Electrochemical impedance spectroscopy (EIS) is promising for biosensor applications due to its capability for complete frequency characterization of an electrochemical interface. This allows selection of the optimum detection frequency for maximum sensitivity. Genetically engineered glucose/galactose receptor (GGR) proteins from E. coli have been bound to an Au electrode through introduction of a cysteine group into this cysteine-free protein. The wild-type GGR protein shows insignificant adsorption onto an Au electrode. We have demonstrated a reagentless glucose impedance biosensor through exploitation of the large-amplitude “hinge-bending” motion of this protein upon ligand binding. For the GGR A1C mutant, the response of this biosensor is linear in the micromolar range, but for the GGR Q26C and E309C mutants, the impedance signal is unstable.