Compartmentation of mitochondrial metabolism requires specific membrane translocators
The mitochondrial inner membrane is impermeable for metabolites. Specific translocators enable a specific transport of metabolites between the mito-chondrial matrix and the cytosol in a counter-exchange mode (Fig. 5.22).
The role of the ATP/ADP and the phosphate translocators (Fig. 5.20) has been discussed. Malate and succinate are transported into the mitochondria in counter-exchange for phosphate by a dicarboxylate trans-locator. This transport is inhibited by butylmalonate. α-Ketoglutarate, cit-rate, and oxaloacetate are transported in counter-exchange for malate. By these translocators, substrates can be fed into the citrate cycle. Glutamate is transported in counter-exchange for aspartate, and pyruvate in counter-exchange for OH- ions. Although these translocators all occur in plant mitochondria, most of our present knowledge about them is based on stud-ies with mitochondria from animal tissues. A comparison of the amino acid sequences known for the ATP/ADP, phosphate, citrate, and glutamate/ aspartate translocators shows that they are homologous; the proteins of these translocators represent a family deriving from a common ancestor. All these translocators are composed of 2 x 6 transmembrane helices.
The malate-oxaloacetate translocator is a special component of plant mitochondria and has an important function in the malate-oxaloacetate cycle. It also transports citrate and is involved in providing the carbon skeletons for nitrate assimilation (Fig. 10.11). The oxaloacetate translocator and, to a lesser extent, the -ketoglutarate translocator are inhibited by the dicarboxylate phthalonate. The transport of glycine and serine, involved in the photorespiratory pathway , has not yet been characterized. Although final proof is still lacking, it is expected that this transport is mediated by one or two mitochondrial translocators.