Dicarboxylic acid transporter

The PBM is the primary interface between the host cell cytoplasm and the microsymbiont, and forms the main control point for nutrient and signal exchanges. It has been well documented that the carbon sources required for the function of bacteroids are provided by the host plant primarily in the form of dicarboxylates (see Dilworth Glenn, 1984). This is strongly supported by the fact that mutations in the bacterial dicarboxylic acid transporter result in ineffective nodules (Birkenhead et al., 1988 Watson et al., 1988). In addition, the flow of many other... 

Carboxylic acids such as pyruvate, succinate, and citrate are transported into the matrix by the pyruvate transporter, the dicarboxylic acid transporter, and the tricarboxylic acid transporter, respectively. Pymvate transport operates as an antiporter with hydroxide ion. The other transporters are driven by concentration gradients for their substrates. For example, high concentrations of citrate in the matrix lead to export of citrate to the cytoplasm, where it can inhibit phospho-fmctokinase (see Chapter 9). 

In the cytoplasm, oxaloacetate is reduced by NADH to malate via cytoplasmic malate dehydrogenase. Malate can pass into the mitochondrial matrix via special dicarboxylic acid transporters that are in the inner mitochondrial membrane. Once in the matrix, oxaloacetate is reoxidized to malate by mitochondrial malate dehydrogenase. Thus this series of enzyme and transporter reactions has effectively shuttled reducing equivalents in NADH from the cytoplasm to the matrix. This shuttle constitutes the most efficient way of transferring hydrogen atoms from NADH in the cytoplasm to the mitochondrial matrix, and hence into the electron transport chain (see Sec. 10.12 for the glycerol 3-phosphate shuttle). 

The transport mechanism of fumaric acid in Rhizopus strains has not been studied yet. Increasing the number or activity of the dicarboxylic acid transporters could lower the intracellular fumarate concentration and could therefore have a positive effect on the production yield. 

Compartmentation of these reactions to prevent photorespiration involves the interaction of two cell types, mescrphyll cells and bundle sheath cells. The meso-phyll cells take up COg at the leaf surface, where Og is abundant, and use it to carboxylate phosphoenolpyruvate to yield OAA in a reaction catalyzed by PEP carboxylase (Figure 22.30). This four-carbon dicarboxylic acid is then either reduced to malate by an NADPH-specific malate dehydrogenase or transaminated to give aspartate in the mesophyll cells. The 4-C COg carrier (malate or aspartate) then is transported to the bundle sheath cells, where it is decarboxylated to yield COg and a 3-C product. The COg is then fixed into organic carbon by the Calvin cycle localized within the bundle sheath cells, and the 3-C product is returned to the mesophyll cells, where it is reconverted to PEP in preparation to accept another COg (Figure 22.30). Plants that use the C-4 pathway are termed C4 plants, in contrast to those plants with the conventional pathway of COg uptake (C3 plants). 

Toad bufadienolides occur not only by themselves but also in a conjugated form, sulfates, dicarboxylic esters and amino acid-dicarboxylic acid esters have all been reported (Steyn and van Heerden 1998). Because of the activity of the bufadienolides in inhibiting active monovalent cation transporters, it is suggested that these compounds have a role in maintaining sodium homeostasis in toads that migrate between fresh and salt water environments (Flier, I idwards, Daly and Myers 1980). 

Satsuniabayashi, H., H. Kurita, Y. Yokouchi, and H. Ueda, Photochemical Formation of Particulate Dicarboxylic Acids under Long-Range Transport in Central Japan, Atmos. Environ., 24A, 1443-1450 (1990). 

Organelles within cells have their own ion-concentrating mechanisms. Thus, mitochondria can concentrate K+, Ca2+, Mg2+, and other divalent metal ions as well as dicarboxylic acids (Chapter 18). The entrance and exit of many substances from mitochondria appear to occur by exchange diffusion, i.e., by secondary active transport. Such ion exchange processes may also occur in other membranes. 

Cation-anion cotransport was effected by an optically active macrotricyclic cryp-tand that carried simultaneously an alkali cation and a mandelate anion and displayed weak chiroselectivity [4.23a], as did also the transport of mandelate by an optically active acyclic ammonium cation [6.39]. Employing together a cation and an anion carrier should give rise to synergetic transport with double selection by facilitating the flow of both components of a salt (see the electron-cation symport below). Selective transport of amino acids is effected by a convergent dicarboxylic acid receptor [4.24b]. 

For an effective symbiotic state, the plant and the microsymbiont must maintain a constant metabolic flow of carbon and nitrogen. While the bacteroids function as an engine for nitrogen fixation the fuel comes from the plant. Dicarboxylic acids are the primary carbon sources fed to the bacteroids by the plant. This unidirectional flow of carbon must be controlled by the PBM. Recently, several specific carbon and amino acid transport systems have been identified in the PBM using isolated peribac-teroid units (PBU Day et al., 1990). Thus, in order for the host plant to house endosymbiotic bacteria and support their metabolic needs, a number of nodulin genes must be induced to support the ontogeny and function of the nodules. 

Twenty-seven two-year-old scaleless hens were transported to Visalia, CA on October 1, 1979, housed in cages overnight with food and water provided ad lib, and exposed to field applications of DEF on October 2. Some birds were repeatedly treated over the next five days. The mature cotton fields (Diversified Farming Inc.) were sprayed from a ground vehicle that treated 8 rows simultaneously with DEF-6 (0.72 kg/l, 6 lb/gal Mobay Chemical Co.) at 0.37 gal/acre and Accelerate (amine salt of endothall (7-oxabicyclo (2.2.1) heptane-2,3-dicarboxylic acid, 0.06 kg/l, 0.5 lbs/gal) at 0.19 gal/acre in 25 gal water/acre. 

Carrigan CN, Esslinger CS, Bartlett RD, Bridges RJ, Thompson CM (1999) Quinoline-2,4-dicarboxylic acids synthesis and evaluation as inhibitors of the glutamate vesicular transport system. Bioorg Med Chem Lett 9 2607-2612. 

A direct K+ requirement for translocation has, however, been reported for glutamic acid transport in brain (Kanner and Schuldiner, 1987 Carlson et al., 1989). The dicarboxylic amino acids appear to be transported largely by specific transporters which do not participate in neutral amino acid transport. Recent studies, both in reconstituted systems and the expression of the cloned transporter, have confirmed the K+ requirement (see below). 

The second pathway is called the C4 cycle because COj is initially converted to the four-carbon dicarboxylic acids, malic or aspartic acids (Fig. 3.3). Phos-phoenolpyruvic acid (I) reacts with one molecule of CO2 to form oxaloacetic acid (II) in the mesophyll of the biomass, and then malic or aspartic acid (III) is formed. The Q acid is transported to the bundle sheath cells, where decarboxylation occurs to regenerate pyruvic acid (IV), which is returned to... 

Electron transport across organised bilayers is an integral part of biological energy storage systems such as photosynthesis and provides a means of controlling back electron transfer and of separation of the products of redox reactions. Esters of 2,l,3-benzothiadiazole-4,7-dicarboxylic acid (481) have been used to study the transfer of electrons from 2-(morpholino)ethanesulfonic acid (MES) to 1,5-anthraquinone disulfonate in micelles or across vesicle bilayers. The esters absorb the light, accept an electron from MES and transfer it to the... 

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