We describe a phage display methodology for engineering synthetic antigen binders (sABs) that recognize either the apo or the ligand-bound conformation of maltose-binding protein (MBP). sABs that preferentially recognize the maltose-bound form of MBP act as positive allosteric effectors by substantially increasing the affinity for maltose. A crystal structure of a sAB bound to the closed form of MBP reveals the basis for this allosteric effect. We show that sABs that recognize the bound form of MBP can rescue the function of a binding-deficient mutant by restoring its natural affinity for maltose. Furthermore, the sABs can enhance maltose binding in vivo, as they provide a growth advantage to bacteria under low-maltose conditions. The results demonstrate that structure-specific sABs can be engineered to dynamically control ligand-binding affinities by modulating the transition between different conformations.