Predicted Structural Alterations in Proinsulin during Its Interactions with Prohormone Convertases

The intracellular conversion of proinsulin to insulin occurs via cleavage at the two dibasic sites: Arg31-Arg32, B chain-C-peptide (BC) junction; and Lys64-Arg65, A chain-C-peptide (CA) junction, catalyzed by the subtilisin-like prohormone convertases SPC3 (PC1/PC3) and SPC2 (PC2), respectively. In this report we propose a possible conformational variant of proinsulin that would facilitate the formation of enzyme-substrate complexes at the BC and AC junctions of proinsulin with the substrate binding groove of the two closely related convertases. Productive convertase interaction requires extended peptide conformations in both the CA junction (residues 62-67, LQKRGI) and the BC junction (residues 29-34, KTRREA) and leads to significant perturbations in the normally alpha-helical N-terminal region of the A chain and the extended C-terminal region of the B chain of the insulin moiety of proinsulin. In this model of the reactive conformation of human proinsulin, both processing sites assume positions that are relatively far apart. The C-peptide was then modeled in an unobtrusive conformation relative to the convertases and the remainder of the substrate, forming an extended loop of length approximately 40 A with a short alpha-helical segment rather than a random coil. A model of the stereochemical transformations that occur during the processing of proinsulin by SPC2 is presented.