Which of the following molecules can be transported by Aquaporin?
Selectivity of aquaporin pores illustrates precise molecular sieving. Aquaporin-1 monomer six helical bundle creates hourglass pore length approximately twenty angstroms with two constrictions: extracellular aromatic arginine filter formed by Arg195 guanidinium, His180 imidazole, Phe56 phenyl providing size filter diameter two point eight angstroms allowing water kinetic diameter two point zero angstroms but excluding hydrated sodium diameter seven point two angstroms, glucose eight angstroms, urea larger, and electrostatic barrier preventing proton conductance via positive Arg repulsion breaking continuous water wire. Second NPA constriction at center forces water reorientation interrupting Grotthuss hopping. Water moves single file driven by osmotic gradient direction high to low chemical potential up to billions per second maintaining kidney proximal reabsorption ninety percent filtered water, red cell volume regulation, lung alveolar fluid clearance. Sodium requires ENaC epithelial sodium channel, glucose requires SGLT and GLUT transporters, ATP requires ABC transporters. Therefore water is uniquely transported molecule by classic aquaporins distinguishing from ionic and metabolite transporters with larger pores and coupled mechanisms.
Ref: Verkman et al., Aquaporin Water Channels – Physiology and Selectivity, Nature Reviews Mol Cell Biol.