Oxaloacetate formation

Aspartate aminotransferase 2.6.1.1 L-Aspartate Oxaloacetate Formation of a hydrazone... 

Pyruvate is derived from phosphoenolpyruvate may be inter-converted into lactate, alanine, oxaloacetate. Formation ofacetyl-CoA from pyruvate is essentially irreversible... 

The reaction will proceed toward oxaloacetate formation in the cell if low product concentration is maintained. Oxidation of NADH by the mitochondrial electron transport system and utilization of oxaloacetate in the formation of citrate shifts the malate-oxaloacetate reaction toward oxaloacetate production. 

When the nitrogen concentration was less than 0.04 % or more than 0.4 %, production of griseofulvin was inhibited. The retarding effect of the high nitrogen concentration levels was explained by a specific inhibition of NH/ on oxaloacetate formation [17],... 

The carboxylation of pyruvate condenses the tricarbon compound with a molecule of acetyl Co A consequently, oxaloacetate formation drives acetyl CoA out of the metabolic traffic. 

Anaplerotic reactions refer to C3-carboxylation and C4-decarboxylation around the phosphoenolpyruvate-pyruvate-oxaloacetate node, which interconnect the TCA cycle with glycolysis. These reactions result in direct oxaloacetate formation or depletion. Carboxylation of phosphoenolpyruvate catalyzed by phosphoenolpyruvate carboxylase and that of pyruvate by pyruvate carboxylase contribute to its formation. Accordingly, decarboxylation of oxaloacetate catalyzed by phosphoenolpyruvate carboxykinase and oxaloacetate decarboxylase form phosphoenolpyruvate and... 

The key reaction was recognized by Utter to be the phosphorylation of oxaloacetate. Formation of the enol is facilitated in this compound and phosphoenol-pyruvate is then formed by phosphorylation with inosine triphosphate (ITP) (decarboxylation occurs at the same time). Oxaloacetate itself is provided by the direct carboxylation discussed above via a biotin enzyme. Alternately, the reductive carboxylation yields malate which is converted to oxaloacetate. The first reaction, although requiring additional ATP, appears to be the more important one, and, of course, the equilibrium favors C02-fixation more strongly in this way. The following... 

During the next reaction, fumarate is hydrated by another stereospecific enzyme, fumarate hydra-tase, to form L-malate which is oxidized to oxalo-acetate by an NAD -linked malate dehydrogenase (EC 1.1.1.37). Although the AG value implies that the reverse reaction is thermodynamically more favourable, oxaloacetate formation is promoted by its role in the highly exergonic initial reaction of the cycle. 

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... 

One of these alternate models, postulated by Gunter Wachtershanser, involves an archaic version of the TCA cycle running in the reverse (reductive) direction. Reversal of the TCA cycle results in assimilation of CO9 and fixation of carbon as shown. For each turn of the reversed cycle, two carbons are fixed in the formation of isocitrate and two more are fixed in the reductive transformation of acetyl-CoA to oxaloacetate. Thus, for every succinate that enters the reversed cycle, two succinates are returned, making the cycle highly antocatalytic. Because TCA cycle intermediates are involved in many biosynthetic pathways (see Section 20.13), a reversed TCA cycle would be a bountiful and broad source of metabolic substrates. 

It is worth noting that the reaction catalyzed by PEP carboxykinase (Eigure 20.25) could also function as an anaplerotic reaction, were it not for the particular properties of the enzyme. COg binds weakly to PEP carboxykinase, whereas oxaloacetate binds very tightly K, = 2 X 10 Af), and, as a result, the enzyme favors formation of PEP from oxaloacetate. 

Preparation o the key intermediate for the chloroquinoline series starts with Shiff base formation of metachloroaniline with ethyl oxaloacetate (66). Heating of the intermediate leads to cyclization into the aromatic ring and consequent formation of the quinoline ring (67). Saponification of the ester to the acid... 

Glycolysis, the pentose phosphate pathway, and fatty acid synthesis are all found in the cytosol. In gluconeo-genesis, substrates such as lactate and pyruvate, which are formed in the cytosol, enter the mitochondrion to yield oxaloacetate before formation of glucose. 

