Improving the sensitivity and dynamic range of reagentless fluorescent immunosensors by knowledge-based design.
The variable fragment (Fv) of an antibody can be transformed into a reagentless fluorescent biosensor by mutating a residue into a cysteine in the neighborhood of the paratope (antigen-binding site) and then coupling an environment-sensitive fluorophore, e.g., N-((2-(iodoacetoxy)ethyl)-N-methyl)amino-7-nitrobenz-2-oxa-1,3-diazole (IANBD ester), to the mutant cysteine. For some residues, named operational, the formation of the conjugate does not affect the affinity of the Fv fragment for the antigen, and the binding of the antigen generates a measurable variation in the fluorescence intensity of the conjugate. We tested if this signal variation could be increased by coupling several molecules of fluorophores to the same molecule of Fv. Seven operational residues have been previously identified in the single-chain Fv (scFv) of monoclonal antibody D1.3 (mAbD1.3), directed against lysozyme. Ten double mutants of scFvD1.3, involving these residues, were constructed and coupled to the IANBD ester. The fluorescence of the double conjugates revealed a transfer of resonance energy between the two identical fluorescent groups. This homotranfer could be more important in the free state of the conjugate than in its antigen-bound state and increase its sensitivity for the detection of the antigen by up to 2.9-fold. A poorly sensitive conjugate could be improved by coupling a second molecule of fluorophore to residues located far from the paratope. Mutations altering the affinity of scFvD1.3 for lysozyme were introduced into one of its fluorescent conjugates. Using a mixture of three mutant derivatives of this unique conjugate, we could titrate lysozyme with precision in a concentration range encompassing 3 orders of magnitude.
Renard
Martial
M
Unit of Molecular Prevention and Therapy of Human Diseases, CNRS URA 2185, Institut Pasteur, 28 rue Docteur Roux, 75724 Paris Cedex 15, France.
Bedouelle
Hugues
H
D001665:Binding Sites / Q000235:genetics; D015374:Biosensing Techniques / Q000379:methods*; D003198:Computer Simulation; D004306:Dose-Response Relationship, Immunologic; D015195:Drug Design; D000939:Epitopes / Q000737:chemistry / Q000235:genetics; D031541:Fluorescence Resonance Energy Transfer / Q000379:methods; D005456:Fluorescent Dyes / Q000138:chemical synthesis*; D007118:Immunoassay / Q000379:methods*; D007135:Immunoglobulin Variable Region / Q000737:chemistry / Q000235:genetics; D007202:Indicators and Reagents; D007700:Kinetics; D008956:Models, Chemical; D008958:Models, Molecular; D009113:Muramidase / Q000737:chemistry / Q000235:genetics; D016297:Mutagenesis, Site-Directed; D010069:Oxadiazoles / Q000737:chemistry; D011485:Protein Binding / Q000235:genetics; D012680:Sensitivity and Specificity; D013816:Thermodynamics
D000939:Epitopes; D005456:Fluorescent Dyes; D007135:Immunoglobulin Variable Region; D007202:Indicators and Reagents; D010069:Oxadiazoles; C035573:4-(N-(iodoacetoxy)ethyl-N-methyl)amino-7-nitrobenz-2-oxa-1,3-diazole; D009113:Muramidase