Anaplerotic pathways

The intermediates of glycolysis and the citric acid cycle are used as precursors of biosynthetic pathways. To maintain the energy-yielding processes of glycolysis and the citric acid cycle these lost intermediates must be replenished through anaplerotic reactions. Three carbon compounds are carboxylated to form oxaloacetate. Both pyruvate and phosphoenol  pyruvate can  be  used in these reactions, for example oxaloacetate can  be formed from the enzymatic addition of carbon dioxide to pyruvate or PEP.