Molecular dynamics and circular dichroism studies of human and rat C-peptides

Proinsulin C-peptide has been recently described as an endogenous peptide hormone, responsible for important physiological functions others than its role in proinsulin processing. Accumulating evidences that C-peptide exerts beneficial effects in the treatment of long term complications of patients with type 1 diabetes mellitus indicate that this molecule may be administered together with insulin in future therapies. Despite its clear pharmacological interest, the secondary and three-dimensional (3D) structures of human C-peptide are still points of controversy. In the present work we report molecular dynamics (MD) simulations of human, rat I and rat II C-peptides. A common experimental strategy applied to all peptides consisted of homology building followed by multinanosecond MD simulations in vacuum and water. Circular dichroism (CD) experiments of each peptide in the absence and presence of 2,2,2-trifluoroethanol (TFE) were performed to support validation of the theoretical models. A multiple sequence alignment of 23 known mammalian C-peptides was constructed to identify significant conserved sites that would be important for the maintenance of secondary and tertiary structures. The analysis of the molecular dynamics trajectories for the human, rat I and rat II molecules have shown quite different general behavior, being the human C-peptide more flexible than the two others. Human and rat C-peptides exhibit very stable turn-like structures at the middle and C-terminal regions, which have been described as potential active sites of C-peptides. Human C-peptide also presented a short alpha-helix throughout the MD, which was not found in the rat molecules. CD data is in very good agreement with the MD results and both methods were able to identify a greater structural stability and potential in rat C-peptides when compared to the human C-peptide. The simulation results are discussed and validated in the light of multiple sequence alignment, recent experimental data from the literature and our own CD experiments.