Citric acid cycle malate

Formation of Oxaloacetate in a Mitochondrion In the last reaction of the citric acid cycle, malate is dehydrogenated to regenerate the oxaloacetate necessary for the entry of acetyl-CoA into the cycle ... 

Reaction 8. In the final step of the citric acid cycle, malate dehydrogenase catalyzes the reduction of NAD+ to NADH and the oxidation of malate to oxaloacetate. Because the citric acid cycle "began" with the addition of an acetyl group to oxaloacetate, we have come full circle. 

While the citric acid cycle takes place in mitochondria, many anabolic reactions take place in the cytosol. Oxaloacetate, the starting material for gluconeogenesis, is a component of the citric acid cycle. Malate, but not oxaloacetate, can be transported across the mitochondrial membrane. After malate from mitochondria is carried to the cytosol, it can be converted to oxaloacetate by malate dehydrogenase, an enzyme that requires NAD+. Malate, which crosses the mitochondrial membrane, plays a role in lipid anabolism, in a reaction in which malate is oxidatively decarboxylated to pyruvate by an enzyme that requires NADP+, producing NADPH. 

The conversion of oxaloacetate to succinate is catalyzed by enzymes of the citric acid cycle malate dehydrogenase, fumarase and succinate dehydrogenase. These enzymes were isolated from the cells of P. shermanii... 

Acid-catalyzed hydration of isolated double bonds is also uncommon in biological pathways. More frequently, biological hydrations require that the double bond be adjacent to a carbonyl group for reaction to proceed. Fumarate, for instance, is hydrated to give malate as one step in the citric acid cycle of food metabolism. Note that the requirement for an adjacent carbonyl group in the addition of water is the same as that we saw in Section 7.1 for the elimination of water. We ll see the reason for the requirement in Section 19.13, but might note for now that the reaction is not an electrophilic addition but instead occurs... 

Steps 7-8 of Figure 29.12 Hydration and Oxidation The final two steps in the citric acid cycle are the conjugate nucleophilic addition of water to fumarate to yield (S)-malate (L-malate) and the oxidation of (S)-malate by NAD+ to give oxaloacetate. The addition is cataiyzed by fumarase and is mechanistically similar to the addition of water to ris-aconitate in step 2. The reaction occurs through an enolate-ion intermediate, which is protonated on the side opposite the OH, leading to a net anti addition. 

The final step is the oxidation of (S)-malate by NAD+ to give oxaloacetate, a reaction catalyzed by malate dehydrogenase. The citric acid cycle has now returned to its starting point, ready to revolve again. The overall result of the cycle is... 

Theoretically, a fall in concentration of oxaloacetate, particularly within the mitochondria, could impair the ability of the citric acid cycle to metabolize acetyl-CoA and divert fatty acid oxidation toward ketogenesis. Such a fall may occur because of an increase in the [NADH]/[NAD+] ratio caused by increased P-oxida-tion affecting the equilibrium between oxaloacetate and malate and decreasing the concentration of oxaloacetate. However, pyruvate carboxylase, which catalyzes the conversion of pyruvate to oxaloacetate, is activated by acetyl-CoA. Consequently, when there are significant amounts of acetyl-CoA, there should be sufficient oxaloacetate to initiate the condensing reaction of the citric acid cycle. 

The criteria for gene displacement in this study were strict. The reactions catalyzed were required to have the same EC (Enzyme Commission) number, which implies that the same cofactors had to be involved. In the example of reactions involved in the citric acid cycle given previously, when only the carbohydrate substrate and product of the reaction were the same, we could identify gene displacements at 6 of the 11 steps included in the analysis. Only two of those (malate dehydrogenase and fumarase) met the criteria in Galperin et al. (1998). 

Many enzymes in the mitochondria, including those of the citric acid cycle and pyruvate dehydrogenase, produce NADH, aU of which can be oxidized in the electron transport chain and in the process, capture energy for ATP synthesis by oxidative phosphorylation. If NADH is produced in the cytoplasm, either the malate shuttle or the a-glycerol phosphate shuttle can transfer the electrons into the mitochondria for delivery to the ETC. Once NADH has been oxidized, the NAD can again be used by enzymes that require it. 

Althoi alanine is the major gluconeogenic amino acid, 18 of the 20 (all but leucine and lysine) are also gluconeogenic. Most of these are converted by individual pathways to citric acid cycle intermediates, then to malate, following the same path from there to glucose. 

The standard free-energy change for this reaction is quite high, but under physiological conditions (including a very low concentration of oxaloacetate) AG 0 and the reaction is readily reversible. Mitochondrial malate dehydrogenase functions in both gluconeogenesis and the citric acid cycle, but the overall flow of metabolites in the two processes is in opposite directions. 

Table 16-2 shows the most common anaplerotic reactions, all of which, in various tissues and organisms, convert either pyruvate or phosphoenolpyruvate to ox-aloacetate or malate. The most important anaplerotic reaction in mammalian liver and kidney is the reversible carboxylation of pyruvate by C02 to form oxaloacetate, catalyzed by pyruvate carboxylase. When the citric acid cycle is deficient in oxaloacetate or any other intermediates, pyruvate is carboxylated to produce more oxaloacetate. The enzymatic addition of a carboxyl group to pyruvate requires energy, which is supplied by ATP—the free energy required to attach a carboxyl group to pyruvate is about equal to the free energy available from ATP. 

