Oxaloacetate

Bcamples of metal-ion catalysed organic reactions in water where the catalyst acts exclusively as Lewis acid are the hromination of diketones" " and the decarboxylation of oxaloacetate. The latter reaction has been studied in detail. In 1941 it was demonstrated that magnesium(II) ions catalyse this reaction" Later also catalysis by other multivalent metal ions, such as Zn(II), Mn(II), Cu(II), Cd(ir), Fe(II), Pb(II), Fe(III)... 

The rate of the Nf -catalysed Diels-Alder reaction is barely sensitive to the presence of ligands. Apparently no significant effect due to -back donation is observed, in contrast to the effect of aromatic diamines on the metal-ion catalysed decarboxylation reaction of oxaloacetate (see Section 3.1.1). 

Cltra.te The citrate reaction is followed by monitoring the decrease in the concentration of NADH. Oxaloacetate instantiy decarboxylates to pymvate. 

Oxidation. Succinic acid reacts with hydrogen peroxide, giving different products that depend on the experimental conditions peroxysuccinic acid [2279-96-1] (CH2COOOH)2, oxosuccinic acid [328-42-7] (oxaloacetic acid) malonic acid [141-82-2] or a mixture of acetaldehyde, malonic acid, and make acid [6915-15-7]. Succinic anhydride in dimethylformamide (DMF) with H2O2 gives monoperoxysuccinic acid [3504-13-0], HOOCCH2CH2COOOH, mp 107°C (70). 

Potassium permanganate oxidizes succinic acid to a mixture of malic and tartaric acid [133-37-9]. 3-Hydroxypropionic acid [503-66-2] is obtained with sodium perchlorate. Cerium(IV) sulfate in sulfuric acid medium oxidizes succinic acid to oxaloacetic acid (71). 

Fig. 2. Kiebs (citiic acid) cycle. Coenzyme A is lepiesented CoA—SH. The cycle begins with the combination of acetyl coenzyme A and oxaloacetic acid to...

Oxoglutaric acid (2-oxopentane-l,5-dioic, a-ketoglutaric acid) [328-50-7] M 146.1, m 114 , 115-117 , (pK ,( see oxaloacetic acid above). Crystd repeatedly from Me2CO/ benzene, EtOAc or ethyl propionate. 

Citrate synthase catalyzes the metabolically important formation of citrate from ace-tyl-CoA and oxaloacetate [68]. Asp-375 (numbering for pig CS) has been shown to be the base for the rate-limiting deprotonation of acetyl-CoA (Fig. 5) [69]. An intennediate (which subsequently attacks the second substrate, oxaloacetate) is believed to be formed in this step the intermediate is thought to be stabilized by a hydrogen bond with His-274. It is uncertain from the experimental data whether this intermediate is the enolate or enol of acetyl-CoA related questions arise in several similar enzymatic reactions such as that catalyzed by triosephosphate isomerase. From the relative pK values of Asp-375... 

Wiegand, G., ef al. Crysfal strucfure analysis and molecular model of a complex of cifrafe synthase with oxaloacetate and S-acetonyl-coenzyme A. 

FIGURE 14.22 Glutamate aspartate aminotransferase, an enzyme conforming to a double-displacement bisnbstrate mechanism. Glutamate aspartate aminotransferase is a pyridoxal phosphate-dependent enzyme. The pyridoxal serves as the —NH, acceptor from glntamate to form pyridoxamine. Pyridoxamine is then the amino donor to oxaloacetate to form asparate and regenerate the pyridoxal coenzyme form. (The pyridoxamine enzyme is the E form.)... 

In 1932 Krebs was studying the rates of oxidation of small organic acids by kidney and liver tissue. Only a few substances were active in these experiments —notably succinate, fumarate, acetate, malate, and citrate (Figure 20.2). Later it was found that oxaloacetate could be made from pyruvate in such tissues, and that it could be further oxidized like the other dicarboxylic acids. 

Another important piece of the puzzle came from the work of Carl Martius and Franz Knoop, who showed that citric acid could be converted to isocitrate and then to a-ketoglutarate. This finding was significant because it was already known that a-ketoglutarate could be enzymatically oxidized to succinate. At this juncture, the pathway from citrate to oxaloacetate seemed to be as shown in Figure 20.3. Whereas the pathway made sense, the catalytic effect of succinate and the other dicarboxylic acids from Szent-Gyorgyi s studies remained a puzzle. 

In 1937 Krebs found that citrate could be formed in muscle suspensions if oxaloacetate and either pyruvate or acetate were added. He saw that he now had a cycle, not a simple pathway, and that addition of any of the intermediates could generate all of the others. The existence of a cycle, together with the entry of pyruvate into the cycle in the synthesis of citrate, provided a clear explanation for the accelerating properties of succinate, fumarate, and malate. If all these intermediates led to oxaloacetate, which combined with pyruvate from glycolysis, they could stimulate the oxidation of many substances besides themselves. (Kreb s conceptual leap to a cycle was not his first. Together with medical student Kurt Henseleit, he had already elucidated the details of the urea cycle in 1932.) The complete tricarboxylic acid (Krebs) cycle, as it is now understood, is shown in Figure 20.4. 

FIGURE 20.3 Martius and Knoop s observation that citrate could be converted to isocitrate aud then a-ketoglutarate provided a complete pathway from citrate to oxaloacetate. 

FIGURE 20.5 Citrate is formed in the citrate synthase reaction from oxaloacetate and acetyl-CoA. The mechanism involves nncieophiiic attack by the carbanion of acetyl-CoA on the carbonyl carbon of oxaloacetate, followed by thioester hydrolysis. 

Citrate synthase in mammals is a dimer of 49-kD subunits (Table 20.1). On each subunit, oxaloacetate and acetyl-CoA bind to the active site, which lies in a cleft between two domains and is surrounded mainly by a-helical segments (Figure 20.6). Binding of oxaloacetate induces a conformational change that facilitates the binding of acetyl-CoA and closes the active site, so that the reactive carbanion of acetyl-CoA is protected from protonation by water. 

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... 

This is a good point to pause in our trip through the TCA cycle and see what has happened. A two-carbon acetyl group has been introduced as acetyl-CoA and linked to oxaloacetate, and two COg molecules have been liberated. The cycle has produced two molecules of NADH and one of GTP or ATP, and has left a molecule of succinate. 

The TCA cycle can now be completed by converting succinate to oxaloacetate. This latter process represents a net oxidation. The TCA cycle breaks it down into (consecutively) an oxidation step, a hydration reaction, and a second oxidation step. The oxidation steps are accompanied by the reduction of an [FAD] and an NAD. The reduced coenzymes, [FADHg] and NADH, subsequently provide reducing power in the electron transport chain. (We see in Chapter 24 that virtually the same chemical strategy is used in /3-oxidation of fatty acids.)... 

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