Phosphoenolpyruvate carboxykinase (PEPCK) catalyses the conversion of oxaloacetate to pyruvate in the first step of gluconeogenesis. Since oxaloacetate is impermeable to the inner mitochondrial membrane, the localisation of PEPCK within the cell plays a major role in defining substrate preferences for gluconeogenesis. As a result, the immunochemically distinct isoenzymes of PEPCK found within the mitochondrial and the cytosolic cell fractions vary in their proportion to one another between organs and species and with prandial state.

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