Molecular switches toggle between
Many signaling proteins function as binary molecular switches that toggle between inactive and active conformations. Guanine-nucleotide-binding proteins switch by exchanging GDP for GTP; kinases and phosphatases switch by reversible phosphorylation. These switches allow cells to make rapid, decisive transitions in response to external cues while remaining fully reversible when the stimulus is withdrawn.
Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)