The Dynamics and Infrared Spectrocopy of Monomeric and Dimeric Wild Type and Mutant Insulin

TL;DR

This study uses molecular dynamics simulations and infrared spectroscopy to analyze how mutations and dimerization affect insulin's structure and vibrational signatures, revealing differences in dynamics and spectroscopy between monomers and dimers.

Contribution

It introduces a combined computational and experimental approach to characterize insulin aggregation and dimerization through infrared spectral analysis.

Findings

Spectroscopic signatures differ between monomeric and dimeric insulin, especially at residues involved in dimerization.

Mutations at position B24 significantly alter the infrared spectra, indicating changes in aggregation behavior.

Monomers in dimers are dynamically asymmetric despite symmetric crystal structures.

Abstract

The infrared spectroscopy and dynamics of -CO labels in wild type and mutant insulin monomer and dimer are characterized from molecular dynamics simulations using validated force fields. It is found that the spectroscopy of monomeric and dimeric forms in the region of the amide-I vibration differs for residues B24-B26 and D24-D26, which are involved in dimerization of the hormone. Also, the spectroscopic signatures change for mutations at position B24 from phenylalanine - which is conserved in many organisms and known to play a central role in insulin aggregation - to alanine or glycine. Using three different methods to determine the frequency trajectories - solving the nuclear Schr\"odinger equation on an effective 1-dimensional potential energy curve, instantaneous normal modes, and using parametrized frequency maps - lead to the same overall conclusions. The spectroscopic response of monomeric WT and mutant insulin differs from that of their respective dimers and the spectroscopy of the two monomers in the dimer is also not identical. For the WT and F24A and F24G monomers spectroscopic shifts are found to be $\sim 20$ cm$^{-1}$ for residues (B24 to B26) located at the dimerization interface. Although the crystal structure of the dimer is that of a symmetric homodimer, dynamically the two monomers are not equivalent on the nanosecond time scale. Together with earlier work on the thermodynamic stability of the WT and the same mutants it is concluded that combining computational and experimental infrared spectroscopy provides a potentially powerful way to characterize the aggregation state and dimerization energy of modified insulins.