Glucose interactions with a model peptide

Molecular dynamics simulations have been conducted of the helical polypeptide melittin, in concentrated aqueous solutions of the alpha and beta anomers of D-glucopyranose. Glucose is an osmolyte, and it is expected to be preferentially excluded from the surfaces of proteins. This was indeed found to be the case in the simulations. The results indicate that the observed exclusion may have a contribution from an under-representation of hydrogen bonding interactions between glucose groups and exposed side chains, compared to water. However, glucose was found to bind quite specifically to melittin by stacking its hydrophobic face, consisting of aliphatic protons, against the flat hydrophobic face of the indole group of the tryptophan-19 side chain. Although the binding site for this interaction is localized, the binding is weak for both anomers, with a binding free energy estimated as only ∼0.5 kcal/mol (i.e. near k(B)T). The face of the sugar stacked against the Trp indole ring is different for the two anomers of glucose, due to the disruption of the H1-H3-H5 hydrophobic triad of the beta anomer by the axial C1 hydroxyl group in the alpha anomer. The measurable affinity of the sugar for the Trp side chain is consistent with the very frequent occurrence of this group in the binding sites of proteins that complex with sugars.