Isomerization of citrate to isocitrate

Citrate Synthase Is the Gateway to the TCA Cycle Aconitase Catalyzes the Isomerization of Citrate to Isocitrate... 

In some cases, separate proteins have been fused together at the genetic level to create a single multidomain, multifunctional enzyme (Figure 3-20c). For instance, the isomerization of citrate to isocitrate in the citric acid cycle is catalyzed by aconitase, a single polypeptide that carries out two separate reactions (1) the dehydration of citrate to form cis-aconitate and then (2) the hydration of ds-aconitate to yield isocitrate (see Figure 8-9). 

Reaction 2. The enzyme aconitase catalyzes the dehydration of citrate, producing ds-aconitate. The same enzyme, aconitase, then catalyzes addition of a water molecule to the ds-aconitate, converting it to isocitrate. The net effect of these two steps is the isomerization of citrate to isocitrate ... 

A wide range of metal ions is present in metalloenzymes as cofactors. Copper zinc snperoxide dismntase is a metalloenzyme that nses copper and zinc to help catalyze the conversion of snperoxide anion to molecnlar oxygen and hydrogen peroxide. Thermolysin is a protease that nses a tightly bonnd zinc ion to activate a water atom, which then attacks a peptide bond. Aconitase is one of the enzymes of the citric acid cycle it contains several iron atoms bonnd in the form of iron-sulfur clusters, which participate directly in the isomerization of citrate to isocitrate. Other metal ions fonnd as cofactors in metalloenzymes include molybdenum (in nitrate rednctase), seleninm (in glutathione peroxidase), nickel (in urease), and vanadinm (in fungal chloroperoxidase). see also Catalysis and Catalysts Coenzymes Denaturation Enzymes Krebs Cycle. 

In the next step of the TCA cycle, the hydroxyl (alcohol) group of citrate is moved to an adjacent carbon so that it can be oxidized to form a keto group. The isomerization of citrate to isocitrate is catalyzed by the enzyme aconitase, which is named for an intermediate of the reaction. The enzyme isocitrate dehydrogenase catalyzes the oxidation of the alcohol group and the subsequent cleavage of the carboxyl group to release CO2 (an oxidative decarboxylation). 

Step 2. Isomerization of Citrate to Isocitrate The second reaction of the citric acid cycle, the one catalyzed by aconitase, is the isomerization of citrate to isocitrate. The enzyme requires Fe +. One of the most interesting features of the reaction is that citrate, a symmetrical (achiral) compound, is converted to isocitrate, a chiral compound, a molecule that cannot be superimposed on its mirror image. 

Aconitic add an unsaturated tricarboxylic acid, usually occurring in the cis form (m.p. 130°C), but sometimes in the irons (m.p. 194-195 °C). Free A. a. was first discovered in aconite, Aconitum napellus. The anionic form of cis-A. a. (propene-cis-l,23-trioic acid) is important as an intermediate in the isomerization of citrate to isocitrate in the Tricarboxylic add cycle (see). 

Fig. 3. Condensation of acetyl-CoA and oxaloacetate to citrate and isomerization of citrate into isocitrate. The numbers next to the carbon atoms indicate the enumeration of the metabolite carbon atoms. OAA oxaloacetate AcCoA acetyl-coenzyme A...

Aconitase catalyzes the isomerization of citrate to isodtrate, isocitrate dehydrogenase catalyzes the oxidative decarboxylation of isocitrate to a-ketoglutarate, and a-ketoglutarate dehydrogenase catalyzes the oxidative decarboxylation of a-keto-glutarate to succinyl-CoA. Succinyl-CoA and the remaining intermediates are the 4-carbon intermediates of the Krebs cycle. Succinyl thiokinase catalyzes the release of coenzyme A from succinyl-CoA and the production of GTP. Succinate dehydro-... 

The tertiary hydroxyl group is not properly located in the citrate molecule for the oxidative decarboxylations that follow. Thus, citrate is isomerized into isocitrate to enable the six-carbon unit to undergo oxidative decarboxylation. The isomerization of citrate is accomplished by a dehydration step followed by a hydration step. The result is an interchange of a hydrogen atom and a hydroxyl group. The enzyme catalyzing both steps is called aconitase because cis-aconitate is an intermediate. 

Then, continuing in Scheme 11.89, isomerization of citrate (aconitase, aconitate hydratase, EC 4.2.1.3) to isocitrate occurs. This isomerization passes through... 

Step 2 of Figure 29.12 Isomerization Citrate, a prochiral tertiary alcohol, is next converted into its isomer, (2, 35)-isocitrate, a chiral secondary alcohol. The isomerization occurs in two steps, both of which are catalyzed by the same aconitase enzyme. The initial step is an ElcB dehydration of a /3-hydroxy acid to give cfs-aconitate, the same sort of reaction that occurs in step 9 of glycolysis (Figure 29.7). The second step is a conjugate nucleophilic addition of water to the C=C bond (Section 19.13). The dehydration of citrate takes place specifically on the pro-R arm—the one derived from oxaloacetate—rather than on the pro-S arm derived from acetyl CoA. 

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. 

Citrate is isomerized to isocitrate by aconitase (see Figure 9.5). [Note Aconitase is inhibited by fluoroacetate, a compound that is used as a rat poison. Fluoroacetate is converted to fluoroacetyl CoA, which condenses with oxaloacetate to form fluorocitrate—a potent inhibitor of aconitase—resulting in citrate accumulation.]... 

Aconitase, the enzyme that catalyzes this isomerization, is named for the fact that the unsaturated compound formed by removing H20 from either citrate or isocitrate, cA-aconitate, can also serve as substrate or product. Aconitase catalyzes the attainment of equilibrium between citrate, isocitrate, and civ-acon itate. The three compounds may be considered as belonging to the same metabolic pool. 

The cycle starts with the condensation of oxaloacetate (C4) and acetyl CoA (C2) to give citrate (Cg), which is isomerized to isocitrate (Cg). Oxidative decarboxylation of this intermediate gives a-ketoglutarate (C5). The second molecule of... 

Citrate is isomerized to isocitrate by a rearrangement of the molecule. Aconitate serves as an enzyme-bound intermediate. 

Citrate is isomerized to form a secondary alcohol that can be easily oxidized. In the next reaction of the cycle, citrate, which contains a tertiary alcohol, is reversibly converted to isocitrate by aconitase. During this isomerization reaction, an intermediate called cw-aconitate is formed by dehydration. The carbon-carbon double bond of cw-aconitate is then rehydrated to form the more reactive secondary alcohol, isocitrate. 

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