Upper panel: Insulin is secreted by the pancreas in the form of hexamer aggregates with ions (Zn2+) at their center: these aggregates divide to form dimers, which then split to form monomers. Each monomer consists of an A chain (red) and a B chain (gray) with a C-terminal B23-B30 strand. An insulin monomer binds to the L1 domain of an insulin receptor, with the C-terminus of the B chain partially detaching from the central core of the monomer to avoid a clash with the CT peptide (blue) of the receptor. A phenylalanine amino acid residue at the position B24 of insulin (PheB24, black) is crucial for insulin binding. Lower panel: Fish-hunting cone snails secrete insulin-like molecules that do not form aggregates. These molecules induce low blood sugar levels in fish, which stops them escaping. Although cone snail insulin lacks both the C-terminal of the B-chain (gray) and PheB24, it can still bind to fish insulin receptors because it contains two tyrosine amino acid residues (TyrB16 and TyrB20, black) that mimic the missing interactions. The A-chain is shown in beige.