Appendix 1—figure 1. Schematic representation of the free energy profile of a SNARE assembly.

(Left) The free energy profile at a reference calcium level $[C{a}^{2+}]={[C{a}^{2+}]}_0$. The activation barrier $\mathrm{\Delta }{G}^{‡}$ and the average number $n_{Ca}^{‡}$ of calcium ions bound to the SNARE assembly at transition state are indicated. (Right) An elevation of the calcium level lowers the free energy barrier and thereby increases the rate k₁ of the SNARE conformation transition.