Aconitases

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

The enzyme aconitase catalyzes the hydration of aconitic acid to two products citric acid and isocitric acid. Isocitric acid is optically active citric acid is not. What are the respective constitutions of citric acid and isocitric acid ... 

In the presence of the enzyme aconitase, the double bond of aconitic acid undergoes hydration. The reaction is reversible, and the following equilibrium is established ... 

AceCyl-CoA + oxaloacetate + HgO. CoASH + citrate 2. Citrate. isocitrate 3. Isocitrate + NAD. a-ketoglntarate + NADH + CO, + 4. a-Ketoglntarate + CoASH + NAD. snccinyl-CoA + NADH + CO, + H Citrate synthase Aconitase Isocitrate dehydrogenase u-Ketoglutarate dehydrogenase complex... 

Citrate is isomerized to isocitrate by aconitase in a two-step process involving aconitate as an intermediate (Figure 20.7). In this reaction, the elements... 

FIGURE 20.7 (a) The aconitase reaction converts citrate to cis-aconitate and then to isocitrate. Aconitase is stereospecific and removes the pro-/ hydrogen from the pro-/ arm of citrate, (b) The active site of aconitase. The iron-sulfur cluster (red) is coordinated by cysteines (yellow) and isocitrate (white). 

Fumarate is hydrated in a stereospecific reaction by fumarase to give L-malate (Figure 20.17). The reaction involves fraw5-addition of the elements of water across the double bond. Recall that aconitase carries out a similar reaction. 

Wachtershanser has also suggested that early metabolic processes first occurred on the surface of pyrite and other related mineral materials. The iron-sulfur chemistry that prevailed on these mineral surfaces may have influenced the evolution of the iron-sulfur proteins that control and catalyze many reactions in modern pathways (including the succinate dehydrogenase and aconitase reactions of the TCA cycle). 

Aconitase catalyzes the citric acid cycle reaction citrate isocitrate... 

In contrast to laboratory reactions, enzyme-catalyzed reactions often give a single enantiomer of a chiral product, even when the substrate is achiral. One step in the citric acid cycle of food metabolism, for instance, is the aconitase-catalyzed addition of water to (Z)-aconitate (usually called ris-aconitate) to give isocitrate. 

Problem 9.26 The aconitase-catalyzed addition of water to ds-aconitate in the citric acid cycle occurs with the following stereochemistry. Does the addition of the OH group occur on the Re or the Si face of the substrate What about the addition of the H Does the reaction have syn or anti stereochemistry ... 

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. 

Enzymes a) citrate synthase b) aconitase c) isocitrate dehydrogenase d) a-oxoglutarate dehydrogenase e) succiny CoA synthetase f) succinate dehydrogenase g) fumarase h) malate dehydrogenase i) nucleoside diphosphokinase. 

Figure 5.3 Major control points of glycolysis and the TCA cycle. Enzymes I, hexokinase II, phosphofructokinase III, pyruvate kinase IV, pyruvate dehydrogenase V, citrate synthase VI, aconitase VII, isocitrate dehydrogenase VIII, a-oxoglutarate dehydrogenase.

A further way in which metabolic control may be exercised is the artificial deprivation of required ions and cofactors, for example aconitase must have ferrous ions for activity. Conversely, addition of toxic ions is possible, for example aconitase is inhibited by cupric ions. Finally the use of metabolic analogues is possible. If monofluoroacetate is added to cells then monofluorocitrate is produced by titrate synthase and this compound inhibits the activity of aconitase. Great care has to be taken when using metabolic analogues, however, they are often less than 100% specific and may have unexpected and unwanted serious side effects. 

The first problem is that if ritric add is removed, there is apparently no way of regenerating oxaloacetate. The second problem is that to accumulate ritric add, aconitase has to be blocked to avoid dtric add being converted to aconitate. 

In fad the aconitase enzyme in A. niger is active even when dtric add is accumulating. This aconitase, if allowed to come to equilibrium, yields 90% dtrate, 3% ris-aconitate ferrous ions 7% isocitrate. To lower the activity of the enzyme, ferrous ions (essential for... 

The diagram looks very promising in terms of citric acid formation in that a-oxoglutarate dehydrogenase is inactive, isodtrate dehydrogenase has veiy low activity and aconitase equilibrates 90% towards dtric add. 

The effects of copper ions on the process were elucidated as follows. It was known that copper ions inhibit aconitase activity. 

With B as the major pathway the yield would fall dramatically if aconitase was totally inactivated. 

With A as the major pathway, it is reasonable to assume that there would be no effect on itaconic add production since aconitase is not involved. In fact it was shown that tiie presence of copper ions increases the yield of itaconic add by a factor of up to 3. 

When induced in macrophages, iNOS produces large amounts of NO which represents a major cytotoxic principle of those cells. Due to its affinity to protein-bound iron, NO can inhibit a number of key enzymes that contain iron in their catalytic centers. These include ribonucleotide reductase (rate-limiting in DNA replication), iron-sulfur cluster-dependent enzymes (complex I and II) involved in mitochondrial electron transport and cis-aconitase in the citric acid cycle. In addition, higher concentrations of NO,... 

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