Mitochondria malate dehydrogenase

In common with cholesterol synthesis described in the next section, fatty acids are derived from glucose-derived acetyl-CoA. In the fed state when glucose is plentiful and more than sufficient acetyl-CoA is available to supply the TCA cycle, carbon atoms are transported out of the mitochondrion as citrate (Figure 6.8). Once in the cytosol, citrate lyase forms acetyl-CoA and oxaloacetate (OAA) from the citrate. The OAA cannot re-enter the mitochondrion but is converted into malate by cytosolic malate dehydrogenase (cMDH) and then back into OAA by mitochondrial MDH (mMDH) Acetyl-CoA remains in the cytosol and is available for fatty acid synthesis. 

The tricarboxylic acid cycle not only takes up acetyl CoA from fatty acid degradation, but also supplies the material for the biosynthesis of fatty acids and isoprenoids. Acetyl CoA, which is formed in the matrix space of mitochondria by pyruvate dehydrogenase (see p. 134), is not capable of passing through the inner mitochondrial membrane. The acetyl residue is therefore condensed with oxaloacetate by mitochondrial citrate synthase to form citrate. This then leaves the mitochondria by antiport with malate (right see p. 212). In the cytoplasm, it is cleaved again by ATP-dependent citrate lyase [4] into acetyl-CoA and oxaloacetate. The oxaloacetate formed is reduced by a cytoplasmic malate dehydrogenase to malate [2], which then returns to the mitochondrion via the antiport already mentioned. Alternatively, the malate can be oxidized by malic enzyme" [5], with decarboxylation, to pyruvate. The NADPH+H formed in this process is also used for fatty acid biosynthesis. 

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 ... 

Oxaloacetate, the product of the first step in gluconeogenesis, must leave the mitochondrion and enter the cytosol where the subsequent enzyme steps take place. Since the inner mitochondrial membrane is impermeable to oxaloacetate, it is converted to malate by mitochondrial malate dehydrogenase. This leaves the mitochondrion and is converted back to oxaloacetate in the cytosol by cytoplasmic malate dehydrogenase. 

A similar shuttle, the malate-aspartate shuttle, operates in heart and liver (Fig. 6). Oxaloacetate in the cytosol is converted to malate by cytoplasmic malate dehydrogenase, reoxidizing NADH to NAD+ in the process. The malate enters the mitochondrion via a malate-a-ketoglutarate carrier in the inner mitochondrial membrane. In the matrix the malate is reoxidized to oxaloacetate by NAD+ to form NADH. Oxaloacetate does not easily cross the inner mitochondrial membrane and so is transaminated to form aspartate which then exits from the mitochondrion... 

Fig. 5.4. Two types of energy metabolism in cestodes. (a) Type 1 homolactate fermentation, (b) Type 2 Malate dismutation. Reaction 3 involves a carboxylation step decarboxylation occurs at 6, 7 and 10. Reducing equivalents are generated at reactions 6 and 7 one reducing equivalent is used at reaction 9. Thus, when the mitochondrial compartment is in redox balance and malate is the sole substrate, twice as much propionate as acetate is produced. Key 1, pyruvate kinase 2, lactate dehydrogenase 3, phosphoenolpyruvate carboxykinase 4, malate dehydrogenase 5, mitochondrial membrane 6 malic enzyme 7, pyruvate dehydrogenase complex 8, fumarase 9, fumarate reductase 10, succinate decarboxylase complex. indicates reactions at which ATP is synthesised from ADP cyt, cytosol mit, mitochondrion. (After Bryant Flockhart, 1986.)...

Answer Malate dehydrogenase catalyzes the conversion of malate to oxaloacetate in the citric acid cycle, which takes place in the mitochondrion, and also plays a key role in the transport of reducing equivalents across the inner mitochondrial membrane via the malate-aspartate shuttle (Fig. 19-29). This shuttle requires the presence of malate dehydrogenase in the cytosol and the mitochondrial matrix. 

