Reversible binding with stochastic gating. (A) A macromolecule fluctuates between the open, M1 (red), and closed, M2 (blue), states. A ligand L reversibly binds to the macromolecule in the open state M1 and forms a complex C. (B) A ligand fluctuates between the reactive, L1 (red), and unreactive, L2 (blue), states. The ligand in the reactive state L1 binds to the macromolecule M and forms a complex C. The open (or reactive) states are labeled by the index “1”.