Malate-aspartate cycle

The second electron shuttle system, called the malate-aspartate shuttle, is shown in Figure 21.34. Oxaloacetate is reduced in the cytosol, acquiring the electrons of NADH (which is oxidized to NAD ). Malate is transported across the inner membrane, where it is reoxidized by malate dehydrogenase, converting NAD to NADH in the matrix. This mitochondrial NADH readily enters the electron transport chain. The oxaloacetate produced in this reaction cannot cross the inner membrane and must be transaminated to form aspartate, which can be transported across the membrane to the cytosolic side. Transamination in the cytosol recycles aspartate back to oxaloacetate. In contrast to the glycerol phosphate shuttle, the malate-aspartate cycle is reversible, and it operates as shown in Figure 21.34 only if the NADH/NAD ratio in the cytosol is higher than the ratio in the matrix. Because this shuttle produces NADH in the matrix, the full 2.5 ATPs per NADH are recovered. 

But in mammalian species, several mechanisms exist for transferring NADH reducing equivalents, i.e., the malate-aspartate cycle (10) and the a-glycerophosphate-dihydroxy acetone phosphate cycle (16). It seems unlikely that the magnitude of NADH redox transfers due to the interconversions of proline - P5C would alter the N AD+/NADH balance. The oxidation of NADPH by P5C reductase, on the other hand, may play a major role in regulating NADP+/NADPH. Importantly, the of P5C reductase for NADPH is markedly lower than that for NADH (86,117). Although the V .. activities are higher with NADH than with NADPH the conversion of P5C to proline by PC reductase would affect NADP / NADPH more than NAD+/NADH if one considers the respective in vivo concentrations and the respective redox ratios of the two pyridine nucleotides. 

Because the 2 NADH formed in glycolysis are transported by the glycerol phosphate shuttle in this case, they each yield only 1.5 ATP, as already described. On the other hand, if these 2 NADH take part in the malate-aspartate shuttle, each yields 2.5 ATP, giving a total (in this case) of 32 ATP formed per glucose oxidized. Most of the ATP—26 out of 30 or 28 out of 32—is produced by oxidative phosphorylation only 4 ATP molecules result from direct synthesis during glycolysis and the TCA cycle. 

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. 

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. 

Citrin is an aspartate-glutamate antiporter that has a role both in the urea cycle and in the malate aspartate shuttle. It is necessary for the transport of aspartate produced in the mitochondria into the cytosol, where it is used by AS. Its role in the malate-aspartate shuttle is to transport cytosolic NADH reducing equivalents into the mitochondria, where they are used in oxidative phosphorylation. Defects in citrin cause citrullinemia type II. Patients manifest later-onset intermittent hyperammonemic encephalopathy as in HHH syndrome. 

The overall effect of the malate-aspartate shuttle is to transfer the equivalent of two electrons from the cytoplasm to the mitochondrion. The cycle is thought to be driven by cytoplasmic acid (H ). The concentration of protons in the cytoplasm is greater than that in the mitochondrion, which has an alkaline interior. This concentration gradient is thought to drive the membrane-bound glutamate/aspartate exchanger. 

Lanoue, K. F., and Williamson, J. R. (1971). Interrelationships between malate-aspartate shuttle and citric add cycle in rat heart mitochondria. Metabolism 20,119-140. 

Malate-aspartate shuttle for the transport of cytoplasmic reducing equivalents across the inner membrane of mitochondria. Malate, which carries the reducing equivalents, is oxidized to oxaloacetate with the generation of NADH in the matrix. To complete the unidirectional cycle, oxaloacetate is transported out of the matrix as aspartate. Mai = malate OAA = oxaloacetate a-KG = a-ketoglutarate Glu = glutamate ... 

As noted above, however, NAD must be regenerated from the NADH produced or the glycolytic cycle would cease. Under aerobic conditions regeneration of cytosohc NAD+ from cytosolic NADH is accomplished by transferring electrons across the mitochondrial membrane barrier to the electron transfer chain where the electrons are transferred to oxygen. There are two different shuttle mechanisms whereby this transfer of electrons across the membrane to regenerate cytosohc NAD+ can be accomplished, the glycerol 3-phosphate shuttle and the malate-aspartate shuttle. 

See also Malate/Aspartate Shuttle, Transamination in Amino Acid Metabolism (from Chapter 20), Citric Acid Cycle Intermediates in Amino Acid Metabolism (from Chapter 21)... 

Aspartate is involved in the control point of pyrimidine biosynthesis (Reaction 1 below), in transamination reactions (Reaction 2 below), interconversions with asparagine (reactions 3 and 4), in the metabolic pathway leading to AMP (reaction 5 below), in the urea cycle (reactions 2 and 8 below), IMP de novo biosynthesis, and is a precursor to homoserine, threonine, isoleucine, and methionine (reaction 7 below). It is also involved in the malate aspartate shuttle. 

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. 

Malate is converted to oxaloacetate in the citric acid cycle, which takes place in the mitochondria. In the cytoplasm, as a component of the malate-aspartate shuttle, it serves as an electron carrier to transfer electrons from NADH to the inner mitochondrial membrane. Malate can also be used as a source of electrons for the generation of NADPH in the reaction catalyzed by malic enzyme. 

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