Aspects of the disclosure relate to compositions of enzyme-binding polypeptides (EBPs) that modify the substrate specificity of an enzyme and a method for identifying an EBP that modifies substrate specificity of an enzyme binding at least one substrate, the method comprising: contacting the enzyme with a polypeptide library comprising a plurality of EBPs that bind different epitopes of the enzyme; identifying EBPs that bind to the enzyme to form an EBP-enzyme complex; assaying for the activity level and substrate specificity of the EBP-enzyme complex; and identifying EBPs that modify the substrate specificity of the enzyme by identifying EBPs that, when in an EBP-enzyme complex, have a different substrate specificity than un-complexed EBP; wherein the catalytic rate constant of the EBP-enzyme complex is ≥50% of the un-complexed enzyme for at least one substrate and/or wherein the EBP-enzyme complex retains binding to a substrate.