Molecular Determinants of Affinity and Isoform Selectivity in Protein─Small Molecule Hybrid Inhibitors of Carbonic Anhydrase.

Multiple studies have demonstrated the benefit of engineering hybrid ligands that combine the unique benefits of small molecules and proteins or peptides. However, the molecular complexity of hybrid ligands generates a parameter space so large it cannot be exhaustively explored. We systematically evaluated the impact of one molecular design element, conjugation site, on the discovery of functional protein-small molecule hybrids (PriSMs). We utilized a library of yeast-displayed fibronectin domain variants with amino acid and loop length diversity in the paratope and a single cysteine at one of 18 possible conjugation sites. The protein variants were coupled with maleimide-functionalized acetazolamide and sorted via competitive flow cytometry to discover potent and selective inhibitors of three isoforms of carbonic anhydrase. Deep sequencing of the resultant populations of functional PriSMs revealed an isoform-dependent distribution of conjugation site preferences. The top PriSMs showed potency and selectivity gains up to 23- and 100-fold (in this case, for CA-II vs CA-XII, with a 43-fold selectivity gain for CA-II vs CA-IX) relative to PEG2-acetazolamide alone. The presented study expands our fundamental understanding of the role of conjugation site in PriSM function and informs future PriSM engineering efforts by highlighting the benefit of conjugation site diversity in PriSM libraries.