Molecular Cell BiologyFifth Edition

Chapter 7:Transport of Ions and Small Molecules

Across Cell Membranes

Copyright © 2004 by W. H. Freeman & Company

Harvey Lodish • Arnold Berk • Paul Matsudaira • Chris A. Kaiser • Monty Krieger • Matthew P. Scott •

Lawrence Zipursky • James Darnell

Cell membrane

Barrier to the passage of most polar

molecule

Maintain concentration of solute

Aquaporin, the water channel, consists of four identical transmembrane polypeptides

Relative permeability pf synthetic lipid bilayer to different classes of molecule

Diffusion rate depends on :

1. Concentration gradient or electrochemical gradient

2. Hydrophobicity

i.e. higher partition coefficient

3. Particle size

Three main class of membrane protein

1.ATP- power pump( carrier, permease)

couple with energy source for active transport

binding of specific solute to transporter which

undergo conformation change

2. Channel protein

formation of hydrophilic pore

allow passive movement of small inorganic

molecule

3. Transporters

uniport

symport

antiport

1. All transmembrane

proteins

2. ATP binding sites

3. Move molecules uphill against its gradient

Kinetics of simple diffusion and carrier mediated diffusion

Unique features for Uniport transport:

1. Higher diffusion rate for uniport

2. Irrelevant to the partition coefficient

3. Transport rate reach Vmax when each uniport working at its maximal rate

4. Each uniport transports only a single species of molecules or single or closely related molecules

Liposome containing a single type of ytransport protein are useful in studying functional properties of transport protein

Families of GLUT proteins( 1-12)

GLUT1

GLUT2: express in liver cell ( glucose storage)

and ß cell( glucose uptake) pancrease

GLUT4: found in intracellular membrane,

increase expression by insulin, lowers

the blood glucose

ATP powered pump

1. P- class

2, 2 subunit

i.e. Na+-K+ ATP ase, Ca+ATP ase, H+pump

2. F-class

locate on bacterial membrane , chloroplast and mitochondria

pump proton from exoplasmic space to cytosolic for ATP synthesis

3. V-class

maintain low pH in plant vacuole

Operational model of the Ca+-ATP ase in the SR membrane of skeletal muscle cells

Higher Ca+2

Lower Ca+2

Structure of the catalytic subunit of the muscle Ca+2 ATP ase

Phosphorylation site

-helix

Operational model of the Na+/K+ ATP ase in the plasma membrane

Higher affinity for Na+

V-class H+ ATP ase pump protons across lysosomal and vacuolar membrane

Effect of proton pumping by V-class ion pumps on H+ concentration gradients and electric potential gradients across cellular membrane

Generation of electrochemical gradient

Electrochemical gradient combines the membrane potential and concentration gradient which work additively to increase the driving force

ABC transporter

2 T ( transmembrane ) domain

6 - helix

form pathways for transported substance

2A ( ATP- binding domain)

30-40% homology for membranes

i.e. bacterial permease

use ATP hydrolysis

transport a.a ,sugars, vitamines, or peptides

inducible, depend on the environmental condition

i.e. mammalian ABC transporter ( Multi Drug Resistant)

export drug from cytosol to extracellular medium

mdr gene amplified by drus stimulation

mostly hydrophobic for MDR proteins

Structural model for E.coli flippase

6 - helix

Flippase model of transport by MDR1 and similar ABC proteins

Diseases linked with ABC proteins

1. ALD( X-link adrenoleukodestrophy)

defect in ABC transport protein( ABCD1)

located on peroxisome, used for transport for very long fatty acid

2. Tangiers disease

Dificiency in plasma ABCA1 proteins, which is used for transport of phospholipis and cholesterol

3. Cystic fibrosis

mutation of CTFR( cyctic fibrosis transmenbrane regulator; a Cl- transporter in the apical membrane of lung, sweat gland and pancrease)

Ion Channel

Generation of electrochemical gradient across plasma membrane

i.e. Ca+ gradient

regulation of signal transduction , muscle

contraction and triggers secretion of digestive

enzyme in to exocrine pancreastic cells

i.e. Na+ gradient

uptake of a.a , symport, antiport

Q: how does the electrochemical gradient

formed?

