Aquaporins Structure and Function

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Aquaporins - Structure and Function

• Adam Thompson
• University of Tennessee, Knoxville
• 11.15.2005

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What is an Aquaporin?

• Transmembrane channel found in all life forms
• They facilitate the efficient permeation of water
  in a bidirectional nature across the cell
  membrane
• They have high specificty for water and a select
  few solutes but are able to exclude proton
  transfer

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Aquaporins in Different Life Forms

• Human AQP 10 different genes
• Bacterial AQP 2 different genes
• Plant AQP over 30 differernt genes
• Why all the variation in humans and even more so
  in plants?

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General Homology of Aquaporins

• Despite species diversity, all aquaporins (AQP)
  exhibit a remarkable similarity in their general
  composition
• This suggests a common evolutionary ancestor
  amoung the aquaporin family

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AQP Conserved Structural Elements

• NPA motif
• Asn-Pro-Ala
• Restriction Pore
• H2, H5, LE1, LE2
• Hourglass Fold

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Hourglass Fold

• Bacterial AQP GlpF with adopted fold

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Bacterial Aquaporins A Model System

• There are two classes of bacterial AQP
• AQPZ an explicit water channel
• very similar to human AQP1 which is also an
  expilict water channel
• GlpF channel capable of conduction glycerol
• aquaglycoporins
• What makes bacteria a model system?
• What constitutes the differences between these
  two channels?

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Bacterial GlpF

• NPA motif is consistant within all AQP
• vital to the exclusion of protons transport
• Restriction Pore
• Consists of a tetrad of residues from the H2, H5,
  LE1, and LE2 regions
• constituents vital to glycerol conduction

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Role of the NPA Motif

• binds water molecule in center of pore
• similar orientation of hydroxyl dipoles in
  glycerol
• initiates bipolar arrangement of
  molecules above below bound region
• favorable for the conduction of solute
• unfavorable for protons

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NPA Positioning and Location

• The NPA motif is held in place with its two
  Asparagine residues pointed toward the center
  of the channel by four strong hydrogens bonds

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Role of the Restriction Pore

• Suprise! It restricts the entry of molecules
  primarily based on size
• A secondary mechanism entails exclusion based on
  the charges of potential transfer molecules
• Both of these mechanisms are dictated by the
  amino acid composition of the pore

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Restriction Pore, con't

• Atoms
• H2 Trp48
• H5 Gly191
• LE1 Phe200
• LE2 - Arg206
• Water and glycerol bound in the pore
• Increased hydrophobicity
• Phe200/Trp48
• Smaller Pore Size
• Gly191

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Glycerol Water How does it Happen?

• glycerol and water are not transported to fulfill
  any stoiciometric requirements within the cell
• the glycerol moves down the channel by competing
  with water for hydrogen bonds

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Glycerol is Much Larger Than Water

• The z-axis of aquaporins is very narrow which
  also helps to limit proton hopping along with
  the bipolar orientation of dipole moments
• This narrowness makes glycerol conduction
  difficult
• GlpF contains a periplasmic vestibule
• located between the restriction pore and NPA
  motif
• allows glycerol molecules to orient themselves
  for entry into the pore
• this same vestibule is not present in AQPZ

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Glycerol Water con't

• There are loops which constitue the vestibule
  formed in GlpF arising from its asymmetric nature
• these sames loops are much shorter in AQPZ
  leading to no vestibule and consequently no
  glycerol conduction
• the vestibule in GlpF has a -4kcal/mol energy
  well in the proton motive force (PMF)

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GlpF Vestibule Sequence
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AQP1 vs. GlpF

• Composition of the restriction pore
• H2 Phe58
• H5 His182
• LE1 Cys191
• LE2 Arg197

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Human Aquaporins
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