Malonic acid inhibition

Malonic acid is a classical example of a true competitive inhibitor. Malonic acid inhibits succinic dehydrogenase, which catalyzes the oxidation of succinic acid to fumaric acid, as shown below. 

Condensation of coumaric acid with malonic acid yields the basic chalcone and stilbane skeletons (see Fig. 3.6). Stilbenes are found in most vascular plants, where they exhibit fungicidal and to a lesser extent antibiotic properties. They function as both constitutive and inducible defense substances. Some stilbenes inhibit fungal spore germination and hyphal growth, whereas others are toxic to insects and parasitic nematodes (round-worms). They also possess antifeeding and nematicide properties in mammals. For example, resveratrol (a stilbene in red wine) suppresses tumor formation in mammals. 

The most recent advances on enzymatic inhibition are related to endonucleases and polymerases. A tris-malonic acid fullerene derivative can interfere with DNA restrictive enzymatic reactions, demonstrating a dose-dependent inhibition of these enzymes, with an IC50 in the micromolar range. The addition of ROS scavenger does not revert the enzymatic activity, indicating that the fullerene action should be exerted in a direct way (Yang et al., 2007d). 

At the start, the cycle begins with a certain amount of Ce+4 ions. The second reaction provides Br- ions, which inhibit the first reaction. This leads to an increase in concentration of Ce+3. After reaching a certain amount of Ce+3, the oxidation reaction starts, since little Ce+4 remains. The system can no longer produce sufficient Br to inhibit the reaction, and Ce+3 decreases rapidly, producing Ce+4 until the cycle is completed. It is possible to maintain indefinite oscillations with constant frequency in a continuous flow stirred reactor into which bromate, malonic acid, and cerium catalyst are being supplied at a uniform rate. 

An oscillatory cycle can be qualitatively described in the following way. Suppose that a high enough ceric ion concentration is present in the system. Then, bromide ion will be produced rapidly, and its concentration will also be high. As a result, autocatalytic oxidation is completely inhibited, and the ceric ion concentration decreases owing to the reduction of ceric ion by malonic acid. The bromide ion concentration decreases along with that of ceric ion. When the ceric ion concentration reaches its lower threshold, the bromide ion concentration drops abruptly. The rapid autocatalytic oxidation starts and raises the ceric ion concentration. When this concentration reaches its upper threshold, the bromide concentration increases sharply, completely inhibiting the autocatalytic oxidation. The cycle then repeats. Pulse injections of Br, Ag, and Ce " result in phase shifts in oscillations of the Ce " " concentration (Fig. 3), which confirm the mechanism. 

Wood and coworkers have demonstrated that pigeon liver mince poisoned with malonate is unable to convert C 02 into isotopic succinate but can direct the isotope into malate and fumarate as well as into the a-carboxyl of a-ketoglutarate (see Fig. 5). On the other hand, non-poisoned liver possesses the ability to anaerobically fix C 02 in succinic acid as well as fumaric and malic acids. The dicarboxylic acids invariably contain the isotopic carbon predominantly in their respective carboxyl groups. These important experiments provide conclusive proof for two main concepts. First, that malonate specifically inhibits succinic acid dehydrogenase and hence blocks completely the formation of succinic from fumaric acid. Secondly, there must be two routes by which succinate is formed. By the first pathway, pyruvate can condense with... 

The Belousov-Zhabotinsky reaction demonstrated here is set in train by the reduction of potassium bromate to elemental bromine by malonic acid and manganese(II) sulfate this is shown by the orange coloration. The reaction of the bromine with malonic acid to give mono or dibromomalonic acid leads to decolorisation. At the same time more bromine is formed in the initial redox process, and this again replaces one or two hydrogen atoms of the malonic acid. The process is repeated many times the start reaction is inhibited by complexa-tion of the brominated malonic acid by Mn(ll) ions, so that the oscillation slowly comes to an end. ... 

The permanent inhibition of concentration oscillations by the addition of trace amounts of chloride to bromate-cerium-malonic acid systems was found to be of temporary duration only. The chloride inhibition mechanism was considered to involve oxidation of chloride to chlorous acid which then reduced Ce to Ce " to prevent oscillation. ... 

Since the main reactants, bromate and malonic acid, are in surplus, inhibition and negative feedback in the BZ reaction are quite different from that in the previous example of thermokinetic oscillations. Inhibition is provided in part by a direct decomposition of HBr02 (analogous to heat removal in the previous example) but mainly by a chain of reactions of oxidized ion catalyst with brominated malonic acid (BrMA), summarized as... 

