Malate dehydrogenase transfer

A FIGURE 8-10 The malate shuttle. This cyclical series of reactions transfers electrons from NADH in the cytosol (intermembrane space) across the inner mitochondrial membrane, which is impermeable to NADH itself. StepH Cytosolic malate dehydrogenase transfers electrons from cytosolic NADH to oxaloacetate, forming malate. StepH An antiporter (blue oval) in the inner mitochondrial membrane transports malate into the matrix in exchange for a-ketoglutarate. StepH Mitochondrial malate dehydrogenase converts malate back to oxaloacetate, reducing NAD in the matrix to NADH in the process. StepH Oxaloacetate, which cannot directly cross the inner membrane, is converted to... 

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... 

Water is added across the double bond of fumarate in a reaction which is catalyzed by the enzyme fumarase. The reaction is reversible, although slightly exer-gonic. At equilibrium there is 82% immolate. In the ninth and last step of the cycle, the secondary hydroxyl group of malate is dehydrogenated. The enzyme malate dehydrogenase transfers the hydrogen to NAD. The product of this reaction is oxaloacetate, the primer for the whole chain of reactions. The cycle is closed and we have finished one trip around it. 

NAD (P) " -dependent enzymes are stereospecific. Malate dehydrogenase, for example, transfers a hydride to die pro-/ position of NADH, whereas glyceraldehyde-3-phosphate dehydrogenase transfers a hydride to die pro-5 position of the nicotinamide. Alcohol dehydrogenase removes a hydride from the pro-i position of edianol and transfers it to die pro-i position of NADH. 

The malate-aspartate shuttle is the most important pathway for transferring reducing equivalents from the cytosol to the mitochondria in brain. This shuttle involves both the cytosolic and mitochondrial forms of aspartate aminotransferase and malate dehydrogenase, the mitochondrial aspartate-glutamate carrier and the dicarboxylic acid carrier in brain (Fig. 31-5) [69]. The electrogenic exchange of aspartate for glutamate and a... 

In the malate shuttle (left)—which operates in the heart, liver, and kidneys, for example-oxaloacetic acid is reduced to malate by malate dehydrogenase (MDH, [2a]) with the help of NADH+HT In antiport for 2-oxogluta-rate, malate is transferred to the matrix, where the mitochondrial isoenzyme for MDH [2b] regenerates oxaloacetic acid and NADH+HT The latter is reoxidized by complex I of the respiratory chain, while oxaloacetic acid, for which a transporter is not available in the inner membrane, is first transaminated to aspartate by aspartate aminotransferase (AST, [3a]). Aspartate leaves the matrix again, and in the cytoplasm once again supplies oxalo-acetate for step [2a] and glutamate for return transport into the matrix [3b]. On balance, only NADH+H"" is moved from the cytoplasm into the matrix ATP is not needed for this. 

A key structural and mechanistic feature of lactate and malate dehydrogenases is the active site loop, residues 98-110 of the lactate enzyme, which was seen in the crystal structure to close over the reagents in the ternary complex.49,50 The loop has two functions it carries Arg-109, which helps to stabilize the transition state during hydride transfer and contacts around 101-103 are the main determinants of specificity. Tryptophan residues were placed in various parts of lactate dehydrogenase to monitor conformational changes during catalysis.54,59,60 Loop closure is the slowest of the motions. 

A reversible covalent modification that plants use extensively is the reduction of cystine disulfide bridges to sulf-hydryls. Many of the enzymes of photosynthetic carbohydrate synthesis are activated in this way (table 9.3). Some of the enzymes of carbohydrate breakdown are inactivated by the same mechanism. The reductant is a small protein called thioredoxin, which undergoes a complementary oxidation of cysteine residues to cystine (fig. 9.5). Thioredoxin itself is reduced by electron-transfer reactions driven by sunlight, which serves as a signal to switch carbohydrate metabolism from carbohydrate breakdown to synthesis. In one of the regulated enzymes, phosphoribulokinase, one of the freed cysteines probably forms part of the catalytic active site. In nicotinamide-adenine dinucleotide phosphate (NADP)-malate dehydrogenase and fructose-1,6-bis-... 

Oxaloacetate formed in the transfer of acetyl groups to the cytosol must now be returned to the mitochondria The inner mitochondrial membrane is impermeable to oxaloacetate. Hence, a series of bypass reactions are needed. Most important, these reactions generate much of the NADPH needed for fatty acid synthesis. First, oxaloacetate is reduced to malate by NADH. This reaction is catalyzed by a malate dehydrogenase in the cytosol. 

As outlined above, Bash et al. concluded that in the similar malate dehydrogenase reaction, proton transfer preceded hydride transfer. 

PEP carboxykinase is found within the mitochondria of some species and in the cytoplasm of others. In humans this enzymatic activity is found in both compartments. Because the inner mitochondrial membrane is impermeable to OAA, cells that lack mitochondrial PEP carboxykinase transfer OAA into the cytoplasm by using, for example, the malate shuttle. In this process, OAA is converted into malate by mitochondrial malate dehydrogenase. After the transport of malate across mitochondrial membrane, the reverse reaction is catalyzed by cytoplasmic malate dehydrogenase. 

The anabolic reactions of gluconeogenesis take place in the cytosol. Oxaloacetate is not transported across the mitochondrial membrane. Two mechanisms exist for the transfer of molecules needed for gluconeogenesis from mitochondria to the cytosol. One mechanism takes advantage of the fact that phosphoenolpyruvate can be formed from oxaloacetate in the mitochondrial matrix (this reaction is the next step in gluconeogenesis) phosphoenolpyruvate is then transferred to the cytosol, where the remaining reactions take place (Figure 19.12). The other mechanism relies on the fact that malate, which is another intermediate of the citric acid cycle, can be transferred to the cytosol. There is a malate dehydrogenase enzyme in the cytosol as well as in mitochondria, and malate can be converted to oxaloacetate in the cytosol. 

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. 

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