Membranes CH339K. This has nothing to do with anything, but you all seemed appalled by the picture...
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Transcript of Membranes CH339K. This has nothing to do with anything, but you all seemed appalled by the picture...
Membranes
CH339K
This has nothing to do with anything, but you all seemed appalled by the picture of the New Zealand weta, I figured a really big arthropod was worth a slide.
Theraphosa blondi – the Goliath Bird-Eater of South America can reach a legspan of 11 inches and a weight of 6-7 ounces. Fangs up to 2 cm.
Membrane Proteins
Transmembrane Helices
• Thickness of hydrophobic layer ~30Å
• Rise of helix 1.5 Å/aa• ~20 aa helix to span
membrane• Example glycophorin
Transmembrane Helices (cont.)
• Several characteristic motifs for helical membrane proteins
Bacteriorhodopsin
Purple membrane of halobacteriaLight-powered proton pumpTrimer – each monomer (above) contains 7 helices + retinal
Action of retinal (for anyone who cares)
Inside Outsidehν
H+
Hydrophobicity Plots
Beta Barrels
OmpG Protein of Gram-negative Bacteria• Adjacent b-sheets can also form membrane-spanning proteins• Common in bacteria, mitochondria, and chloroplasts• “Omp” = Outer membrane protein• 14-stranded -barrel• 34 kDaltons• Allows mono-, di-, and trisaccharides through outer membrane
Glycosylphosphatidylinisotol (GPI) anchored proteins
functionality ranges from enzymatic to antigenic and adhesion
What HAS to cross the membrane
(1) Water*, gases (O2, CO2), inorganic ions (Na+ , K+ , Cl- , Mg2+ , PO4
-3 , Ca++ , HCO3-,
NH3 / NH4+)
(2) small molecular weight organic molecules (sugars, amino acids, small lipids and lipid precursors, urea)
(3) large organic molecules and cell fragments
What DOES cross?
1. Impermeable to charged (inorganic ions, charged organic molecules etc); interior of membrane is very hydrophobic and resists penetration by charged atoms and molecules
2. Permeable to lipid-soluble (non-polar) molecules; permeability decreases as size of molecule increases (here we mean molecular volume).
3. Generally permeable to small slightly polar but uncharged molecules like water and dissolved gases (O2, CO2, CH3CH2OH); these molecules are literally small enough to pass beween phospholipid molecules.
4. Impermeable to proteins, carbohydrates, nucleotides, any “molecules of size.”
For the more visually oriented
Rate of movement depends on 2 factors
Partition coefficient Diffusion coefficient
Rates and Permeability Constants
where:P = permeability coefficient (cm-sec-1)K = partition coefficient (dimensionless)D = diffusion coefficient (cm2-sec-1)r = membrane thickness (cm)
The actual rate of transfer across the membrane then becomes:
J = -P*A*C
where:J = rate of passive diffusionP = permeability coefficient for the substance in questionA = surface area of the membraneC = difference in substance concentrations, [destination] - [source]
(usually in mol-cm-3.)
r
KD P
(Fick’s Law)
Correlation of K and P
Permeability varies with
temperature
D is temperature dependent, and thus so is P
Plot is the change in P for glycerol with T for two kinds of mouse sperm.
From Biology of Reproduction October 1, 1999 vol. 61 no. 4 1031-1041
Permeability versus Diffusion Coefficient
Correlation between permeability and diffusion coefficients of selected aliphatic and aromatic constituents of JP-8 jet fuel.From: Singh, S. and Singh, J. (2003) Percutaneous absorption, biophysical, and macroscopic barrier properties of porcine skin exposed to major components of JP-8 jet fuel, Env, Tox. Pharm. 14: 77-85.
P for some molecules of interest
Should be 1(you get pictures from the internet, you take your chances)
Sugar permeabilities
Membrane Preparation D-Glucose D-Mannitol
Synthetic Lipid Bilayer 2.4 x 10-10 4.4 x 10-11
Calculated Passive Diffusion 4 x 10-9 3 x 10-9
Intact Human Erythrocyte (red blood cell)
2.0 x 10-4 5 x 10-9
Anomalous Point!!! – Something must be letting glucose through the membrane!
Facilitated DiffusionIonophores– Valinomycin (K+ carrier) from Streptomyces
Facilitated Diffusion - Channels and Transporters (Permeases)
Facilitated diffusion is saturable
Kinetics of Facilitated Diffusion
Channels
Gramicidin from Bacillus brevis• 15 amino acids• Passage for monovalent cations
Gramicidin channel formation.
Lundbæk J A et al. J. R. Soc. Interface 2010;7:373-395
©2010 by The Royal Society
Channels
•-latrotoxin from Latrodectes mactans • channel for cations through presynaptic membrane• triggers neurotransmitter release• 130 kDal, forms tetramers
Pore Electron Micrographs
From: Zh. I. Andreeva-Kovalevskaya, A. S. Solonin, E. V. Sineva, and V. I. Ternovsky (2008) Pore-Forming Proteins and Adaptation of Living Organisms to Environmental Conditions, Biochemistry (Moscow)73: 1473-1492.