Figure 28-3. Formation of alanine by transamination of pyruvate. The amino donor may be glutamate or aspartate. The other product thus is a-ketoglutarate or oxaloacetate.

The synthesis of this aminoquinoline starts with one of the standard sequences for preparation of 4-hydroxyquinolines, i.e., with the formation of the Shiff base (5) from the appropriately substituted aniline and diethyl oxaloacetate. Thermal cycliza-tion gives the quinolone (6) this then spontaneously tautomerizes to the enol form (7). Saponification followed by decarboxylation gives the desired quinolol... 

The Jirst indirect route in glucose synthesis involves the formation of phosphoenolpyruvate from pyruvate without the intervention of pyruvate kinase. This route is catalyzed by two enzymes. At first, pyruvate is converted into oxaloacetate. This reaction occurs in the mitochondria as the pyruvate molecules enter them, and is catalyzed by pyruvate carboxylase according to the scheme... 

It was observed that glutamate and aspartate are diverted predominantly to the synthesis of cell substance rather than to the formation of oxalate. It is not inconsistent to see oc-ketoglutarate being formed from glutamate, while no oxaloacetic acid can be detected in the medium containing aspartate, as the oxaloacetic acid is known to be extremely unstable (2), (62), (Hi). The relatively low yields of oxalic acid, derived... 

The formation of oxaloacetic acid by dehydrogenation implies that this acid may be dissimilated by two mechanisms. It is known (62), (114) that oxaloacetic acid is subject to decarboxylation under acid conditions, and that higher pH is favorable to its stability. Thus, alkaline media enable the add to remain unchanged long enough to be split, yielding acetate and oxalate, while acidic media cause decarboxylation. 

Another observation on oxalate formation is that other a-keto acids, such as oxalosuccinic acid (74) and a-ketoglutaric acid (106) do not seem to yield oxalate directly but indirectly (123). This appears to be due to the fact that only oxaloacetic acid can function as an acetate donor. In this connection the intervention of Coenzyme A may be considered, since it is reported to function in the acetylation of sulfanilamide and choline (73) and recently was shown to take part in the enzymatic synthesis of citric acid. This concept may be illustrated as follows ... 

These considerations allow for a cyclic mechanism to occur in the formation of oxalic acid, where oxaloacetic acid yields either oxalate and acetate, which may be reoxidized, or pyruvate, which will then in turn yield acetate. This is illustrated in the following phase sequence of oxalate formation by these organisms ... 

There are two unusual aspects to the regulation of gluconeogenesis. The first step in the reaction, the formation of oxaloacetate from pyruvate, requires the presence of acetyl-CoA. This is a check to make sure that the TCA cycle is adequately fueled. If there s not enough acetyl-CoA around, the pyruvate is needed for energy and gluconeogenesis won t happen. However, if there s sufficient acetyl-CoA, the pyruvate is shifted toward the synthesis of glucose. 

Reactions of cyanide with the salts or esters of some amino acids (e.g., pyruvate, a-ketoglutarate, oxaloacetate) lead to formation of cyanohydrin intermediates and their incorporation into intermediary metabolism. 

The reaction mixture for a coupled assay includes the substrates for the initial or test enzyme and also the additional enzymes and reagents necessary to convert the product of the first reaction into a detectable product of the final reaction. The enzyme aspartate aminotransferase (EC 2.6.1.1), for instance, results in the formation of oxaloacetate, which can be converted to malic acid by the enzyme malate dehydrogenase (EC 1.1.1.37) with the simultaneous conversion of NADH to NAD+, a reaction which can be followed spectropho-tometrically at 340 nm ... 

The oxidative decarboxylation reaction above is part of the TCA cycle and leads to the formation of oxaloacetate, which maybe used to synthesize citrate (with acetyl-CoA) or may be used as a substrate by phosphoenol pyruvate carboxykinase, PEPCK. It should be noted that the phosphoenolpyruvate generated by PEPCK reaction shown above is... 

It is possible to put forward a biochemical advantage for this unusual reaction in the activation of acetoacetate. The succinate that is produced in the reaction is further metabohsed to oxaloacetate, via the reactions of the Krebs cycle. Since acetyl-CoA requires oxaloacetate for formation of citrate and oxidation in the cycle, one product of... 

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