Chapter 10Membrane Transport

Biochemistry IDr. Loren Williams

Revised 03/11/2013

Thermodynamics of Transport

Mammalian Cell

inside 12 mM Na+

140 mM K+

outside150 mM Na+

4 mM K+

Passive TransportMovement of molecules across membranes driven by electochemical potentialMechanism is either simple diffusion or facilitated (mediated) diffusionNot thermodynamically linked to other processesNo ATP hydrolysis

Mediation of transport across membranesrequired for charged, polar or large molecules

1) Ionophoreslipid-soluble molecules synthesized by microorganisms facilitate transport of ions across membrane.

Valinomycin a potassium-specific passive transporter a dodecadepsipeptide antibioticproduced by several Streptomyces strains highly selective for potassium ions over sodium and other

ionsKd for potassium is 106

Kd for sodium is 10.facilitates transport of K+ down the electrochemical

potential gradient.Deadly

Figure 10-1

Sodium (Na+)Oxidation state: +1Ionic radius: 0.95 ÅPreferred ligands: ONumber of ligands: 6-7Preferred geometry: octahedral (not a strong preference)

Potassium (K+)Oxidation state: +1Ionic radius: 1.33 ÅPreferred ligands: ONumber of ligands: 4-7Preferred geometry: variable, octahedral

Calcium (Ca2+)Oxidation state: +2Ionic radius: 0.99 ÅPreferred ligands: ONumber of ligands: 6-10Preferred geometry: variable

Passive Transport

2) Ion Channelsintegral membrane proteins found in the membranes of all cells (necessary to keep cells from

exploding)highly selective for specific ions (K+ vs Na+ vs Ca2+ etc)fast: rate of transport is close to the diffusion limit very tightly regulatedgated: flow of ions across the cell membrane is turned off or no in

response to stimuliSensors: pH, ligands, voltage, etcK+ channels and anion channels hyperpolarize cells (cause

the membrane potential to become more negative), Na+ and Ca2+ channels and non-selective cation channels depolarize

cells (cause the membrane potential to become more positive).

Ion channels form pores that permit the flux of ions down their electrochemical gradient.

Mammalian Cell

outside (high Na+)150 mM Na+

4 mM K+

inside (high K+) 12 mM Na+

140 mM K+

Ion channels form pores that permit the flux of ions down their electrochemical gradient.

KcsA K+ channel

KcsA K+ Ion Channel10,000-fold selectivity of K+ over Na+

homo tetramerfour identical protein subunits

two transmembrane helices, central pore

three parts: a selectivity filter

(extracellular side), a dilated water-filled cavity

(center),gate

(cytoplasmic side, proton-activated,

opens at acidic pH)

How does ion selectivity work? How is K+ distinguished from Na+?

Ion Dehydration: disrupts favorable molecular interactions (DH>0)Ion Coordination: forms favorable molecular interactions (DH<0)

To enter the selectivity channel the ions must dehydrate. The enthalpy of ion coordination by the selectivity channel has to offset the unfavorable dehydration enthalpy.

Na+ is smaller (ionic radius 0.95 Å) than K+ (ionic radius 1.33 Å). The coordination geometry in the selectivity channel is bad for Na+: The O-Na+ distances are too long. The O-K+ distances are just right.

Optimum O - Na+ distance = 2.4 Å (0.95 + 1.5)Optimum O - K+ distance = 2.8 Å (1.33 + 1.5)

K+ is about the same size as water, Na+ is smaller than water.

Free energy landscape for K+ is featureless throughout the channel.

Channels are gated (they open and close)

Mechanics (touch, sound, etc): Channels open and close in response to membrane deformation.

Ligands (neurotransmitters…): Channels open and close in response to ligand binding

Signals (Ca2+…): Channels open and close in response to Ca2+ binding

Voltage (changes in membrane potential): Channels open and close in response to Ca2+ binding

Voltage Gating (Kv Channel): Gate # 1S4 helix (+++++ charged)

At the resting potential (-60mV), the gate is closed. Depolarization moves S4 toward the outside (extracelluar side) of the membrane, and opens the channel.

Voltage Gating (Kv Channel): Gate # 2inactivation ball

Closes the channel a few msec after the S4 helix opens it. The channel does not reopen until the potential is reset,

Figure 10-9

Outside

Inside

openclosed

Figure 10-9a

Outside

Figure 10-9b,c

Action Potential

o stimulation of a neurono open Na+ channelo depolarizeo close Na+ channelo open K+ channelo hyperpolarizeo close K+ channel

(refractory)o Slow reset to resting

potential by ionpumps

10 m/sec

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Glucose Transporter: has two exclusive conformational statesis specific for glucoseis driven by chemical potential (concentration)is the basis of selectivity is indicatedis similar to systems of amino acid transport, etc

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The Na+/K+ pump (ATPase)

maintains resting potentialregulates cell volume,signal transducer/integrator

20% of cell's energy expenditure.50-70% cell's energy expenditure for neurons

Pumps 3 Na+ out for every 2 K+ in (hydrolyzes 1 ATP in the process).

o unphosphorylated pump binds 3 intracellular Na+ and an ATP,o phosphate is transfered from ATP to aspartate of the pump, release of ADP o conformational change exposes the Na+ ions to the outside, all Na+ are released outside o pump binds 2 extracellular K+ ions. o dephosphorylation of the pump, o both K+ are released on the insideo cycle completed

start

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