Figure 1.

Insulin is stored in dense-core secretory granules. Transmission electron micrographs of mouse β-cells depicting dense-core secretory granules (A) or a single insulin secretory granule from a human β-cell (B). Crystal structure of hexameric insulin with central Zn²⁺ ions in gray (C) coordinated by histidine (A10) side chains (D). (C,D) Paired insulin dimers within the hexamer are colored separately for visual clarity. Lavender or green denote the A chains; purple or green denote the B chains. Images were created using Mol* of 2INS from the Research Collaboratory for Structural Bioinformatics Protein Data Bank (RCSB PDB) [14,15].