Novel Polystyrene-Binding Nanobody for Enhancing Immunoassays: Insights into Affinity, Immobilization, and Application Potential

Abstract

Nanobodies, which represent the next generation of antibodies due to their unique properties, face a significant limitation in their poor physical adsorption on solid supports. In this study, we successfully discovered polystyrene binding nanobodies from a synthetic nanobody library. Notably, bivalent nanobody B2 exhibited high affinity for polystyrene (0.7 nM for ELISA saturation binding analysis and 15.6 nM for isothermal titration calorimetry), displaying a pH-dependent behavior. Remarkably, hydrophobic and electrostatic interactions contribute minimally to the binding process. Molecular modeling provided insights into the interaction between B2 and polystyrene, revealing that the Trp51 residue within the CDR2 loop formed an aromatic H-bond with polystyrene at a distance of 2.74 Å, thus explaining the observed reduction in B2 affinity caused by Trp51 mutations. To explore B2's potential in protein immobilization, we constructed a bispecific nanobody by fusing B2 to an anticarcinoembryonic antigen nanobody 11C12, which cannot be immobilized on polystyrene through passive adsorption. Remarkably, the fusion construct achieved effective immobilization on polystyrene within 5 min by passing the need for periplasmic protein purification despite its low expression level. Moreover, the fusion construct demonstrated excellent linearity in the chemiluminescent enzyme immunoassay. For the first time, this study reports a simplified and seamless platform for the oriented immobilization of nanobody. Importantly, the entire process eliminated the need for protein purification, enabling efficient and rapid immobilization of fusion proteins directly from crude cell extracts, even when the expression level was low. Our developed process dramatically reduced the processing time from 2.5 days to just 5 min.