In animals and fungi there is a similar dichotomy. NADPH can be generated by cytosolic malic enzyme which catalyses the reaction malate + NADP+ — pyruvate + COg + NADPH. Cytosolic malate derives from the following successive reactions the pyruvate/ citrate shuttle on the mitochondrial inner membrane takes pyruvate to the mitochondrion in exchange for citrate cytosolic ATP citrate lyase catalyses ATP + citrate + CoA-SH —> acetylCoA (CH3CO-S-C0A) + oxaloacetate and cytosolic malate dehydrogenase, which catalyses NADH + oxaloacetate NAD+ + malate. This scheme provides both acetylCoA and NADPH for subsequent long chain fatty acid synthesis (see section on Fatty acid synthesis ). 

Pyruvate carboxylase is a mitochondrial enzyme, vhereas the other enzymes of gluconeogenesis are cytoplasmic. Oxaloacetate, the product of the pyruvate carboxylase reaction, is reduced to malate inside the mitochondrion for transport to the cytosol. The reduction is accomplished by an NADH-linked malate dehydrogenase. When malate has been transported across the mitochondrial membrane, it is reoxidized to oxaloacetate by an NAD+-linked malate dehydrogenase in the cytosol (Figure 16.28). 

Pyruvate carboxylase is a mitochondrial enzyme, whereas the other enzymes of gluconeogenesis are present primarily in the cytoplasm. Oxaloacetate, the product of the pyruvate carboxylase reaction, must thus be transported to the cytoplasm to complete the pathway. Oxaloacetate is transported from a mitochondrion in the form of malate oxaloacetate is reduced to malate inside the mitochondrion by an NADH-linked malate dehydrogenase. After malate has been transported across the mitochondrial membrane, it is reoxidized to oxaloacetate by an NAD -linked malate dehydrogenase in the cytoplasm (Figure 16.26). The formation of oxaloacetate from malate also provides NADH for use in subsequent steps in gluconeogenesis. Finally, oxaloacetate is simultaneously decarboxylated and phospho-ry lated by phosphoenolpyruvate carboxy kinase to generate phosphoenol pyruvate. The phosphoryl donor is GTP. The GO2 that was added to pyruvate by pyruvate carboxylase comes off in this step. 

To move the oxaloacetate (released after the last step) back into the mitochondrion, Oxaloacetate + NADH + H+ -> L-Malate + NAD+ (catalyzed by Malate Dehydrogenase)... 

Opperdoes, F. R., Markos, A. and Steiger, R. F. (1981) Localization of malate dehydrogenase, adenylate kinase and glycolytic enzymes in glycosomes and the threonine pathway in the mitochondrion of cultured procyclic trypomastigotes of Trypanosoma brucei. Mol. Biochem. Parasitol. 4 291-309. 

A more complex and more efficient shutde mechanism is the malate-aspartate shuttle, which has been found in mammalian kidney, liver, and heart. This shuttle uses the fact that malate can cross the mitochondrial membrane, while oxaloacetate cannot. The noteworthy point about this shuttle mechanism is that the transfer of electrons from NADH in the cytosol produces NADH in the mitochondrion. In the cytosol, oxaloacetate is reduced to malate by the cytosolic malate dehydrogenase, accompanied by the oxidation of cytosolic NADH to NAD+ (Figure 20.24). The malate then crosses the mitochondrial membrane. In the mitochondrion, the conversion of malate back to oxaloacetate is catalyzed by the mitochondrial malate dehydrogenase (one of the enzymes of the citric acid cycle). Oxaloacetate is converted to aspartate, which can also cross the mitochondrial membrane. Aspartate is converted to oxaloacetate in the cytosol, completing the cycle of reactions. 

Cytosolic malate dehydrogenase transfers the electrons and protons from NADH to oxaloacetate forming malate. Malate enters the mitochondrion via the dicarboxylate carrier in exchange for... 

Figure 29.1 shows that the high ratio of NADH NAD in the mitochondrion favours reduction of oxaloacetate to malate in the malate dehydrogenase reaction. It also restricts oxidation in the a-ketoglutarate dehydrogenase and isocitrate dehydrogenase reactions. The result is that Krebs cycle is inhibited. 

