Hydroxide and proton migration in aquaporins

¹ MEMPHYS Center for Biomembrane Systems
² EPFL Lausanne
³ Parc Científic de Barcelona

^* To whom correspondence should be addressed. E-mail: crovira{at}pcb.ub.es.

Submitted on December 17, 2004
Revised on January 13, 2005
Accepted on 10 May 2005

	Abstract

Hypothetical hydroxide and proton migration along the linear water chain in Aquaporin GlpF from E. coli are studied by ab initio Car-Parrinello molecular dynamics simulations. It is found that the protein stabilizes a bipolar single-file of water. The single-file features a contiguous set of water-water hydrogen bonds in which polarization of the water molecules vary with position along the channel axis. De-protonation of the water chain promotes water molecules to reorient while the hydroxide ion rapidly migrates by sequentially accepting protons from the neighboring water molecules. The hydroxide ion is not attracted by a conserved, channel lining arginine residue but immobilized at two centrally located, conserved asparagine-proline-alanine motifs where fourfold coordination stabilizes the ion. Hydroxide transition from the channel vestibules into the channel lumen is strongly influenced by electrostatic coupling to two conserved oppositely aligned macro-dipoles. This suggests that the macro-dipole's negative poles play a role in preventing hydroxide ions from entering into the channel's inner vestibules. Water protonation within the lumen facilitates water reorientation and subsequent proton expelling occur. In the periplasmic half-channel, expelling occurs via the Grotthuss mechanism. Protonation within the cytoplasmic half-channel implies wire-breakage at the NPA motifs. The proton is here diffusively rejected as [H5O2]+.

Key Words: Car-Parrinello molecular dynamics, ab initio computer simulation, aquaglyceroporin, hydroxide transport, membrane protein, proton transport

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