Aspartate translocator

For a proper understanding of the role of the translocators in the regulation of metabolism, knowledge of their kinetic constants is indispensable. Table 1 summarises these parameters. Because of technical difficulties, in most cases it has not been possible to determine these parameters at 37°C. It is also important to stress that the kinetic constants have been determined in isolated mitochondria. It is likely that the kinetic constants in the intact cell are different, one reason being that there is an inhibitory interaction of cytosolic anions with the various translocators. Some of these effects are given in Table 2. With the exception of a few cases (the a-oxoglutarate translocator in heart [18] and the carnitine and aspartate translocators see Sections If, iii and iv), little is known about the values of the metabolites to be transported from the matrix side of the mitochondrial membrane. In the case of citrate and ATP transport such information is difficult to obtain because most of the intramitochondrial citrate and ATP is chelated with Mg " and only the free anions are transported. Likewise, little is known about possible competition between metabolites present in the matrix for export out of the mitochondria. The complexity of the complete kinetic analysis of a translocator, in which both the external and internal concentrations have been taken into account, is illustrated by the studies of Sluse et al. [18] on a-oxoglutarate transport in heart mitochondria. 

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... 

Figure 3. Schematic representation of the reaction cycle of SERCA pumps. The SERCA pumps exist in two conformational state El binds Ca2+ with high affinity at the cytoplasmic site of the SER membrane, while E2 has low affinity for Ca2+ and thus releases it on the opposite site of the membrane. ATP phosphorylates a highly conserved aspartic acid residue allowing for the translocation of Ca2+ in the SER lumen...

Cord development is triggered not only by connection between resources, but also by the nutrient status of the mycelium. Developmental responses in saprotrophic cord formers may be caused by the changes in C/N balance in individual hyphae resulting from a local rise in nutrient levels due to uptake and translocation of nutrients within some hyphae but not others. Development and biomass production are differentially affected by carbon and nitrogen content and the C/N ratio in defined media (Watkinson, 1999 Fig. 7.4). Biomass increased with both sucrose and aspartate, while... 

Zinc efflux is mediated by a zinc exporter known as ZntA (Zn + transport or tolerance), a membrane protein which was identified through studies of bacterial strains that were hypersensitive to zinc and cadmium. Sequence inspection revealed that ZntA was a member of the family of cation transport P-type ATPases, a major family of ion-translocating membrane proteins in which ATPase activity in one portion of the protein is used to phophorylate an aspartate within a highly conserved amino acid sequence, DKTG, in another portion of the protein. The cysteine rich N-terminus of these soft metal transport proteins contains several metal-binding sites. How the chemical energy released by ATP hydrolysis results in metal ion transport is not yet known, in part because there is only partial information about the structures of these proteins. The bacterial zinc exporter also pumps cadmium and lead and is therefore also involved in protection from heavy metal toxicity (see Metal Ion Toxicity). 

A model for the mechanism of action of the enzyme is shown in Figure 12-11. It proposes that Na", K+-ATPase can exist in two (or more) conformational states one binding Na+ or ATP (or both) and the other binding K" " or phosphate (or both). On the cytoplasmic side, Na" " binding initiates transient phosphorylation of an aspartate residue at the active site, resulting in a cyclic process with translocation of Na" " from inside to outside and of K+ from outside to inside. The vectorial equation for the transport is... 

Model for transport of Na+ and K+ by Na+,K+-ATPase. Inside the cell, Na+ initiates the one-way ion exchange cycle by the phosphorylation of an aspartate residue at the active site by ATP, which eventually leads to the translocation of Na" " and K+. Conformational changes of the enzyme occur during the exchange of ions. [Reproduced, with permission, from K. J. Sweadner and S. M. Goldin, Active transport of sodium and potassium ions. New Engl. J. Med. 302, 111 (1980).]... 

Translocation systems of the inner mitochondrial membrane are listed in Table 14-5. Anion translocators are responsible for electroneutral movement of dicarboxylates, tricarboxylates, a-ketoglutarate, glutamate, pyruvate, and inorganic phosphate. Specific electrogenic translocator systems exchange ATP for ADP, and glutamate for aspartate, across the membrane. The metabolic function of the translocators is to provide appropriate substrates (e.g., pyruvate and fatty acids) for mitochondrial oxidation that is coupled to ATP synthesis from ADP and Pj. 

If the PEP carboxykinase is located in the mitochondrion, the formation of PEP will take place in the mitochondrion and it will be translocated to the cytosol. If the PEP carboxykinase is located in the cytosol, oxaloacetate will be converted to either malate or aspartate and then transported to the cytosol where it will be reconverted to oxaloacetate, which, via the action of PEP carboxykinase, can be converted to PEP. In cases where the enzyme is located both in the mitochondrion and cytosol, as in humans, some of both of these processes take place. To maintain electroneutrality, the transport of these compounds via PEP, aspartate, or malate out of the... 

As already discussed, the model proposed by Tsukihara et alP depends on the so-scalled H-pathway of proton transfer and on the function of an aspartic acid that is not conserved in the bacterial oxidases. This model also depends crucially on the formyl group and on the hydroxyethyl farnesyl side chain of heme a. Many proton-pumping bacterial oxidases, such as cytochrome boj, from E. coll, have replaced heme a with a protoheme (heme B) that lacks both the formyl and the farnesyl side chain. Therefore, this model is restricted to heme a - containing oxidases, and implies different mechanisms of proton translocation in different members of the heme-copper oxidase superfamily. 

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