Unraveling C-Peptide’s Role in MIDY: A Structural Perspective

Proinsulin folding requires dynamic positioning of the C-peptide to guide A- and B-chain alignment and disulfide pairing. Mutant INS-gene-induced diabetes of youth (MIDY) arises when single-residue substitutions disrupt this process. We mapped the conformational free-energy landscapes of wild-type (WT) proinsulin and seven MIDY variants using metadynamics and molecular dynamics simulations. WT exhibits a deep free-energy minimum at compact conformations. In contrast, MIDY mutants display a continuum of destabilization: E­(A4)K retains near-WT stability, Akita (C­(A7)­Y), V­(B18)­A, and R­(Cpep + 2)C show moderate loss of the native basin, while H­(B5)­D, L­(A16)­P, and Y­(B26)C collapse the closed–open barrier and populate misfolded open states >50% of the time. Structural analyses reveal that WT and E­(A4)­K preserve robust A–C docking, with the C-peptide flexibly engaging the A-chain groove. Destabilizing mutants progressively erode these native A–C contacts while forming compensatory, non-native B–C interactions. Per-residue energy decomposition highlights the loss of canonical salt bridges and emergence of aberrant electrostatic and hydrophobic hot spots, correlating with the collapse of the folding free-energy barrier. Secondary-structure analysis further shows that mutants rigidify the normally disordered C-peptide, increasing helical or strand propensity in a mutation-specific manner. Collectively, these findings establish a continuum from near-native stability to overt misfolding, mechanistically linking single-site mutations to altered folding landscapes and aggregation risk in MIDY. The results highlight the C-peptide as a dynamic linchpin of proinsulin folding and suggest that restoring its flexible docking could provide a therapeutic avenue.