Download AQUAPORINS – USEFUL LEADS TO LOW ENERGY

desalination and membranes
refereed paper
AQUAPORINS – USEFUL LEADS TO
LOW ENERGY DESALINATION
MEMBRANES?
B Bolto, M Hoang, T Tran
Abstract
The 2003 Nobel Prize in Chemistry was
awarded to Agre and MacKinnon, who
contributed to the fundamental chemical
knowledge on how cells function. They
discovered a system of molecular
reticulation in cell membranes: channels,
gates and valves all of which are needed
for cells to function. It is an excellent
model for designing the ultimate in
practical low energy, high flux
desalination membranes that transport
water, but not salt.
Introduction
Water crosses cell membranes by two
routes: diffusion through a liquid bilayer,
or diffusion through water channels
called aquaporins. Progress in
understanding the structure and function
of aquaporins has been rapid, with the
topic being the subject of detailed
reviews (Agre et al., 2002; Beitz, 2009).
Aquaporins transport solute-free water
across cell membranes via exclusive
water channels that are not permeable to
ions or other small molecules. More than
ten different mammalian aquaporins have
been identified to date (Nobel
Foundation, 2003). They are composed
of proteins of molecular weight
~28,000 Da, are very widely distributed
and have different important specific
functions. Aquaporin-1 from human red
blood cells was the first to be discovered
and is the most studied; it exists in the
kidney, eye, brain and lung. Each pore
facilitates water transport through the
cell membrane at the rate of three billion
water molecules per second, in a
movement that appears to be bidirectional depending on the osmotic
gradient. Based on hydrophobicity and
plots of the amino acid sequences of the
proteins, aquaporin units are predicted to
have six membrane-spanning segments,
as shown in Figure 1 for aquaporin-1,
which is known as AQP1. This aquaporin
appears to exist as a tetramer or four
such units, with each aquaporin
monomer containing two hemi-pores
which fold onto each other in a unique
80 AUGUST 2010 water
Figure 1. Postulated structure of aquaporin-1 (adapted from King and Agre, 1998).
way to form the water channel. There are
thus four channels per structure.
To maintain an even pressure in the
cells it is important that water can pass
through the cell wall, as has been known
for a long time. The appearance and
function of these pores remained one of
the classical unsolved problems of
biochemistry. It was not until around
1990 that the first water channel was
discovered. Like so much else in the
living cell, a protein was the dominant
factor. Water molecules are not the only
entities that pass into and out of the cell.
For billions of cells to be able to function
collaboratively, coordination is required,
so communication between the cells is
necessary. The signals sent in and
between cells involve ions or small
molecules. These start cascades of
chemical reactions that control all our
bodily functions. The signals in the brain
also involve such chemical reactions.
As early as the middle of the
nineteenth century it was understood
that there must be openings in the cell
membrane to permit a flow of water and
salts. In the middle of the 1950s it was
discovered that water can be rapidly
transported into and out of cells through
pores that admit water molecules only.
During the next 30 years this was studied
in detail and the conclusion was that
there must be some type of selective
filter that prevents ions from passing
through the membrane, while uncharged
water molecules flow freely. Although
this was known, it was not until 1992
Can man emulate nature?
that the molecular machinery was
identified as to which protein or proteins
formed the actual channel. In the mid1980s Agre studied various membrane
proteins from red blood cells and cells in
the kidney (Nobel Foundation, 2003). The
hypothesis was tested in a simple
experiment where cells that contained
the protein in question were compared
with cells that did not have it. When the
cells were placed in a water solution,
membranes containing the protein
absorbed water by osmosis and became
swollen, while those that lacked the
protein were not affected. Trials were
also run with artificial cells, or liposomes,
which are really surfactant bubbles
surrounded on the outside and inside by
water. It was found that the liposomes
became permeable to water if the protein
was planted within their membranes.
In 2000, together with other research
teams, Agre reported the first highresolution images of the threedimensional structure of the aquaporin.
With these data it was possible to map in
detail how a specific water channel
functions which admits water molecules
but not other molecules or ions.
Selectivity is a central property of the
channel. Water molecules worm their
way through the narrow channel of the
hour glass structure by orienting
themselves in the local electrical field
formed by the charged functional groups
in the channel wall. Cations are stopped
on the way because of the positive
charge at the centre of the channel
(Figure 2), which rejects like-charged
species such as hydrated protons which
would otherwise cause a pH change.
technical features
desalination and membranes
Studies of the water channel
aquaporin-Z from E. coli
have shown that it
transports less than one ion
per 109 water molecules
(Pohl et al., 2001).
Structural Details of
Water Channels
Hundreds of proteins have
been identified in water
channels from all forms of
life (Fujiyoshi et al., 2002).
The strict selectivity for
water raised many questions
about the structural basis
Figure 2. Passage of water molecules through an aquaporin
responsible for these
from Nobel Foundation, 2003).
remarkable properties. The
structure is based on a motif
centre of the pore, thus inhibiting the
or sequence of three specific amino
permeation of cations (Law and Sansom,
acids (asparaginine, proline and alanine,
2002).
abbreviated as NPA) and the unique
During the past ten years, water
aquaporin fold.
channels have developed into a highly
The three amino acids are of different
topical research field. The aquaporins
types: the first is hydrophilic and polar,
have proved to be a large protein family.
and the other two are hydrophobic.
