Functional characterization of the first aquaporin membrane protein was reported in 1992, but most membrane physiologists felt that there must be openings (pores or channels) in cell membranes to permit a flow of water, because the osmotic permeability of some epithelial cells was much too large to be accounted for by simple diffusion through the plasma membrane. It is predicted that a single human aquaporin-1 channel protein facilitates water transport at a rate of roughly 3 billion water molecules per second. Such transport appears to be bidirectional, in accordance with the prevailing osmotic gradient. In 1992 a "water channel" was identified by Peter Agre, M.D. (professor of Biological Chemistry and Medicine - John Hopkins Med School - 2003 Nobel Laureate in Chemistry) and what its molecular machinery might look like was suggested; that is, proteins were identified that formed an actual channel in membranes that facilitated water movement. In the mid-1980s Agre studied various membrane proteins isolated from the red blood cells. He also found one of these in the kidney cells. Having determined both its peptide sequence and the corresponding DNA sequence, he speculated that this might be the protein of the so-called cellular water channel. He termed this channel protein - aquaporin.
Agre
tested his hypothesis that aquaporin might be a water channel protein in a
simple experiment (fig. 1 - below). He compared cells which contained the
protein in question with cells which did not have it. When the cells were placed
in a water solution, those that had the protein in their membranes absorbed
water by osmosis and swelled up while those that lacked the protein were not
affected at all. Agre also ran trials with artificial cell
membranes, termed liposomes,
which are a simple lipid bound droplets of water. He found that the
liposomes became permeable to water only if the
aquaporin protein was implanted in their
Agre also knew that mercury ions often prevent cells from taking up and releasing water, and he showed that water transport through his artificial membrane sacs with the aquaporin protein was prevented in the same way by mercury. This was further evidence that aquaporin might actually be a water channel. How might a water channel work*? In 2000, together with other research teams, Agre reported the first high-resolution images of the three-dimensional structure of the aquaporin. With these data, it was possible to map in detail how a water channel might function. How is it that aquaporin only admits water molecules and not other molecules or ions? The membrane is, for instance, not allowed to leak protons. This is crucial because the difference in proton concentration between the inside and the outside of the cell is the basis of the cellular energy-storage system. Aquaporins form tetramers in the cell membrane, and facilitate the transport of water and, in some cases, other small solutes, as glycerol, across the membrane. However, the water pores are completely impermeable to charged species, such as protons, a remarkable property that is critical for the conservation of membrane's electrochemical potential. Based on hydrophobicity plots of their amino acid sequences , the aquaporins are predicted to have six membrane-spanning segments, as depicted in the model of aquaporin-1 below. Aquaporins exist in the plasma membrane as homotetramers. Each aquaporin monomer contains two hemi-pores, which fold together to form a water channel (fig 3.). The
probable mechanism of action of aquaporin channels
is studied using supercomputer simulations. In the April 2002 issue of
Science, the simulations run by researchers at the University of Illinois (Morten
Jensen,
The physiological and medical significance of possible water channels. The aquaporin proteins have been found to be a large protein family. More than 10 different mammalian aquaporins have been identified to date. Closely related water channel proteins have been isolated from plants, insects and bacteria. Aquaporin-1 from human red blood cells was the first to be discovered and is probably the best studied. In the human body alone, at least eleven different aquaporin protein variants have been found. The kidney removes waste substances the body wishes to dispose of. In the kidney water, ions and other small molecules leave the blood as ‘primary’ urine. Over 24 hours, about 170 liters of primary urine can be produced. Most of the water from this is reabsorbed so that finally about one liter of urine a day leaves the body. From the glomerulus of the kidney, primary urine is passed on through a winding tube where about 70% of the water is reabsorbed to the blood by the aquaporin AQP1 protein. At the end of the glomerulus tube, another 10% of water is reabsorbed with a similar aquaporin, AQP2. Apart from this, sodium, potassium and chloride ions are also reabsorbed into the blood. Antidiuretic hormone (vasopressin) stimulates the transport of AQP2 to cell membranes in the tube walls and hence increases the water resorption from the urine. People with a deficiency of this hormone might be affected by the disease diabetes insipidus with a daily urine output of 10-15 liters. References: 1. R.
Bowen, Colorado State University, January 2, 2002
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