The glyoxylate cycle is active in the germinating seeds of some plants and in certain microorganisms that can live on acetate as the sole carbon source. In plants, the pathway takes place in glyoxysomes in seedlings. It involves several citric acid cycle enzymes and two additional enzymes isocitrate lyase and malate synthase. 

Balance Sheet for the Citric Acid Cycle The citric acid cycle has eight enzymes citrate synthase, aconitase, isocitrate dehydrogenase, a-ketoglutarate dehydrogenase, succinyl-CoA synthetase, succinate dehydrogenase, fumarase, and malate dehydrogenase. 

Oxaloacetate is formed in the last step of the citric acid cycle by the NAD+-dependent oxidation of L-malate. Can a net synthesis of oxaloacetate from acetyl-CoA occur using only the enzymes and cofactors of the citric acid cycle, without depleting the intermediates of the cycle Explain. How is oxaloacetate that is lost from the cycle (to biosynthetic reactions) replenished ... 

Synthesis of L-Malate in Wine Making The tartness of some wines is due to high concentrations of L-malate. Write a sequence of reactions showing how yeast cells synthesize L-malate from glucose under anaerobic conditions in the presence of dissolved C02 (HCO3 ). Note that the overall reaction for this fermentation cannot involve the consumption of nicotinamide coenzymes or citric acid cycle intermediates. 

In the third step, 1, -/3-hydroxyacyl-CoA is dehydrogenated to form /3-ketoacyl-CoA, by the action of /3-hydroxyacyl-CoA dehydrogenase NAD+ is the electron acceptor. This enzyme is absolutely specific for the l stereoisomer of hydroxyacyl-CoA The NADH formed in the reaction donates its electrons to NADH dehydrogenase, an electron carrier of the respiratory chain, and ATP is formed from ADP as the electrons pass to 02. The reaction catalyzed by /3-hydroxyacyl-CoA dehydrogenase is closely analogous to the malate dehydrogenase reaction of the citric acid cycle (p. XXX). 

Compartmentalization of Citric Acid Cycle Components Isocitrate dehydrogenase is found only in the mitochondrion, but malate dehydrogenase is found in both the cytosol and mitochondrion. What is the role of cytosolic malate dehydrogenase ... 

The following is the sum of three steps in the citric acid cycle A + B + FAD + H20 — C + FADH2 + NADH Reactant A Reactant B Reactant C A. Succinyl CoA GDP Succinate B. Succinate NAD+ Oxaloacetate C. Fumarate NAD Oxaloacetate D. Succinate NAD Malate E. Fumarate GTP Malate Correct answer = B. Succinate + NAD" + FAD oxaloacetate + NADH + FADH2... 

Figure 10-6 Reactions of the citric acid cycle (Krebs tricarboxylic acid cycle). Asterisks designate positions of isotopic label from entrance of carboxyl-labeled acetate into the cycle. Note that it is not the two carbon atoms from acetyl-CoA that are immediately removed as C02 but two atoms from oxaloacetate. Only after several turns of the cycle are the carbon atoms of the acetyl-CoA completely converted to C02. Nevertheless, the cycle can properly be regarded as a mechanism of oxidation of acetyl groups to C02. Green daggers (+) designate the position of 2H introduced into malate as 2H from the medium. FADS designates covalently bound 8-histidyl-FAD (see Chapter 15).

One of the first persons to study the oxidation of organic compounds by animal tissues was T. Thunberg, who between 1911 and 1920 discovered about 40 organic compounds that could be oxidized by animal tissues. Salts of succinate, fumarate, malate, and citrate were oxidized the fastest. Well aware of Knoop s (3 oxidation theory, Thunberg proposed a cyclic mechanism for oxidation of acetate. Two molecules of this two-carbon compound were supposed to condense (with reduction) to succinate, which was then oxidized as in the citric acid cycle to oxaloacetate. The latter was decarboxylated to pyruvate, which was oxidatively decarboxylated to acetate to complete the cycle. One of the reactions essential for this cycle could not be verified experimentally. It is left to the reader to recognize which one. 

The primary substrate of the citric acid cycle is acetyl-CoA. Despite many references in the biochemical literature to substrates "entering" the cycle as oxaloacetate (or as one of the immediate precursors succinate, fumarate, or malate), these compounds are not consumed by the cycle but are completely regenerated hence the term regenerating substrate, which can be applied to any of these four substances. A prerequisite for the operation of a catalytic cycle is that a regenerating substrate be readily available and that its concentration... 

Fig. 1.2 Intermediates of the citric acid cycle showing the relationship between glutamate and aspartate. Pyruvate dehydrogenase complex (1) citrate synthase (2) aconitase (3) isocitrate dehydrogenase (4) a-ketoglutarate dehydrogenase (5) succinyl-CoA synthetase (6) fumarate (7) fumarase dehydratase (8) malate dehydrogenase (9) and aspartate aminotransferase (10)...

This transfer of reducing equivalents is essential for maintaining the favorable NAD+/NADH ratio required for the oxidative metabolism of glucose and synthesis of glutamate in brain (McKenna et al., 2006). The malate-aspartate shuttle is considered the most important shuttle in brain. It is particularly important in neurons. It has low activity in astrocytes. This shuttle system is fully reversible and linked to amino acid metabolism with the energy charge and citric acid cycle of neuronal cells. 

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