From the fact that the glyoxylate and TCA cycles have several enzymes (citrate synthetase, aconitase, malate dehydrogenase) in common it appeared axiomatic that they must operate together in the same intracellular compartment, the mitochondrion. Early experiments with crude particulate pellets containing mitochondria gave only partial support to this view since, although malate synthetase was present in such preparations, most of the isocitrate lyase was present in the supernatant fraction (Yamamoto and Beevers, 1961 Marcus and Velasco, 1960). 

ATP-dependent carboxylation to oxaloacetate, which is hydrogenated by NADH-dependent malate dehydrogenase (EC 1.1.1.37). The resulting malate leaves the mitochondrion and is decarboxylated to pyruvate in the cytoplasm by NADP-linked malate dehydrogenase (EC 1.1.1.40). Operation of this cycle results in the export of reducing equivalents from the mitochondrion to the cytoplasm. 

Lactate and alanine enter as pyruvate following the activities of lactate dehydrogenase (Figure 11.4) and alanine aminotransferase (Section 16.3). The first of the bypass reactions, the objective of which is to overcome the unfavourable energetics of a reversal of the pyruvate kinase reaction, seems a tortuous route (Figure 11.11). The reaction sequence relies on two important enzymes pyruvate carboxylase and phosphoenolpyruvate carboxykinase. Since pyruvate carboxylase is located exclusively in the mitochondrion, pyruvate must cross the inner mitochondrial membrane (Section 12.2). Oxaloacetate produced by pyruvate carboxylase cannot traverse the inner membrane and is reduced by malate dehydrogenase into l-malate. This step is the reversal of the tricarboxylate cycle reaction (Section 12.4). Malate may, of... 

Unlike glycolysis, which occurs strictly in the cell cytosol, gluconeogen-esis involves a complex interaction between the mitochondrion and the cytosol. This interaction is necessitated by the irreversibility of the pyruvate kinase reaction, by the relative impermeability of the inner mitochondrial membrane to oxaloacetate, and by the specific mitochondrial location of pyruvate carboxylase. Compartmentation within the cell has led to the distribution of a number of enzymes (aspartate and alanine aminotransferases, and NAD -malate dehydrogenase) in both the mitochondria and the cytosol. In the classical situation represented by the rat, mouse, or hamster hepatocyte, the indirect "translocation" of oxaloacetate—the product of the pyruvate carboxylase reaction—into the cytosol is effected by the concerted action of these enzymes. Within the mitochondria oxaloacetate is converted either to malate or aspartate, or both. Following the exit of these metabolites from the mitochondria, oxaloacetate is regenerated by essentially similar reactions in the cytosol and is subsequently decarboxylated to P-enolpyruvate by P-enol-pyruvate carboxykinase. Thus the presence of a membrane barrier to oxaloacetate leads to the functioning of the malate-aspartate shuttle as an important element in gluconeogenesis. 

To understand why isocitrate dehydrogenase is so intensely regulated we must consider reactions beyond the TCA cycle, and indeed beyond the mitochondrion (fig. 13.15). Of the two compounds citrate and isocitrate, only citrate is transported across the barrier imposed by the mitochondrial membrane. Citrate that passes from the mitochondrion to the cytosol plays a major role in biosynthesis, both because of its immediate regulatory properties and because of the chain of covalent reactions it initiates. In the cytosol citrate undergoes a cleavage reaction in which acetyl-CoA is produced. The other cleavage product, oxaloacetate, can be utilized directly in various biosynthetic reactions or it can be converted to malate. The malate so formed can be returned to the mitochondrion, or it can be converted in the cytosol to pyruvate, which also results in the reduction of NADP+ to NADPH. The pyruvate is either utilized directly in biosynthetic processes, or like malate, can return to the mitochondrion. 

Fatty acids are generated cytoplasmically while acetyl-CoA is made in the mitochondrion by pyruvate dehydrogenase.This implies that a shuttle system must exist to get the acetyl-CoA or its equivalent out of the mitochondrion. The shuttle system operates in the following way Acetyl-CoA is first converted to citrate by citrate synthase in the TCA-cycle reaction. Then citrate is transferred out of the mitochondrion by either of two carriers, driven by the electroos-motic gradient either a citrate/phosphate antiport or a citrate/malate antiport as shown in Figure 2-2. 

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