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#aquaporin

2 public questions tagged with this topic.

Plasma Membrane Proteins

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.

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Plasma Membrane Proteins

Which protein functions as a water channel and facilitates rapid movement of water across membranes?

Water permeability of plasma membranes exceeds lipid diffusion requiring dedicated proteinaceous pores. Aquaporin family comprising thirteen human isoforms forms homotetramers in kidney proximal tubules, red cells, and brain astrocytes each monomer about twenty eight kilodaltons six transmembrane helices arrangement N-terminus cytosolic with two half-helices HB and HE entering membrane containing invariant NPA motifs meeting at center forming aqueous pathway hourglass shape. Extracellular vestibule leads to aromatic arginine constriction formed by Arg195 His180 Phe56 limiting diameter to two point eight angstroms permitting single file waters hydrogen bonded to carbonyl oxygens and asparagine side chains while excluding hydronium via electrostatic barrier and interrupting Grotthuss proton wire. Transport occurs passive facilitated diffusion rate up to three billion molecules per second per channel driven by osmotic gradient without conformational change measured by stopped-flow light scattering. Band 3 performs anion exchange, GPCR transduces signals, spectrin provides elasticity, establishing aquaporin uniquely as water channel with dual NPA filter ensuring high selectivity.

Ref: Agre et al., Aquaporin Water Channels – Structure and Function, Nobel Lecture 2003.

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