ATP determines the direction and an enzyme lowers the activation energy says a textbook. But the forward and backward direction of a bi-directional reaction requires different enzymes. Also some tumor cells/proliferating cells express a low affinity form of pyruvate kinase (dimer form of PKM2) to branch off intermediates from glycolysis. It looks to me that the enzyme determines the fate of a substrate whereas ATP can't change the fate of a substrate. Or, is there a way that a substrate with an enzyme X can be involved in multiple reactions?