Unraveling Determinants of Affinity Enhancement in Dimeric Aptamers for a Dimeric Protein.

High-affinity aptamers can be derived de novo by using stringent conditions in SELEX (Systematic Evolution of Ligands by EXponential enrichment) experiments or can be engineered post SELEX via dimerization of selected aptamers. Using electrophoretic mobility shift assays, we studied a series of heterodimeric and homodimeric aptamers, constructed from two DNA aptamers with distinct primary sequences and secondary structures, previously isolated for VEGF-165, a homodimeric protein. We investigated four factors envisaged to impact the affinity of a dimeric aptamer to a dimeric protein: (1) length of the linker between two aptamer domains, (2) linking orientation, (3) binding-site compatibility of two component aptamers in a heterodimeric aptamer, and (4) steric acceptability of the two identical aptamers in a homodimeric aptamer. All heterodimeric aptamers for VEGF-165 were found to exhibit monomeric aptamer-like affinity and the lack of affinity enhancement was attributed to binding-site overlap by the constituent aptamers. The best homodimeric aptamer showed 2.8-fold better affinity than its monomeric unit (K _d  = 13.6 ± 2.7 nM compared to 37.9 ± 14 nM), however the barrier to further affinity enhancement was ascribed to steric interference of the constituent aptamers. Our findings point to the need to consider the issues of binding-site compatibility and spatial requirement of aptamers for the development of dimeric aptamers capable of bivalent recognition. Thus, determinants highlighted herein should be assessed in future multimerization efforts.