Analysis of insulin allostery in solution and solid state with FTIR.

The insulin hexamer acts as an allosteric unit mediated by homotropic and heterotropic effects shifting the equilibrium between three distinct conformational states (T(6), R(3)T(3) and R(6)). The homotropic ligand phenol stabilises the R(6) state by binding to hydrophobic pockets only present in the R(6) state and shifts the equilibrium towards the R(6) state. The structural difference between the T(6) and R(6) state is primarily a change in the B1-B8 residues from extended conformation (T(6)) to alpha-helix (R(6)). The aim of this study was to investigate FTIR as an alternative method to monitor the T-R transition in the insulin hexamer upon phenol binding, and in addition to explore the advantage of infrared spectroscopy to measure solid state samples, and support the ability to maintain an allosteric state upon drying. The FTIR spectra of insulin in solution showed an increase in alpha-helix upon phenol binding and correlated well with the transition measured by CD yielding similar dissociation constants. Furthermore it was possible to maintain the increase in alpha-helix upon phenol binding after lyophilisation. The overall structure of the FTIR spectra changed upon lyophilisation but an increase in alpha-helix content was retained. Reconstitution of lyophilised insulin resulted in a change in structure resembling the structure of insulin prior to lyophilisation. Principal component analysis of all spectra was computed resulting in distinct clusters, and most variation in the data set could be explained by PC1 corresponding to a change in alpha-helix.