Glutamate/aspartate shuttle

The latter may take place in the mitochondria and the phosphoenolpyruvate then passes into the cell cytoplasm. Alternatively, oxalacetate may be transferred into the cell cytoplasm by means of the glutamate-aspartate shuttle and converted to phosphoenolpyruvate by cytoplasmic phosphoenolpyruvate carboxykinase. In the ruminant animal, pyruvate carboxylase is located in the cell cytoplasm as well as the mitochondria, and pyruvate can be changed to phosphoenolpyruvate entirely in the cytoplasm. 

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).

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... 

The interconversion of o -ketoglutarate to glutamate involves the malate-aspartate shutde. This shuttle translocates a-ketoglutarate from mitochondria into the cytoplasm and then converts it to glutamate by the catalytic action of aspartate aminotransferase (McKenna et al., 2006). As part of the malate-aspartate shuttle, NADH is oxidized during reduction of oxaloacetate to malate. Malate diffuses across the outer mitochondrial membrane (Fig. 1.6). From the intermembrane space, the malate-a-ketoglutarate antiporter in the inner membrane transports malate into the matrix. For every malate molecule entering the matrix compartment, one molecule of... 

Fig. 1.6 Reactions of the malate-aspartate shuttle showing the transport of reducing equivalents from cytoplasm to mitochondria. a-KG, a-Ketoglutarate Asp, aspartate Glu, glutamate OAA, oxaloacetate aspartate aminotransferase (1) and malate dehydrogenase (2)...

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. 

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 a-ketoglutarate/malate exchange carrier and the glutamate/aspartate carriers also have a wide distribution. These two carriers are on the pathway of the malate/aspartate shuttle, which transports reducing equivalents from the cytosol into the mitochondria [6]. Reducing equivalents (NADH) are generated in the cytosol by glycolysis but NADH is impermeable to the mitochondrial membrane in... 

The overall effect of the malatc-asparlate shuttle is to transfer the equivalent of tw o 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. 

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 ... 

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. 

By decreasing the mitoehondrial concentration of glutamate, an inhibitor of pyruvate carboxylase, through stimulation of the TCA cycle (secondary to the increase in mitochondrial acetyl-CoA) and the aspartate shuttle (secondary to the increase in cytosolic PEPCK induced by glucagon). 

Operation of the most widespread shuttle—the malate-aspartate shuttle is depicted in Figure 8 10. Critical to the shuttle are two antiport proteins in the inner mitochondrial membrane, a malate/a-ketoglutarate antiporter and a glutamate/aspartate antiporter, that permit transport of their... 

Aminooxyacetate, an inhibitor of glutamate— oxalacetate transaminase, inhibits the formation of aspartate. Soling Kleinicke (1976) observed that aminooxyacetate did not inhibit the formation of glucose from lactate and, therefore, concluded that the malate-aspartate shuttle was not essential for the lactate gluconeogenesis in avian liver. However, Ochs Harris (1980) found that aminooxyacetate did block lactate gluconeogenesis when lower concentrations of pyruvate were used and incubation was for longer than 15 min. They concluded that the malate-aspartate shuttle was required. 

Oxaloacetate is an intermediate of many metabolic pathways. It also plays a role in the malate-aspartate shuttle, which transfers high energy electrons into mitochondria. Citrate is formed by the condensation of oxaloacetate with acetyl CoA. A transamination reaction transfers an amino group from an amino acid to an a-keto acid. Transfer of the amino group from aspartate to a-ketoglutarate forms oxaloacetate and glutamate. In gluconeogenesis, pyruvate is carboxylated in mitochondria to form oxaloacetate. After transfer to the cytosol, the enzyme phosphoenolpyruvate carboxykinase catalyses the conversion of oxaloacetate to phosphoenolpyruvate. 

The malate—aspartate shuttle is the mechanism by which electrons from NADH produced in the cytosol are transported into mitochondria, as the inner membrane is impermeable to NADH itself. Oxaloacetate is reduced to malate in the cytosol by malate dehydrogenase, in the process oxidizing NADH to replenish cytosolic NAD. The malate-aspartate shuttle is found mainly in cardiac muscle and liver cells, while the glycerol 3-phosphate shuttle operates mainly in brain and skeletal muscle cells. Once malate has entered the mitochondria it is oxidized to oxaloacetate, generating NADH within the mitochondrial matrix. Oxaloacetate is then converted to aspartate, which is transported out of the mitochondria in exchange for glutamate. 

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