Practice question

Ion channels achieve selective conduction despite high throughput by incorporating a narrow selectivity filter that discriminates on basis of size, dehydration energy and electrostatic coordination. Pore domain S5, P-loop, S6 forms inverted teepee; P-loop folds as two short pore helices P1 and P2 scaffolding filter. In potassium channels carbonyl oxygens from TVGYG sequence replace hydration waters precisely matching potassium radius but requiring too large cavity for smaller sodium. In sodium channels DEKA motif and outer ring carboxylates create high field site preferring partially hydrated sodium with one water and lysine blocking divalents. Calcium channels use EEEE locus binding calcium with high affinity enabling double occupancy knock-off. Once optimal ion enters filter it is stabilized, non-matching ions face energetic barrier and are excluded. Gate opening controlled by voltage sensor S4 or ligand binding occurs elsewhere, filter itself remains rigid. No ATP hydrolysis occurs. Hence filter ensures only compatible ion type passes, establishing basis for electrical signaling, selective reabsorption and action potential generation.