Oxaloacetate has two main roles. It is an intermediate which is consumed in gluconeogenesis and it is also a key intermediate in the citric acid cycle where it fuses with acetyl CoA to form citrate, finally being regenerated by the cycle. Thus pyruvate carboxylase produces oxaloacetate for gluconeogenesis but also must maintain oxaloacetate stages for citric acid cycle function.  Intended for the latter reason the  activity  of  pyruvate  carboxylase   depends  absolutely  on  the  presence  of acetyl  CoA; the biotin  prosthetic  set  of the enzyme  cannot  be carboxylated unless acetyl  CoA is bound  to the enzyme.  This allosteric activation by acetyl CoA ensures which more oxaloacetate is made when excess acetyl CoA is available. In this role of maintaining the level of citric acid cycle intermediates and the pyruvate carboxylase reaction is learn to be anaplerotic which is filling up.