Designed, Functionalized Helix−Loop−Helix Motifs that Bind Human Carbonic Anhydrase II:  A New Class of Synthetic Receptor Molecules

Polypeptides designed to fold into helix-loop-helix motifs and to dimerize to form four-helix bundles were functionalized by the introduction of a sulfonamide derivative known to bind human carbonic anhydrase II (HCAII) and one or both of the dansyl- and methoxycoumarin fluorescent probes. The 42-residue sequence DC that carries all three substituents in solvent-exposed positions was found to bind HCAII with a dissociation constant of 5 nM in aqueous solution at pH 7. At 2 muM concentration, DC was mainly dimeric in aqueous solution but bound HCAII as a monomer. Upon addition of a large excess of a helix-loop-helix motif without a high-affinity ligand, KE2-Q, a ternary complex was formed between HCAII, DC, and KE2-Q. Hydrophobic interactions between DC and HCAII and coordination of the sulfonamide group to the zinc ion of HCAII contributed cooperatively to binding in a demonstration of the usefulness of folded polypeptide-small organic molecule chimera as novel protein receptors. The DC homodimer was found to be a very sensitive biosensor component due to intermolecular quenching of its fluorescence that was inhibited upon binding to HCAII.