Which of the following molecules moves most efficiently through aquaporins?
Aquaporins represent a family of tetrameric channel proteins mediating extremely rapid yet selective water transport essential for osmoregulation, kidney concentration and brain water balance. Each monomer contains six transmembrane helices plus two half helices HB and HE each bearing conserved Asn-Pro-Ala motif that meet centrally forming dipoles. Water moves single file through an hourglass pore constricted to 2.8 angstrom at aromatic arginine filter where histidine, arginine and phenylalanine side chains create size barrier. Permeation rate exceeds one billion molecules per second per monomer driven purely by osmotic gradient without conformational change or ATP hydrolysis. Selectivity depends on three principles: size exclusion prevents hydrated ions and glucose, electrostatic repulsion by arginine carbonyl orientation blocks proton hopping via Grotthuss mechanism by breaking water wire, and precise hydrogen bonding via NPA asparagines forces water reorientation. Sodium ions retain large hydration shell and would require energetic dehydration, while glucose exceeds pore diameter. Some aquaglyceroporins conduct glycerol or ammonia inefficiently, but orthodox water channels show thousand fold preference for water, ensuring efficient osmotic equilibration.
Ref: Agre et al., Nobel Lecture 2003, Aquaporin Water Channels Structure - NPA Motifs and Proton Exclusion.