Fluorescence resonance energy transfer glucose sensor from site-specific dual labeling of glucose/galactose binding protein using ligand protection

Abstract

Background: Site-selective modification of proteins at two separate locations using two diferent reagents is highly desirable for biosensor applications employing fluorescence resonance energy transfer (FRET), but few strategies are available for such modification. To address this challenge, sequential selective modification of two cysteines in glucose/galactose binding protein (GGBP) was demonstrated using a technique we call "ligand protection." Method: In this technique, two cysteines were introduced in GGBP and one cysteine is rendered inaccessible by the presence of glucose, thus allowing sequential attachment of two diferent thiol-reactive reagents. The mutant E149C/A213C/L238S was first labeled at E149C in the presence of the ligand glucose. Following dialysis and removal of glucose, the protein was labeled with a second dye, either Texas Red (TR) C5 bromoacetamide or TR C2 maleimide, at the second site, A213C. Results: Changes in glucose-dependent fluorescence were observed that were consistent with FRET between the nitrobenzoxadiazole and TR fluorophores. Comparison of models and spectroscopic properties of the C2 and C5 TR FRET constructs suggests the greater rigidity of the C2 linker provides more eficient FRET. Conclusions: The ligand protection strategy provides a simple method for labeling GGBP with two diferent fluorophores to construct FRET-based glucose sensors with glucose afinity within the human physiological glucose range (1-30 mM). This general strategy may also have broad utility for other protein-labeling applications. © Diabetes Technology Society.