The rate of ceric oxidation of malonic add and its diethyl ester in acetic acid/sul-furic acid solutions has recently been reported by Vaidya et al. (1987). They find no evidence for precursor complex formation in either system. The reactive Ce(IV) species appear to be Ce(S04)2 ( 2) and CefSO ) " k 2). The second-order rate parameter for the oxidation of malonic add is 40 times greater than that for the ester. Oxidation of the ester is proposed to occur through the enol form yielding a malonyl radical analogous to that identified by Amjad and McAuley. Foersterling et al. (1987) find that the second-order rate constant for malonic acid oxidation by Ce(lV) in sulfuric acid is in excellent agreement with the value of Vaidya et al. They observe that Ce(III) does inhibit the reaction in sulfuric add, which they attribute to a reversible Ce(IV) malonic acid rate-controlling step. 

The next relevant discovery was made in 1910, when it was noted that some enzymes are blocked by substances whose molecular structure resembles that of the normal substrates. Thus amylase, which normally hydrolyses starch, is inhibited by glucose (see Section 9.1). Again, malonic acid 9.3) competitively inactivates the enzyme succinic dehydrogenase by displacing the normal substrate, succinic acid 9.4), from the enzyme (Quastel and Wooldridge, 1927). A similar phenomenon in physiology is the toxic action of carbon monoxide... 

All the component reactions investigated are found to be exothermic. Initial temperature rise of bromide + bromate reaction was found to be the highest (0.55°C/min) while that of cerous + bromate + malonic acid was to be found to be quite low (0.085°C/min). Thus in the first stage, when the reaction was mixed, the latter reaction involving autocatalysis predominates and the temperature rise is very slow. On the other hand, when Br + BrOj reaction involving inhibition reaction becomes dominant, there is a sharp rise in temperature. The thermochemical behaviour is thus in conformity with the FKN mechanism (Br -control mechanism). 

Noyes for bromate-malonic acid reaction and involves formation of BrOg radicals which oxidize the reduced metal complex. Silver(i) inhibition in the gallic acid reaction is observed, presumably by reduction of the Br present... 

Now add 15 mL each of solutions Al and B to a 100-mL beaker equipped with a stir bar. A brown color will appear because of the production of bromine, which disappears as it reacts with the malonic acid. When the solution clears, add 15 mL of solution C, and position the electrodes immediately so that the induction period may be accurately observed and measured. Stir at a low rate, or oscillations may be inhibited. The recorder may be set at any desired speed (typically about 1 cm min ). The voltage scale is usually set at either 250 or 500 mV. 

Oscillating chemical systems, observed in the mixture Ce -BrOs -malonic acid have been investigated. The sequence of intermediates formed in the oxidation of malonic acid by Ce is apparently malonic->tartronic->glyoxylic- forraic acids. Though formic acid is normally inert to oxidation by Ce Br catalyses the reaction with a mechanism which involves Bra and HOBr as active species. In the presence of Br also reacts with malonyl radicals to form bromomalonic acid which is in turn oxidized by Ce by a complex mechanism. Oscillations of these systems only occur if between 0.25 and 0.75 bromide ions are produced for every Ce consumed. This requires that malonyl radicals preferentially attack bromomalonic acid to liberate Br. Inhibition of the oscillations in this system by Cl has been shown to be of only temporary duration. The mechanism involves oxidation of chloride ion to chlorous acid which then reduces to Ce. Oscillations resume when the chlorous acid has been completely oxidized to chlorate. 

Without doubt the main interest here lies in the oscillating reactions involving bromate (the Belousov-Zhabotinskii reaction), i.e., the catalytic oxidative bromination with acidic bromate of (usually) aliphatic polycarboxylic acids or polyketones. The best-studied system involves malonic acid and a catalyst, usually cerium. The oxidation of Ce(III) by acidic bromate is inhibited periodically by bromide whenever the concentration of bromide exceeds a critical value. 

To explain the inhibition of the incorporation of malonic acid into fatty acid in intact plants by the cyclohexane-1,3-diones (Burgstahler 1985) and aryloxy-phenoxypropionic acid-herbicides (Hoppe and Zacher 1982), we suggest the action of a malonate and/or malony1-CoA decarboxylase, able to catalyze the formation of acetate/acetyl-CoA, substrates the incorporation of which into fatty acids can then be blocked by these herbicides. Our model is summarized in Figure 4. 

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