Helen V Hsieh

Direct detection of glucose by surface plasmon resonance with bacterial glucose/galactose-binding protein.

The monitoring and management of blood glucose levels are key components for maintaining the health of people with diabetes. Traditionally, glucose monitoring has been based on indirect detection using electrochemistry and enzymes such as glucose oxidase or glucose dehydrogenase. Here, we demonstrate direct detection of glucose using a surface plasmon resonance (SPR) biosensor. By site-specifically and covalently attaching a known receptor for glucose, the glucose/galactose-binding protein (GGBP), to the SPR surface, we were able to detect glucose binding and determine equilibrium binding constants. The site-specific coupling was accomplished by mutation of single amino acids on GGBP to cysteine and subsequent thiol conjugation. The resulting SPR surfaces had glucose-specific binding properties consistent with known properties of GGBP. Further modifications were introduced to weaken GGBP-binding affinity to more closely match physiologically relevant glucose concentrations (1-30 mM). One protein with a response close to this glucose range was identified, the GGBP triple mutant E149C, A213S, L238S with an equilibrium dissociation constant of 0.5mM. These results suggest that biosensors for direct glucose detection based on SPR or similar refractive detection methods, if miniaturized, have the potential for development as continuous glucose monitoring devices.

Fluorescence Resonance Energy Transfer Glucose Sensor from Site-Specific Dual Labeling of Glucose/Galactose Binding Protein Using Ligand Protection

Background: Site-selective modification of proteins at two separate locations using two different 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 different 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 efficient FRET. Conclusions: The ligand protection strategy provides a simple method for labeling GGBP with two different fluorophores to construct FRET-based glucose sensors with glucose affinity within the human physiological glucose range (1–30 mM). This general strategy may also have broad utility for other protein-labeling applications.

Detection of low-molecular-weight analytes by surface plasmon resonance and receptor conformational changes

Low molecular weight molecules are typically very difficult to detect directly in solution using commercially available SPR (surface plasmon resonance) instruments. This is because the mass change on binding is not sufficient to cause a detectable change in refractive index on binding to surface-bound receptors (e.g., antibodies). Some receptors, however, undergo extensive changes in tertiary structure upon binding ligands. Here we present data suggesting conformational changes in surface-bound receptors such as periplasmic binding proteins and calcium-binding proteins can be detected by SPR. This SPR response can be used to monitor specific binding of carbohydrates and calcium even though the molecular weight of these analytes would be difficult to detect using traditional SPR methods. Therefore this approach has potential applications for developing optical biosensors for such small molecules.