Aspartate transport, mitochondrial

Studies performed by the authors of this review [142,144], suggest that the discrepancies are due to functional microcompartmentation between the aspartate aminotransferase and the aspartate transporter. The apparent for aspartate efflux can be dramatically decreased by generation of intramitochondrial aspartate by the aminotransferase reaction. Detailed isotopic studies using labelled matrix aspartate in liver mitochondrial [142] and labelled intramitochondrial glutamate in kidney mitochondria [144] confirmed the initial suggestion. 

Glutamate-aspartate transporter of mitochondrial inner membrane... 

Transport of adenine nucleotides is presumably fully electrophoretic, and therefore driven by the mitochondrial membrane potential, because ADP exchanges with ATP [9], Aspartate transport is both electrophoretic and -coupled because undissociated glutamate exchanges with the aspartate anion [10]. Since in the intact cell the mitochondrial membrane potential is positive outside and the mitochondrial matrix slightly alkaline with respect to the cytosol, it follows that in vivo movement of ATP and of aspartate is unidirectional, out of the mitochondria. 

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. 

FIGURE 31-7 Mitochondrial carriers. Ions and small molecules enter the intermembrane space, since the outer mitochondrial membrane is not a significant permeability barrier. However, the inner mitochondrial membrane is impermeable to ions except those for which there are specific carriers. Most of the carriers are reversible, as indicated by two-headed arrows. Compounds transported in one direction are indicated in red. The ATP/ADP translocase and the aspartate-glutamate carrier are both electrophoretic their transport is driven in the direction of the mitochondrial membrane potential, as indicated by red arrows. Glutamine is carried into the matrix by an electroneutral carrier. The unimpaired functioning of mitochondrial carriers is essential for normal metabolism. (Adapted with permission from reference [70].)... 

The oxaloacetate is then transported from mitochondrion into the cytosol but not directly, since there is no transporter for oxaloacetate in the mitochondrial membrane. This problem is solved by conversion of oxaloacetate to aspartate, by transamination, and it is the aspartate that is transported across the inner mitochondrial membrane to the cytosol, where oxaloacetate is regenerated from aspartate by a cytosolic aminotransferase enzyme. 

The final reactions to be considered in the metabolism of ethanol in the liver are those involved in reoxidation of cytosolic NADH and in the reduction of NADP. The latter is achieved by the pentose phosphate pathway which has a high capacity in the liver (Chapter 6). The cytosolic NADH is reoxidised mainly by the mitochondrial electron transfer system, which means that substrate shuttles must be used to transport the hydrogen atoms into the mitochondria. The malate/aspartate is the main shuttle involved. Under some conditions, the rate of transfer of hydrogen atoms by the shuttle is less than the rate of NADH generation so that the redox state in the cytosolic compartment of the liver becomes highly reduced and the concentration of NAD severely decreased. This limits the rate of ethanol oxidation by alcohol dehydrogenase. 

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. 

E. There are two shuttle mechanisms, the malate-aspartate shutde and the glycerol 3-phosphate shuttle, that transport electrons to the inner mitochondrial matrix to be used in the electron transport chain. 

Shuttle pathways for the transport of electrons across the inner mitochondrial membrane. A. Glycerophosphate shuttle. B. Malate-aspartate shuttle. 

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... 

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. 

Answer NADH produced in the cytosol cannot cross the inner mitochondrial membrane, but must be oxidized if glycolysis is to continue. Reducing equivalents from NADH enter the mitochondrion by way of the malate-aspartate shuttle. NADH reduces oxaloacetate to form malate and NAD+, and the malate is transported into the mitochondrion. Cytosolic oxidation of glucose can continue, and the malate is converted back to oxaloacetate and NADH in the mitochondrion (see Fig. 19-29). 

Although mitochondria contain both NAD+ and NADH, as does the cytoplasm, the mitochondrial and cytoplasmic pools are unable to exchange their contents directly, as the mitochondrial membranes are impermeable to the cytoplasmic compounds. The shuttle mechanisms allow the H on cytoplasmic NADH to be transported on other compounds into the mitochondria, where it is donated to NAD+ (to form mitochondrial NADH) or to FAD (to form mitochondrial FADH2). There are several shuttle mechanisms that are used by mammalian cells two of the most important are the malate-aspartate shuttle and the glycerol 3-phosphate shuttle. 

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... 

Arginine and fumarate are produced from argininosuccinate by the cytosolic enzyme argininosuccinate lyase. In the final step of the cycle arginase cleaves urea from aspartate, regenerating cytosolic ornithine, which can be transported to the mitochondrial matrix for another round of urea synthesis. 

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