The Nobel Prize in Chemistry The Nobel Prize in Chemistry 2003 2003

The Nobel Prize in Chemistry for 2003 is The Nobel Prize in Chemistry for 2003 is shared between two USA scientists in shared between two USA scientists in honor of their discoveries concerning honor of their discoveries concerning channelschannels in cell membranes. in cell membranes.

Peter Agre is Professor Peter Agre is Professor of of BiologicalBiological Chemistry Chemistry and Professor of and Professor of MediciMedicinene at Johns Hopkins Sc at Johns Hopkins School of Medicine in Baltihool of Medicine in Baltimore, USA. more, USA.

Roderick MacKinnon is Roderick MacKinnon is Professor of Professor of MolecularMolecular NeurobiologyNeurobiology and and Biophysics Biophysics at The at The Rockefeller University in Rockefeller University in New York, USA. New York, USA.

Agre discovered Agre discovered water channelswater channels of the of the cell membrane, and clarified cell membrane, and clarified （弄清） （弄清） important properties such as the selectivityimportant properties such as the selectivity（选择性） （选择性） for water molecules of the water cfor water molecules of the water channels.hannels.

MaKinnon clarified MaKinnon clarified the 3D structure of ththe 3D structure of the K+ channelse K+ channels with high resolution and elucida with high resolution and elucidatedted （解释） （解释） its functional mechanisms such its functional mechanisms such as ion selectivity in detail.as ion selectivity in detail.

All living matter is made up of All living matter is made up of cells cells

They have They have opened our eyesopened our eyes to a fantastic to a fantastic（极好的，难以置信的）（极好的，难以置信的） family of molecular family of molecular machines: machines: channels channels （通道）（通道） which are which are needed for the cell to function.needed for the cell to function.

  

The hunt for the water channelsThe hunt for the water channels

As early as As early as the middle of the nineteenth the middle of the nineteenth centurycentury it was understood that there must it was understood that there must be openings in the cell membrane to be openings in the cell membrane to permitpermit （允许）（允许） a flow of water and salts. a flow of water and salts.

It was not until around 1990 that Peter It was not until around 1990 that Peter Agre discovered the first water channel. Agre discovered the first water channel. Like so much else in the living cell, it was Like so much else in the living cell, it was all all about a protein.about a protein.

Peter Agre studied various membrane proteins from Peter Agre studied various membrane proteins from the red blood cells. He also found one of these in the red blood cells. He also found one of these in the kidneythe kidney （肾）（肾） . Having determined both its . Having determined both its

peptide sequencepeptide sequence （氨基酸顺序）（氨基酸顺序） and the and the corresponding DNA sequencecorresponding DNA sequence （相关（相关 DNADNA 顺顺

序）序） , , he realised that this must be the protein that so he realised that this must be the protein that so

many had soughtmany had sought （寻找）（寻找） before him: the cellular before him: the cellular water channel. water channel. Agre renamed the protein Agre renamed the protein aquaporinaquaporin 。。

ExperimentExperiment Agre tested his hypothesisAgre tested his hypothesis （假设）（假设） in in

a simple experiment where he a simple experiment where he compared cells which compared cells which contained the contained the proteinprotein in question with in question with cells which did cells which did not have itnot have it. When the cells were placed . When the cells were placed in a water solution, those that had the in a water solution, those that had the protein in their membranes absorbed protein in their membranes absorbed water by osmosiswater by osmosis （渗透） （渗透） and swelledand swelled（膨胀）（膨胀） up while those that lacked the up while those that lacked the protein were not affected at all. protein were not affected at all.

ExprimentExpriment

Peter Agre’s experiment with cells containing or lacking aquaporin. The aquaporin is necessary for making the cell absorb water and swell （膨胀） .

SelectivitySelectivity

Billions of water molecules Billions of water molecules can pass through each chacan pass through each channel every second. At the snnel every second. At the same time, H+ are not able tame time, H+ are not able to penetrateo penetrate （透过） （透过） these these open pores.open pores.

How is it that it How is it that it only admits only admits

waterwater moleculesmolecules and not and not other molecules or ions? other molecules or ions?

Because of the Because of the positive positive chargecharge at the center of at the center of the channel, positively the channel, positively charged ions such as charged ions such as H3O+, are deflectedH3O+, are deflected（使转向）（使转向） . This prev. This prevents proton leakageents proton leakage（泄露） （泄露） through the cthrough the channel. hannel.

Mechanism Mechanism

Very near its narrowest point, the wall of tVery near its narrowest point, the wall of the channel contains precisely positioned he channel contains precisely positioned positive chargespositive charges that attract the oxygen a that attract the oxygen atoms of the water molecules as they speetoms of the water molecules as they speed through the constrictiond through the constriction （压缩） （压缩） in the in the protein. This interaction disruptsprotein. This interaction disrupts （破坏） （破坏） the hydrogen bonds that normally link adjthe hydrogen bonds that normally link adjacentacent （临近的） （临近的） water molecules.water molecules.

Ion channels in actionIon channels in action

potassium ionpotassium ion channelschannels

Classify of the channelClassify of the channel Voltage-gated channelVoltage-gated channel whose conformational state depends on the differencwhose conformational state depends on the differenc

e in ionic charge on the two sides of the membrane. e in ionic charge on the two sides of the membrane. Ligand-gated channelLigand-gated channel whose conformational state depends on the binding whose conformational state depends on the binding

of a specific molecule ,which is usually not the solute of a specific molecule ,which is usually not the solute that passes through the channel.that passes through the channel.