Channels
Streptococcus Hemolysin A (heptamer): inserts into susceptible cell membranes, causing cell death by leakage (note -barrel)
A Uniporter
Erythrocyte Glucose Transporter (A permease)
Two states:T1 = open to the outsideT2 = open to the intside
Glucose binding lowers G‡ for T1⇌T2 conversion
An Antiporter – ADP/ATP Exchanger(inner mitochondrial membrane)
Active Transport
Na+-K+ Transporter
Pumps 3 Na+ outPumps 2 K+ inBurns 1 ATP per cycle
Symport (and Secondary or Indirect Active Transport)
Secondary Active Transport- Galactoside Permease of bacteria- Uses extracellular H+ concentration to drive import of lactose
Membrane potentials
Membrane potentials
ZFC
CRTG
1
2ln
What happens when there’s a charge across the membrane?
• R = Gas constant• T = Absolute temperature• Keq = Equilibrium constant• Z = Charge on the ion• F = Faraday’s constant• = charge across the membrane
By convention:• If inside is positive with respect to outside, then > 0• If outside is positive with respect to inside, then < 0
Relationship of and Concentration
2
1lnor
1
2ln
:for Solving1
2ln
:so 0,G m,equilibriuAt 1
2ln
C
C
ZF
RT
C
C
ZF
RT
C
CRTZF
ZFC
CRTG
Nernst Equation
out
in
C
C
zF
RTln
Where:ΔΨ = Membrane Potential (V)R = Gas Constant (Jmol-1K-1)T = Temperature (K)z = Charge on the ionF = Faraday’s Constant (JV-1mol-1)
Goldman Equation
OutOutInIn
InInOutOut
CPCP
CPCP
F
RTln
Where:
ΔΨ = Membrane Potential (V)R = Gas Constant (Jmol-1K-1)T = Temperature (K)F = Faraday’s Constant (JV-1mol-1)P = Permeability (cm-sec-1)
Example – Giant nerve cells of Aplysia
Ion Cin (mM) Cout (mM) P (relative)
K+ 280 10 1
Na+ 61 485 0.12
Cl-1 51 485 0.44
ΔΨ ≈ -45 mV
)485)(.44(.)061)(.12(.)280)(.1(
)051)(.44(.)485)(.12(.)01)(.1(ln
F
RT
Nerve Cell TransmissionVoltage Gated Channels Pore
Voltage Sensors
Voltage sensors change conformation, open and close pore
Nerve Cell TransmissionVoltage Gated Channels
ActionPotential
1) Initial depolarization causes the Na channels to open
2) Na becomes controlling ion3) Potential goes positive4) Na channels close as K channels
open5) K becomes controlling6) Potential goes negative and
overshoots7) K channels close8) Potential returns to normal9) Na/K exchanger restores
concentrations
Action Potential
Scorpion -toxins
Bind to voltage-gated sodium channelsInhibit closure of the channelProlonged action potential disrupts nervous system
Leiurus quinquestriatus – death stalker (North Africa to Western India)
Impact of Scorpion Toxin on Sodium Channel Closure
Tohru Gonoi, Bertil Hille, and William A. Catterall (1984) Voltage clamp analysis of sodium channels in normal and scorpion toxin-resistant neuroblastoma cells, J. Neurosci. 4, 2836 - 2842
Cardiac Glycosides
Foxglove (Digitalis sp.)
The Bad:
Irregular and slow pulse, tremors, various cerebral disturbances, unusual color vision with objects appearing yellowish to green, and blue halos around lights, convulsions, and deadly disturbances of the heart.
The Good:
Positive inotropic* effect; also used in reentrant cardiac arrhythmias and to slow the ventricular rate during atrial fibrillation.
And it’s pretty.
* inotropic: altering the energy of muscle contraction
Cardiac glycoside action
1. Digitalis blocks the Na+/K+ exchanger
2. Intracellular Na+ rises
3. Na+/Ca++ exchanger goes into reverse
4. Intracellular Ca++ rises, stimulating heart muscle (positive inotropic effect)
Cystic FibrosisMost common genetic disease of Europeans.1 in 25 carry the alleleMembrane protein – chloride ion permease.
Cystic FibrosisHigh intracellular ion concentration pulls water from outside the cell.
Thick, sticky mucus; frequent chest infections, shortness of breath, poor nutrient absorption, male infertility, and death (avg age 37 in 2008)
Molecular Formula C129H223N3O54
LD50 <100 ng/kg
Source Palythoa toxica (soft coral)Limu make o hana (Seaweed of Death from Hana)
Palytoxin
Deeds JR, Handy SM, White KD, Reimer JD (2011) Palytoxin Found in Palythoa sp. Zoanthids (Anthozoa, Hexacorallia) Sold in the Home Aquarium Trade. PLoS ONE 6(4): e18235. doi:10.1371/journal.pone.0018235
Maitotoxin (new winner of most toxic molecule)
Molecular Formula C164H256O68S2Na2
LD50 ~50 ng/kg
Source Gambierdiscus toxicus (dinoflagellate)
Effect Activates Ca+2 channels, leading to cell death through complex pathways