.g., dehydration/rehydration, ring opening/closure, bond scission/formation, lead to noncarbohydrate structures which are of known value from petrochemically-derived compounds. One of the most prominent representatives of these sucrose degradation products is 5-hydroxymethylfurfural (HMF). 

In leucine biosynthesis, the intermediate 218 on the valine pathway reacts with acetyl-CoA to yield a-isopropylmalate 225 whose configuration has been shown to be S by X-ray crystallography (200). This reaction is analogous to that catalyzed by citrate synthase, and indeed the subsequent reaction, dehydration/rehydration giving ) -isopropylmalate 226, is analogous to the conversion of citrate to isocitrate. The configuration of / -isopropylmalate 226 had been shown to be 2i ,3S (201), and so the stereochemistry of the citrate and isopropylmalate reactions was identical. j -Isopropylmalate 226 was finally converted to leucine 205 by a 4-pro-R NADH specific dehydrogenase (EC 1.1.1.85) (202) and transamination (Scheme 61). 

DNA, coding for GFP and T7 RNA polymerase, was introduced in liposomes composed by a phospholipid mixture (a), together with the T T machinery (SP6 RNA polymerase and E. coli cell extracts). T7 RNA polymerase was synthesized firstly (the rnapol gene was under SP6 promoter), and in turn it allowed the synthesis of GFP (the gfp gene was under T7 promoter). This is an example of cascade genetic reactions inside large MLVs prepared by the dehydration/rehydration method. 

The main route for glycerol conversion is based on the oxidation and dehydration/rehydration steps. Usually, the oxidation process is performed by a dehydrogenation or an oxidation step with oxidants. On the other hand, a dehydration reaction occurred xmder acidic conditions however, most of the reported reactions are in alkaline media. 

Citrate is isomerized to isocitrate by the enzyme aconitase (aconitate hydratase) the reaction occurs in two steps dehydration to r-aconitate, some of which remains bound to the enzyme and rehydration to isocitrate. Although citrate is a symmetric molecule, aconitase reacts with citrate asymmetrically, so that the two carbon atoms that are lost in subsequent reactions of the cycle are not those that were added from acetyl-CoA. This asymmetric behavior is due to channeling— transfer of the product of citrate synthase directly onto the active site of aconitase without entering free solution. This provides integration of citric acid cycle activity and the provision of citrate in the cytosol as a source of acetyl-CoA for fatty acid synthesis. The poison fluo-roacetate is toxic because fluoroacetyl-CoA condenses with oxaloacetate to form fluorocitrate, which inhibits aconitase, causing citrate to accumulate. 

In the presence of bases, monomeric as well as polymeric sugars are converted into various carboxylates salts [14]. The reaction proceeds through a retro-Michael dehydration step to form an unsaturated aldehyde, followed by rehydration of the aldehyde function and isomerization to acid (Fig. 2.3). The same reaction is also responsible for a stepwise depolymerization of polymeric sugar to carboxylate... 

Citrate is subsequently isomerized to isocitrate this involves dehydration and rehydration via the intermediate cis-aconitate. Both reactions are... 

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. 

---