Malate shuttle

Figure 12-13. Malate shuttle for transfer of reducing equivalents from the cytosol into the mitochondrion. Ketoglutarate transporter , glutamate/aspartate transporter (note the proton symport with glutamate).

Cytoplasmic NADH oxidized using the malate shuttle produces a mitochondrial NADH and yields approximately 3 ATP by oxidative phosphorylation. 

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. 

Gluconeogenesis from several amino acids uses the malate shuttle. 

Pyruvate carboxylase is a mitochondrial enzyme requiring biotin. It is activated by acetyl CoA (fiom p oxidation). The product oxaloacetate (OAA), a citric add cyde intermediate, cannot leave the mitochondria but is reduced to malate that can leave via the malate shuttle. In the cytoplasm, malate is reoxidized to OAA. 

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. 

To initiate gluconeogenesis, oxaloacetate is reduced to malate, which is then transported to the cytosol in the reverse of the malate shuttle. 

Six ATPs will be synthesized if the aspartate-malate shuttle is used to transfer NADH generated through glycolysis to NADH in the mitochondrial matrix four molecules of ATP will be made if the glycerol phosphate shuttle delivers the electrons to ubiquinone in the inner mitochondrial membrane. 

Cleavage of argininosuccinate Argininosuccinate is cleaved to yield arginine and fumarate. The arginine formed by this reaction serves as the immediate precursor of urea. Fumarate produced in the urea cycle is hydrated to malate, providing a link with sev eral metabolic pathways. For example, the malate can be trans ported into the mitochondria via the malate shuttle and reenter... 

It is often necessary to move electrons into mitochondria for disposal via oxidative phosphorylation. However, NADH and FADH2 do not penetrate the inner mitochondrial membrane. Instead, such electrons may first be passed to dihydroxyacetone phosphate or to oxaloacetate to make glycerol-3-phosphate and malate, respectively. These compounds can penetrate the inner mitochondrial membrane via the porters described earlier and oxidized there by mitochondrial NAD+ or FAD. These systems are termed the glycerol-3-phosphate and malate shuttles, respectively, and they are described in greater detail in Chapter 18. 

For Questions 18.9-18.12, choose answers from the following, which represent the numbers of ATPs generated. Use the aspartate-malate shuttle wherever necessary. 

Fatty acid synthesis and degradation. Fatty acids are synthesized in the cytosol by the addition of two-carbon units to a growing chain on an acyl carrier protein. Malonyl CoA, the activated intermediate, is formed by the carboxylation of acetyl CoA. Acetyl groups are carried from mitochondria to the cytosol as citrate by the citrate-malate shuttle. In the cytosol, citrate is cleaved to yield acetyl CoA. In addition to transporting acetyl CoA, citrate in the cytosol stimulates acetyl CoA carboxylase, the enzyme catalyzing the committed step. When ATP and acetyl CoA are abundant, the level of citrate increases, which accelerates the rate of fatty acid synthesis (Figure 30.8). 

Now, let fatty acid biosynthesis starting from glucose. For that look at the integration of the various pathways involved Glycolysis, hexose monophosphate shunt, Pyruvate-malate shuttle, and Fatty acid biosynthesis. It requires reducing equivalents, redox balance and provision of required cytosolic ATP s as well as carbon source. 

In humans, oxaloacetate must be transported out of the mitochondrion to supply the cytosolic PEPCK. Because there is no mitochondrial carrier for oxaloacetate and its diffusion across the mitochondrial membrane is slow, it is transported as malate or asparate (Figure 15-2). The malate shuttle carries oxaloacetate and reducing equivalents, whereas the aspartate shuttle, which does not require a preliminary reduction step, depends on the availability of glutamate and a-ketoglutarate in excess of tricarboxylic acid (TCA) cycle requirements. 

Fatty acid oxidation produces more moles of ATP per mole of CO2 formed than does carbohydrate oxidation. In this case, oxidation of 1 mol of hexose produces at most (assuming malate shuttle operation exclusively) 38 mol of ATP. 

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. 

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... 

Malate and a-glycerolphosphate shuttles. The shuttling of reducing power from cytosolic NADH into the mitochondrion. The malate shuttle (top portion) is more complex, but yields more highenergy phosphates. Malate can move reducing power into or out of the mitochondria. The a-glycerolphosphate shuttle only moves reducing power from the cytosol to the mitochondria. 

Reducing power (i.e., NADH) can be transported into the mitochondrion by the malate shuttle. Malate can transport reducing power out of the mi-... 

Aspartate can be transaminated to form oxaloacetate, an intermediate of the citric-acid cycle. As with most transaminations, this is a reversible reaction, and aspartate can also be synthesized by a transamination reaction with glutamate and oxaloacetate to form aspartate and a-ketoglutarate. Therefore, aspartate is a nonessential amino acid. The aminotransferase with aspartate and a-ketoglutarate is particularly active in most tissues and occurs both in the mitochondria and the cytosol. The importance of this reaction is greater than simply forming the oxaloacetate or aspartate. Aspartate aminotransferase is an important reaction in the malate shuttle (see Chapter 11) wherein, reducing power can be transferred from the cytosol to the mitochondrion. Aspartate also plays a role in purine and pyrimidine synthesis and is particularly important in pyrimidine synthesis, where it donates both carbon and... 

Aspartate can be formed from oxaloacetate and glutamate, via transamination. This is important in urea synthesis, the malate shuttle, purine, and pyrimidine synthesis. 

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