Countertransport system

Acyl carnitine can cross only the inner mitochondrial membrane on a countertransport system that takes in acyl carnitine in exchange for free carnitine being returned to the intermembrane space. Once inside the mitochondrial inner membrane, acyl carnitine transfers the acyl group onto CoA ready to undergo 8-oxidation. This countertransport system provides regulation of the... 

Figure 3. Chloride countertransport system typical LEM separation.

Acylcarnitine can cross only the inner mitochondrial membrane on a countertransport system that takes in acylcarnitine in exchange for free carnitine being returned to the inter-membrane space. Once inside the mitochondrial inner membrane, acylcarnitine transfers the acyl group onto CoA ready to undergo -oxidation. This counter-transport system provides regulation of the uptake of fatty acids into the mitochondrion for oxidation. As long as there is free CoA available in the mitochondrial matrix, fatty acids can be taken up and the carnitine returned to the outer membrane for uptake of more fatty acids. However, if most of the CoA in the mitochondrion is acylated, then there is no need for further fatty uptake immediately and, indeed, it is not possible. 

As examples of coupled countertransport (see Fig. 1.2D) and coupled cotransport (see Fig. 1.2E), the transport of titanium(IV) from low acidity (pH 1) and high acidity ([H+] = 7 M) feed solutions, respectively, using the HLM system may be presented [9, 26]. The di-(2-ethyIhexyl) phosphoric acid (DEHPA) carrier reacts with Ti(IV) ion to form complexes on the feed side at low acidity (pH region) ... 

Pyles TM, mcMalik-Diemer VA, McGavin CA, Whitfield DM, Membrane transport systems. III. A mechanistic study of cation-proton coupled countertransport. Can. j Chem. 1982 60, 2259. 

Zhou, Y., Samson, M., Francon, J., and Blondeau, J. P., Thyroid hormone con-centrative uptake in rat erythrocytes. Involvement of the tryptophan transport system T in countertransport of tri-iodothyronine and aromatic amino acids, Biochem.., 281[Pt. 1], 81, 1992. 

The driving force in this case is the difference between the concentration of the carrier-extracted species complex at the membrane/feed solution interface and the practically zero concentration of the complex at the membrane/receiver solution interface. In the case of extracted ionic species, the driving force for the uphill transport can be the potential gradient generated by the coupled transport of another ionic species across the membrane. The extracted ionic species, in this case, is transported to satisfy the electroneutrality condition within the membrane system. This coupled transport process can be countertransported (Figure 27.5b,c) and cotransporled (Figure 27.5a,d) with respect to the extracted ionic species. 

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