They exist in bacteria, plants and
Mutation of residues around the NPA
animals. In the human body alone at least
motif reduces water permeability,
eleven different variants have been
suggesting that these regions contribute
found. The function of these proteins has
to formation of the aqueous pore (Jung et
now been mapped in bacteria and in
al., 1994). Also relevant among many
plants and animals, with a focus on their
others are seven further amino acids
physiological role. In humans, the water
(valine, isoleucine, leucine, phenylalanine,
channels play an important role in,
cysteine, arginine and histidine).
among other organs, the kidneys. The
Molecular dynamic simulations of water
kidney is an ingenious apparatus for
permeation through AQP1 show that
disposing of substances the body wishes
water molecules are strongly oriented in
to remove. In its windings or glomeruli,
the channel interior, with their dipoles
which function as sieves, water, ions and
rotating about 180° during flow through
other small molecules leave the blood as
the channel (Murata et al., 2000). There is
primary urine. Over 24 h, about 170 L of
a large tilt of about 30° of the α-helices
primary urine is produced. Most of this is
that embrace the central pore region,
re-absorbed by a series of mechanisms
where the NPA motifs contact each other
so that finally about 1 L of urine a day
(Fujiyoshi et al., 2002). The right-handed
leaves the body. From the glomeruli,
arrangement of the α-helices was initially
primary urine is passed on through a
controversial, but other examples of
winding tube where about 70% of the
strongly tilted ones have since emerged.
water is re-absorbed into the blood via
Many helix-helix interactions stabilise the
aquaporin AQP1. At the end of the tube,
system. Water-water hydrogen bonds are
another 10% of water is re-absorbed with
weakened at the narrowest part of the
a similar aquaporin, AQP2.
pore, 0.2 nm across. This suggests that
the NPA region is a major selectivity filter.
The midpoint of the rotation of the water
molecules is located in this NPA region.
About half of the channel wall along the
selectivity filter can be considered
hydrophobic and the other half
hydrophilic (Sui et al., 2001). The
hydrophilic face provides the sites that
are essential for displacing certain waters
of hydration, thereby establishing a
pathway for coordinating water transport.
The terminal amino groups of the NPA
loop might be expected to form a region
of positive electrostatic potential in the
82 AUGUST 2010 water
Channels exist that can admit and
transport ions. There is a well-developed
knowledge of the central functions of ion
channels, which are able to admit one ion
type selectively, but not others. Cells
must also control the opening and
closing of the ion channels. This is
achieved by a gate at the bottom of the
channel which is opened and closed by a
molecular sensor, situated near the gate.
Certain sensors react to certain signals,
such as the binding of a signal molecule
of some kind, an increase in the
concentration of calcium ions, or an
refereed paper
electrical potential over the
cell membrane. For example,
L-glutamate is the major
neurotransmitter in the
mammalian central nervous
system, acting in one way
through ligand-gated ion
channels, or ionotropic
receptors (Bristol University,
2003).
Implications for Water
Treatment
There is undoubtedly much to
learn yet about aquaporins at
a basic mechanistic level.
(adapted
Nevertheless, work is
proceeding on ways of
incorporating aquaporins into
membranes that are aimed at improving
permeability, which for an aquaporin
saturated lipid membrane is claimed to
be more than 100-fold that of normal
water treatment membranes (Jensen et
al., 2006, using data of Pohl et al., 2001),
obtained with aquaporin membranes of
150 μm diameter. Patents have been filed
on water filtration and desalination
applications (Jensen et al., 2006; Kumar
et al., 2009), and one firm is aiming to
release its membrane to the market place
in 2011 (Anon., 2010). The intricacies of
biological membranes mean that manmade versions of them for desalination
purposes are some way off, especially as
they will have to avoid the hydrolytic
instability of proteins under acid or
alkaline conditions. Adequate physical
integrity is another hurdle. There is a
considerable challenge overall.
The open path channel through the
species offers a fast flow that is not
necessarily impeded by the central
constriction in the hour glass pores, as it
is of very short depth. Polyimide
membranes that have been thermally
rearranged have an hour glass
configuration (Park et al., 2010). They are
useful for gas separations such as CO2
removal from mixtures with methane,
where transport of the larger molecule is
facilitated, as it has a long thin shape
versus the larger roughly shaped spheres
for the smaller species.
The nearest unfunctionalised analogues
so far are carbon nanotube structures
that can provide a continuous channel
capable of rapid flows. An example of the
rapid flow possible, which has been
described as slip or frictionless flow, is
that achieved with nanotubes with a pore
size of 2 nm, which have water
permeabilities several orders of
magnitude greater than commercial
technical features
refereed paper
desalination and membranes
polycarbonate membranes, despite
having pore sizes an order of magnitude
smaller (Holt et al., 2006). Their
usefulness in desalination has been
discussed (Corry, 2008). However,
constructing membranes from the
nanotubes is not a trivial problem.
Binding them in a matrix, but ensuring
that there is no leakage between it and
the fibre, is a major challenge. There is
some progress: because of their very
high adsorption capacity for salt,
oxidised carbon nanotube sheets have
been proposed for sea water desalination
(Togfighy and Mohammadi, 2010).
Fujiyoshi, Y., Mitsuoka, K., de Groot, B.,
Philippsen, A., Grubmüller, H., Agre, P. and
Engel, A. (2002). Structure and function of
water channels. Current Opinion in Structural
Biol. 12, 509-515.
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The Authors
Dr Brian Bolto,
Dr Thuy Tran and
Dr Manh Hoang (email:
[email protected];
[email protected];
[email protected])
work for CSIRO Materials Science and
Engineering, Clayton, Victoria.