Practice question

Channel and carrier proteins represent two mechanistically distinct classes of facilitated transport differing in kinetics and conformational dynamics. Channels form transmembrane aqueous pores where ions flow rapidly without strong binding, reaching ten to hundred million per second, gated by voltage, ligand or mechanical stimuli switching between open and closed states. Their selectivity arises from narrow filters where carbonyl oxygens replace hydration, as in potassium channel TVGYG motif, but they avoid large alternating-access changes; binding is weak and transient. Carrier proteins use alternating-access mechanism, alternately exposing binding sites to either side through major helix movements, binding solute with micromolar to millimolar affinity, providing saturable Michaelis-Menten kinetics, competitive inhibition and slower turnover of hundred to ten thousand per second. Saying channels bind and ferry solutes via large conformational change confuses them with carriers. Channels are generally less selective, though highly selective ion channels exist, and their high throughput supports rapid electrical signaling in neurons and epithelial transport requiring fast flux.