Unravelling the mechanism of glucose binding in a protein-based fluorescence probe: molecular dynamics simulation with a tailor-made charge model.

Fluorophores linked to the glucose/galactose-binding protein (GGBP) are a promising class of glucose sensors with potential application in medical devices for diabetes patients. Several different fluorophores at different positions in the protein were tested experimentally so far, but a deeper molecular understanding of their function is still missing. In this work, we use molecular dynamics simulations to investigate the mechanism of glucose binding in the GGBP-Badan triple mutant and make a comparison to the GGBP wild-type protein. The aim is to achieve a detailed molecular understanding of changes in the glucose binding site due to the mutations and their effect on glucose binding. Free simulations give an insight into the changes of the hydrogen-bonding network in the active site and into the mechanisms of glucose binding. Additionally, metadynamics simulations for wild type and mutant unravel the energetics of binding/unbinding in these proteins. Computed free energies for the opening of the binding pocket for the wild-type and the mutant agree well with the experimental data. Further, the simulations also give an insight into the changes of the chromophore conformations upon glucose binding, which can help to understand fluorescence changes. Therefore, the molecular details unravelled in this work may support effective optimisation strategies for the construction of more efficient glucose sensors.