Aconitate hydratase

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. 

Enzymes usually function stereospedfically. In chiral substrates, they only accept one of the enantiomers, and the reaction products are usually also sterically uniform. Aconitate hydratase (aconitase) catalyzes the conversion of citric acid into the constitution isomer isocitric acid (see p.l36). Although citric acid is not chiral, aconitase only forms one of the four possible isomeric forms of isocitric acid (2i ,3S-isocitric acid). The intermediate of the reaction, the unsaturated tricarboxylic acid aconitate, only occurs in the cis form in the reaction. The trans form of aconitate is found as a constituent of certain plants. 

Aconitate hydratase [Fe4S4]— aconitase Phosphopyruvate hyd ratase— enolase ... 

The citrate ion, a very important prochiral metabolic intermediate, has three prochiral centers at C-2, C-3, and C-4, respectively. That at C-3 distinguishes the pro-R and pro-S arms and determines the stereochemical numbering. Citrate containing 14C in the sn-1 position is called s -citrate[l-14C] and is the form of labeled citrate that is synthesized in living cells from oxaloacetate and [l-14C]acetyl coenzyme A (see Fig. 10-6). The first step in the further metabolism of citrate is the elimination of the -OH group from C-3 together with the Hr proton from C-4 through the action of the enzyme aconitate hydratase (aconitase). In this case the proton at C-4 (in the pro-R arm) is selected rather than that at C-2. 

Among the most deadly of simple compounds is sodium fluoroacetate. The LD50 (the dose lethal for 50% of animals receiving it) is only 0.2 mg/kg for rats, over tenfold less than that of the nerve poison diisopropylphosphofluoridate (Chapter 12).a b Popular, but controversial, as the rodent poison "1080," fluoroacetate is also found in the leaves of several poisonous plants in Africa, Australia, and South America. Surprisingly, difluoroacetate HCF2-COO is nontoxic and biochemical studies reveal that monofluoroacetate has no toxic effect on cells until it is converted metabolically in a "lethal synthesis" to 2R,3R-2-fluorocitrate, which is a competitive inhibitor of aconitase (aconitate hydratase, Eq. 13-17).b This fact was difficult to understand since citrate formed by the reaction of fluorooxalo-acetate and acetyl-CoA has only weak inhibitory activity toward the same enzyme. Yet, it is the fluorocitrate formed from fluorooxaloacetate that contains a fluorine atom at a site that is attacked by aconitase in the citric acid cycle. 

Fig. 5.22. Oxidation of acetyl-CoA via the tricarboxylic acid (TCA) cycle. Individual enzymes of the pathway are marked. 1, citrate synthase 2 and 3, cis-aconitate hydratase 4 and 3, isocitrate dehydrogenase 6, a-oxo glutarate dehydrogenas 7, succinate thiokinase 8, succinate...

Aconitase is the trivial name for citrate dehydratase cw-aconitate hydratase (EC 4.2.1.3). It catalyzes the reversible isomerization reaction of citrate into isocitrate via the intermediate cA-aconitate (Figure 2). It is a water-soluble, monomeric protein. In eukaryotic cells aconitase is located in the mitochondrial matrix. In prokaryotes the enzyme occurs in the cytoplasma. The pig heart enzyme consists of 754 amino-acid residues, providing a molecular mass of 83 kDa [27], Aconitase from other sources has similar size. The porcine protein is synthesized with a mitochondrial targeting sequence. The mature, functional protein can be (over)expressed in Escherichia coli [28],... 

Medvedeva, L.V., Popova, T.N., Artiukhov, V.G., Matasova, L.V. (2002). Catalytic properties of cytoplasmic and mitochondrial aconitate hydratase from rat cardiomyocytes. Izv. Akad. Nauk Ser. Biol. 5 528-33. (In Russian)... 

Other C-0 lyase enzymes include aconitate hydratase or aconitase (E. C. 4.2.1.3), an enzyme that catalyzes two tricarboxylic acid cycle steps from isocitric acid to citrate (14)1141 or vice versa, via the intermediate cis-aconitate (13). Citrate dehydratase (E. C. 4.2.1.4) is only capable of converting citrate to cis-aconitate and does not act on isocitrate (15) 115l... 

Formation of IsocItrate via cIs-Aconltate The enzyme aconitase (more formally, aconitate hydratase)... 

A primary intracellular target for the biological actions of nitric oxide ( NO) production is intracellular iron (Hibbs et al., 1990 Henry et al., 1993). In activated macrophages and their tumor cell targets, a characteristic pattern of metabolic dysfunction is observed as a result of -NO synthesis, which includes loss of nonheme iron-containing enzyme function, including aconitate hydratase, complexes I and II of the mitochondrial electron transfer chain (Hibbs et al., 1990) as well as the nonheme iron-containing enzyme ribonucleotide reductase (Lepoivre et al., 1991). 

The operation of the cycle produces large quantities of the reduced forms of the nucleotides of adenine with nicotinamide (NAD and NADP) and ribo-flavine (FP). The regeneration of these coenzymes is effected by a transfer of electrons from the reduced forms to the oxygen of the atmosphere. In almost every kind of living cell, this transfer is mediated by some or all of the cytochrome respiratory chain (Section 5.4.3). Most of the organisms that lack all cytochromes have insignificant aerobic metabolism. Few enzymatic differences in the cycle have been demonstrated in mammals, but in the rat there is six times more aconitate hydratase in the heart than in skeletal muscle (Dixon and Webb, 1979). 

Citrate synthase 2 aconitate hydratase 3 isocitrate dehydrogenase 4 oxoglutarate dehydrogenase 5 nucleoside diphosphate kinase 6 succinate dehydrogenase 7 fumarate hydratase 8 malate dehydrogenase 9 isocitrate lyase 10 malate synthase... 

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

Aconitic acid involved in the TCA and glyoxylate cycles and the acid commonly occurring in nature has the c/5-configuration. The trans- om x has also been isolated from some plant materials - for example, sugarcane Saccharum offi-cinarum) juice (17), tomato (Lycopersicon esculentum) (56) or moss (Bryophyta) (34). However, some of the occurrences might be artifacts of the isolation procedures, for an interconversion between two isomers of aconitic acid has been reported (14, 81). In both cycles, cw-aconitic acid is formed upon dehydration of citric acid catalyzed by aconitase (aconitate hydratase) which also catalyzes the rehydration of cw-aconitate to isocitric acid. 

Inhibits aconitate hydratase in TCA cyde, causing cerebral energy deficit and accumulation of citrate and lact ... 

---