Emad Tajkhorshid, Peter Nollert, Morten O. Jensen, Larry J. W. Miercke,
Joseph O'Connell, Robert M. Stroud, and Klaus Schulten.
Control of the selectivity of the aquaporin water channel family by
global orientational tuning.
Science, 296:525-530, 2002.
TAJK2002
Aquaporins are transmembrane channels found in cell
membranes of all life forms. We examine their
apparently paradoxical property, facilitation of efficient
permeation of water while excluding protons, which is
of critical importance to preserving the electrochemical
potential across the cell membrane. We have
determined the structure of the Escherichia coli
aquaglyceroporin GlpF with bound water, in native (2.7
angstroms) and in W48F/F200T mutant (2.1
angstroms) forms, and carried out 12-nanosecond
molecular dynamics simulations that define the spatial
and temporal probability distribution and orientation of
a single file of seven to nine water molecules inside
the channel. Two conserved asparagines force a
central water molecule to serve strictly as a hydrogen
bond donor to its neighboring water molecules.
Assisted by the electrostatic potential generated by two
half-membrane spanning loops, this dictates opposite
orientations of water molecules in the two halves of the
channel, and thus prevents the formation of a "proton
wire," while permitting rapid water diffusion. Both
simulations and observations revealed a more regular
distribution of channel water and an increased water
permeability for the W48F/F200T mutant.
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