Stretch-gated channel Stretch-gated channel which open their pores in response to mechanical dwhich open their pores in response to mechanical d

eformation of a neuron's plasma membrane. eformation of a neuron's plasma membrane.

As is knownAs is known An action potential iAn action potential i

n the pulvinusn the pulvinus （羽（羽状叶） 状叶） of of Mimosa Mimosa （（含羞草） 含羞草） leaves rleaves results in a reversiblesults in a reversible collapsee collapse （可逆性（可逆性下垂） 下垂） of the leaf.of the leaf.

An action potential as seen in the large intAn action potential as seen in the large internode cells of some algae.ernode cells of some algae.

Three dimensional structure of the bacteThree dimensional structure of the bacterial KcsA channel rial KcsA channel

The entry of a third K+ The entry of a third K+ ion into the selectivity ion into the selectivity filter creates an filter creates an electrostatic repulsion electrostatic repulsion that ejects the ion that ejects the ion bound at the opposite bound at the opposite end of the line .end of the line .

Various types of studies indicate that Various types of studies indicate that there is virtually no energy barrier for an there is virtually no energy barrier for an ion to move from one binding site to the ion to move from one binding site to the next, which accounts for the extremely next, which accounts for the extremely rapid flow of ions across the membrane.rapid flow of ions across the membrane.

Ion-selective channelIon-selective channel Only the potassium ions pass throughOnly the potassium ions pass through The selectivity filter contains four rings of carboThe selectivity filter contains four rings of carbo

nyl O atoms and one ring of threonyl O atoms . nyl O atoms and one ring of threonyl O atoms . Each of these five rings contains four O atoms .Each of these five rings contains four O atoms . The The oxygen atoms in the ion filteroxygen atoms in the ion filter form an envi form an environment which precisely mimics that of the potronment which precisely mimics that of the potassium ion outside the assium ion outside the filterfilter where it is surroun where it is surrounded by water molecules. So the potassium ion ded by water molecules. So the potassium ion can can slip out of its "water coat"slip out of its "water coat" and pass throug and pass through the filter without noticeable resistance.h the filter without noticeable resistance.

The The oxygen atoms in the oxygen atoms in the ion filterion filter form an form an environment which environment which precisely mimics that of precisely mimics that of the potassium ion outside the potassium ion outside the the filterfilter where it is where it is surrounded by water surrounded by water molecules. So the molecules. So the potassium ion can potassium ion can slip slip out of its "water coat"out of its "water coat" and and pass through the filter pass through the filter without noticeable without noticeable resistance.resistance.

The sodium ion, which is smaller than the The sodium ion, which is smaller than the potassium ion, draws water molecules more potassium ion, draws water molecules more closely to itself and is too small to fit snugly closely to itself and is too small to fit snugly between the oxygen atoms in the ion filter. between the oxygen atoms in the ion filter. Sodium ions therefore remain in the water Sodium ions therefore remain in the water outside the channel. outside the channel.

In the closed conformation the M2helics are straight anIn the closed conformation the M2helics are straight and cross over another to form a” helix bundle ”that seals d cross over another to form a” helix bundle ”that seals the cytoplasmic face of the pore. When the channel opthe cytoplasmic face of the pore. When the channel opens the M2 helices bend at a specific hinge point wherens the M2 helices bend at a specific hinge point where a glycine residue is locate. e a glycine residue is locate.

Two alternate models of the movement of Two alternate models of the movement of the S4 helix in orchestrating the opening of the S4 helix in orchestrating the opening of a voltage-gated K+ ion channel.a voltage-gated K+ ion channel.

In the classic model ,the movement of In the classic model ,the movement of the S4 helix is predicted to occur within the S4 helix is predicted to occur within the protected environment of the protein the protected environment of the protein and to move “up and down” like a screw and to move “up and down” like a screw or piston within the membrane. or piston within the membrane.

In the new model the S4 helix is predicteIn the new model the S4 helix is predicted to act like a moving “paddle” that projecd to act like a moving “paddle” that projects into the lipid bilayer and swings betweets into the lipid bilayer and swings between the extracellular and intracellular surfacn the extracellular and intracellular surface of the membrane. e of the membrane.

Conformational states of a Conformational states of a voltage-gated K+ ion channelvoltage-gated K+ ion channel

Inactivation of channel aInactivation of channel activity occurs as the N-tectivity occurs as the N-terminal portion of the polyrminal portion of the polypeptide ,which is called tpeptide ,which is called the inactivation peptide,fithe inactivation peptide,fits into the cytoplasmic ops into the cytoplasmic opening of the channel.ening of the channel.

The inactivity peptide is thought to gain access to the The inactivity peptide is thought to gain access to the pore by passing through the mouth one of four “windpore by passing through the mouth one of four “window” situated between the extended linkers that connow” situated between the extended linkers that connect the S1 heliced in the membrane to a “hanging” cyect the S1 heliced in the membrane to a “hanging” cytoplasmic domain.toplasmic domain.

When one of these dangling inactivation When one of these dangling inactivation peptides moves up into the mouth of the peptides moves up into the mouth of the pore ,the passage of ions is blocked, and pore ,the passage of ions is blocked, and the channels is inactivated. At a the channels is inactivated. At a subsequent stage of the cycle, the subsequent stage of the cycle, the inactivation peptide is released, and the inactivation peptide is released, and the gate to the channel can exist in three gate to the channel can exist in three different states –open, inactivated ,and different states –open, inactivated ,and closed.closed.

"This is what structural biology is for "This is what structural biology is for -- it shows us how extremely simple -- it shows us how extremely simple nature's solutions can be."nature's solutions can be."

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