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Malate-Aspartate Shuttle
描述
The malate-aspartate shuttle (also known as the malate shuttle) is used by mitochondria for translocating electrons produced during glycolysis across the impermeable inner membrane for oxidative phosphorylation. This allows the hydrogen ions of the cofactor NADH produced in the cytosol to reach the electron transport chain in the mitochondria and generate ATP. The shuttle system is required because the inner membrane is impermeable to NADH and its oxidized form NAD+. NAD+/NADH does not cross the membrane, only ions (attached to malate) cross it. In this particular shuttle process, oxaloacetate on the cytoplasmic side is first reduced by NADH, creating malate and NAD+. Malate and the electrons it carries are transported into the mitochondria across the inner mitochondrial membrane, in exchange for alpha-ketoglutarate, which is transported out of the mitochondria. Once inside, the energy in malate is extracted again by reducing NAD+ to make NADH, thereby regenerating oxaloacetate. This NADH is then free to transfer its electrons to the electron transport chain. The oxaloacetate is transaminated with glutamate to make aspartate and alpha-ketoglutarate. Aspartate is returned to the cytosol by the aspartate-glutamate transporter, which moves glutamate into the mitochondria as it transports aspartate out. The net result is that NADH is transported into the mitochondria, generating 3 ATP molecules for every NADH transported in from the cytosol.