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#water channels

2 public questions tagged with this topic.

Plasma Membrane Proteins

Which of the following sequences is highly conserved in Aquaporins?

Highly conserved sequence motif in aquaporin family defines aqueous pore architecture and selectivity. Loop B half-helix and Loop E half-helix each dip into membrane from opposite sides each containing invariant tripeptide Asn-Pro-Ala forming two NPA boxes meeting at center of channel creating electrostatic barrier and orienting water molecules in opposite NPA asparagine carbonyl hydrogen bond donors. Proline introduces kink allowing asparagine side chain to project into pore, alanine stabilizes packing between half-helix and transmembrane helices. Structural alignments show two NPA motifs generate constriction forcing water reorientation one hundred eighty degrees disrupting continuous hydrogen bonded chain required for proton hopping via Grotthuss mechanism, while arginine selectivity filter excludes ions. Mutation NPA to NPG or NPS in aquaporin-2 causes nephrogenic diabetes insipidus reducing water permeability and autosomal dominant cataract. Database search using NPA signature identifies over three hundred aquaporin orthologs across Bacteria E coli aquaporin Z, Archaea, plants tonoplast intrinsic proteins, mammals, distinguishing water channels from glycerol facilitators and ion channels lacking motif.

Ref: Fujiyoshi et al., Structure of Aquaporin Water Channel and NPA Motif, Nature 2002.

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Membrane Transportation

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.

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