Selective movement of Ions Create a

transmembrane electric potential difference

Measuring the electrochemical gradient

Structure of resting K+channel from the bacterium Streptomyces lividans

Important for selection

Replacement of carboxyl backbone from P segment

Smaller Na+ does not fit perfectly

Oocyte expression assay is useful in comparing the function of normal and mutant forms of channel proteins

Cotransport:

Use the energy stored in Na+ or H+ electrochemical gradient to power the transport of another subatance

Symport: the transportd molecules and cotransported ion move in the same direction

Antiport: the transported molecules move in opposited direction

Operation Model for the two-Na+/one glucose symport

Glucose transport against its gradient in the epithelial cells of intestine

1 glucose in

2 Na+ inG=0

Na+ linked antiport Exports Ca+2 from cardiac Muscle Cells

3Na+ out+ Ca+2

in 3Na+ in+ Ca+2

out

maintenance of low cytosolic Ca+2 concentration

i.e. inhibition of Na+/K+ ATPase by Quabain and Digoxin

raises cytosolic Na+

lowers the efficiency of Na+/Ca+2 antiport

increases cytosolic Ca+2

( used in cogestive heart failure)

Cotransporters that regulate cytosolic pH

H2CO3 H+ + HCO-

H+ can be neutrolized by

1.Na+/HCO3-/Cl- antiport

2. Cabonic anhydrase

HCO3- CO2+OH-

3. Na+/H+ antiport

The activity of membrane transport proteins that regulate the cytosolic pH of mammalian cells changes with pH

Plant vacuole membrane

pH 3—6

Low acidity maintained by

V-class ATP-powered pump

PPi -powered pump

Concentration of ions and sucrose by the plant vacuole

Movement of water

Osmosis: movement of water across semipermeable

membrane

Osmotic pressure: hydrostatic pressure uses to stop

the net flow of water

Osmotic pressure

=RT( CB-CA)

Expression of aquaporin by frog oocytes increases their permeability

Aquaporin 1 erythrocyte

Aquaporin2 kidney cells

Water channel pprotein( aquaporin)

tetrameric

6 -helices for

each subunit

2-nm-long water

selective gate

0.28nm gate width

Highly conserved

arginine and histidine

in the gate

H2O for HO bonding

with cystein

Transepithelial transport

Import of molecules on the lumen side of intestinal epithelial cells and their export on the blood facing sides

Transcellular transport of glucose from the intestinal lumen into the blood

Cholera toxin activated by Cl-

Acidification of the stomach lumen by parietal cells in the gastric lining

Typical morphology of two types of mammalian neurons

100m/sec

Neurotransmitters Receptors

1. Ligand gated ion channels

2. G-protein coupled receptors

Synaptic vesicle:

Storage of neurotransmitter.

Low pH of vesicle lumen powers entry of neuritransmitter into lumen by H+/protein antipoter

Structures of small molecules function as neurotransmitters

Exocytosis of synaptic vesicle

1. Action potential

2. Influx of Ca+2 triggers release of neurotransmitter

H+/protein antiport

Cycling of nuerotransmitters and of synaptic vesicles in axon terminals

Signaling at synapse id terminated by degradation or reuptake of neurotransmitter

1. degradation

i.e. acetyocholine

hydrolyzed by acetyocholineaterase

2. reuptake

i.e.transport into axon terminals by Na+/linked

symport transporters for GABA, norepinephrine,

dopamine, and serotonin

Synaptic vesicles in the axon terminal near the region where neurotransmitter release

Sequential activation of gated ion channels at a neurotransmuscular junction

Incoming signals must reach the threshold potential to trigger an

action potential in post synaptic cells