Schematic illustration of the Ca2+ occupancy states for Syt1. The maximal number of Ca2+ ions (symbolized as red ovals) that can bind C2A is 3 (marked as 1,2,3) and 2 (marked as 4, 5) for the C2B. The blue and purple lines represent all potential states ranging from no binding to maximal occupancy by 5 ions. Total of 12 edges representing Syt1 states assuming the actual position of the ions within C2A or C2B is not important. Addition of the positional information increases the number of Ca2+ occupancy states as illustrated by the purple edges. The number of individual states associated with the purple edges summarizes to ten combinations of occupancy of 3 Ca2+ ions. With positional information for Ca2+ ions occupancy the number of individual states reaches 32 (1 state for no occupancy, 5 states for one ion, 10 states for 2 ions, 10 states for 3 ions, 5 states for 4 ions and another state for occupancy of 5 ions).
Brachya et al. BMC Neuroscience 2006 7(Suppl 1):S4 doi:10.1186/1471-2202-7-S1-S4