Citric acid isomerization

Methylmalonyl-CoA mutase 5 -deoxyadenosylco-balamin is part of dimethylbenzimidazolecobamide coenzyme, a constituent of methylmalonyl-CoA mutase. This mutase catalyses the isomerization of methylmalonyl-CoA to succinyl-CoA (anaplerotic reaction of the citric acid cycle). 

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. 

There have been a number of isolated studies of metal-ion catalyzed nucleophilic reactions of other groupings. Particularly interesting is the induced nucleophilic attack on olefins. Hydration is normally very sluggish. Enzymes can speed up such reactions. Aconitase, an iron-containing enzyme, catalyzes the isomerization of citric acid to isocitric acid, through the intermediacy of cis-aconitic acid. A possible mechanism has been suggested based on the following Co(III) model chemistry. Rapid cyclization of the maleate ester produces Ai and AS chelated malate half ester ... 

Isomers are molecules with the same composition (i. e. the same molecular formula), but with different chemical and physical properties. If isomers differ in the way in which their atoms are bonded in the molecule, they are described as structural isomers (cf citric acid and isocitric acid, D). Other forms of isomerism are based on different arrangements of the substituents of bonds (A, B) or on the presence of chiral centers in the molecule (C). 

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. 

The steam-volatility of isomeric thienothiophenes allows separation from the resinous products formed during their synthesis from citric acid or acetylene. Selenophenoselenophenes are also steam-volatile. It is, however, very difficult to separate the isomeric thienothiophenes from each other and from benzo[b]thiophene with which they are also... 

The enzyme aconitase catalyzes the isomerization of citric acid to isocitric acid via the intermediate cis-aconitic acid (Scheme 46),530 and various attempts have been made to model this reaction.21 The cobalt Ill) complexes derived from methyl maleate (171) and methyl fumarate (172) have been prepared531 to study intramolecular attack by coordinated hydroxide on the alkene. Generation of the hydroxo species of the maleic acid complex leads to rapid cyclization to give the... 

The X-ray crystal structure of A1 -tetrahydrocannabinolic acid B is reported,362 and one of the previously reported (Vol. 4, p. 75) dihydrobenzofurans from the citric acid-catalysed condensation of orcinol with menth-4-en-3-ol is shown to be (256) by X-ray analysis.363 G.c.-m.s. assay of A -THC-OMe allows the detection of 1 ng mP1 plasma of A -THC.364 The mass spectral fragmentation of A -THC, A6-THC, and some isomeric cannabinoids to the prominent m/e 231 ion has been examined.365 Miniaturized syntheses of 32 natural, or potentially natural, cannabinoids are reported in connection with their chromatographic analysis.366... 

Perhaps the best characterized example of a subsite differentiated [4Fe-4S] protein is aconitase, which catalyzes the citrate-isocitrate isomerization in the citric acid cycle (257). Aconitase isolated aerobically is inactive and contains a [3Fe-4S] cluster. Activity is restored by incubation with Fe and this also reconstitutes the [4Fe-4S] cluster. Oxidation of the core results in loss of the fourth iron atom, regenerating the [3Fe-4S] form. Mossbauer studies have demonstrated that only one of the four iron sites is exchanged (258). X-ray studies on both [3Fe-4S] and [4Fe-4S] forms of pig heart aconitase 258a) showed that insertion of iron into [3Fe-4S] occurs isomorphously. The positions of the common atoms in the two forms of the core agree to within 0.1 A, supporting the view of the [3Fe-4S] cluster as an iron-voided cubane. A similar result was obtained for the seven iron ferredoxin from Azo-... 

Reaction 4 is, again, a component of the citric acid cycle. This isomerization prepares the molecule for subsequent oxidation and decarboxylation by moving the hydroxyl group of citrate from a tertiary to a secondary position. 

Nicotinamide (118) forms the 1,6-dihydropyridine (119) and the 6,6 -dimer (120) upon polarography in aqueous solution. The 4-isomer (121 Scheme 24) generates the aldehyde (122) when reduced in 0.8 M hydrochloric acid and the alcohol (123) in an alcoholic citric acid buffer solution. The thioamides produce the 4-aminomethyl- or 4-cyano-pyridines under related conditions. The isomeric cyanopyridines... 

Ita-conic Acid.—There is, however, a third acid known of the same composition as the two preceding. It is called ita-conic acid and like the others is obtained from citric acid by distiUation. More than the two isomers just explained are not possible according to geometric isomerism. The isomerism, therefore, of this new acid with the other two must be explained in some other way and has been shown to be structural isomerism due to the different position of the double bond. We have spoken of the fact that both citra-conic and mesa-conic acids, by the addition of hydrogen, are converted into methyl succinic acid. This same result is obtained with ita-conic acid also as may be explained by the following reactions. 

Citric acid is then converted to its isomer, isocitric acid, by way of the enzyme aconitaset which functions, in effect, by removing a molecule of water from the citric acid (producing aconitic acid, which, however, remains bound to the enzyme and is not released into the cell solution at all) and then replacing the water molecule isomerically (18) ... 

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