Lipid Membranes and Drug Targets Flashcards
Hydrophobic Effect
Effect of Oil-drop in Water
- Non-polar molecules stick together in polar media (water)
- in a way to MAXIMIZE # of POLAR interactions
- =EXOERGIC
- in a way to MAXIMIZE # of POLAR interactions
Free Energy Aspects
of Amphiphile Aggregation
- Ordered Water molecules that have fewer interactions with other molecules @ surface of Nonpolar solute
- –> are squeezed out
-
Formation of lipid aggregate (ampiphatic complexes)
-
Favorable both enthapically & Entropically
- VDW
- Enthalpic - interaction amoung water
- Entropic - water ordering
-
Favorable both enthapically & Entropically
Phospholipid Aggregates
Different Forms
-
Micelle / Hexoganal Phase
-
individual units are WEDGE-shape
- head + 1 tail
-
individual units are WEDGE-shape
-
Bilayer
-
indiv units are CYLINDRICAL
- head + 2 tails
-
indiv units are CYLINDRICAL
-
Liposome
- aqueous cavity
Critical Micelle Concentration
CMC
- Specific CONC at which a monomeric amphiphile begins to form micelles
- typically 10-10M for membrane phospholipids
-
HIGHER CMC Values caused by:
- Shorter chains
- Greater negative charge
Phospholipid Structure
-
Polar HydroPHILIC head
- Choline - Phosphate - Glycerol =
-
Non-polar hydroPHOBIC tail
- = 2 hydrocarbon tails
Sphingolipid
*not found in bacteria
- A Polar Membrane Lipid
- a Glycolipid
- Have mostly saturated fatty acid chains
- –> TIGHTER packing in bilayers
- Presence of HB amide bonds –> more rigid
- Consist of:
- Sphingosine + FA + Mono/Oligosaccharide
Glycerophospholipids
-
Glycerol
- 2 FA’s + (PO + Alcohol)
- Not very rigid compared to sphingolipids
- more UNSATURATED = kinks
- BLOOD GROUP ANTIGENS
Galactolipids (Sulfolipids)
- Polar Membrane Lipid - Glycolipid
-
Glycerol
- 2 FA + (Mono/disaccharide + SO4)
Triacylglycerols
- neutral Storage Lipid
-
Glycerol
- 3 FA’s
Blood Groups differ by presense & Type of what?
Glycosyl Transferase
- Blood group antigens are GLycosphingolipids
Cholesterol
-
Eukaryotic membrains contain Cholesterol
- *not found in bacteria
- Orientated w/ -OH facing the aqeuos phase
-
Polycyclic Structure
- –> imparts RIGIDITY to membranes w/ a lot of cholesterol
- Found most in Plasma Membrane
PhosphatidylCHOLINE (PC)
-
found only in prokaryotes
- mainly in Mito
- Inner Membranes
- mainly in Mito
-
distributed @ the OUTER MONOLAYER
- along with sphingomyelin
Cardiolipin
- Lipid found mostly in MITOCHONDRIA
PhosphatidylEthanolamine (PE)
- PE found in both prokaryotes & eukaryotes
- mainly in Mitochondria
-
Negatively charged phospholipid
- located @ INNER leaflet of bilayer
-
involved in RECRUITMENT of POSitive signaling proteins
- to the membrane surface
-
involved in RECRUITMENT of POSitive signaling proteins
- located @ INNER leaflet of bilayer
Flippase
-
Catalyzes TRANSVERSE DIFFUSION
- “FLIP-FLOP” of phospholipid
- from facing outer membrane to inner membrane
- very slow –> very fast
-
Lateral diffusion occurs very fast UNCATALYZED
- also rotation along molecular axis is fast
Liquid-Crystalline Phase
(fluid state)
-
Biological membranes MUST STAY liquid-crystalline
-
TO BE FUNCTIONAL
- Need to be ABOVE transition temperature
-
TO BE FUNCTIONAL
-
if below transition temperature
-
–> GEL STATE (non functional)
- aka paracrystalline state
-
–> GEL STATE (non functional)
How do bacteria adapt to lower temperatures?
- they use more unsaturated fatty acids
- like OLEIC ACID (unsaturated = MORE BENT)
- Enable bacteria membrant to REMAIN liquid crystalline
Traits of Phospholipids with
More Unsaturated Bonds
-
Thinner Bilayers compared to saturated chains
- Cis-double bonds are shorter
- Shorter & Occupy LARGER SURFACE AREA
Passive transport is fastest for molecules with moderate hydrophobicity. Why?
We want the molecule to readily enter the membrane but we also do not want it to leave so quickly
Membranes are refractory to polar molecules.
Why?
Phosphoplipid membrane has polar heads
polarity will block molecule from entering
Topology of membrane proteins can be predicted from hydropathy indices.
What’s that?
2 Dimentional Liquid
or
Liquid Crystalline State
- Rapid conformational & rotational changes in:
- ALKANE chains & head group orientation
- Planar bilayer is maintained
- NO LATERAL Movement observed within 5Nanoseconds**
Uncatalyzed lateral Diffusion
- A motion in liquid-crystalline bilayers
- LATERAL
- occurs very FAST
- because the forces VDW are very weak
Uncatalyzed Transverse Diffusion
“flip-flop”
- Motion in liquid-crystalline bilayer
-
occurs very SLOW
- during transit, e- charged head group must be stripped of solvating water molecules
- *there is also fast rotation along the molecular axis
Glycophorin A
Integral Membrane Protein
- Contains a Single Helical transmembrane segment
- hydrophobic AA side chains allow the segment to sit between the phospholipid bilayer
-
Heavily glycosilated @ extracellular part
- w/ NEGATIVELY charged sialic acid
- –> ensure that eruthrocyte do not adhere to walls of blood vessels
- w/ NEGATIVELY charged sialic acid
Membrane Pore / Channel
Integral Protein
- Formed by assemblies of either a-helical / b-sheeted proteins
- Responsible for:
- signaling
- ligand gated channels
- voltage gated ion channels
- membrane transporters
Bacteriorhodopsin
Integral Protein
-
Seven-Pass Membrane receptor
- important for cell signaling
- GPCR = great target for drug design
- regulate processes from sensing to neuronal transmission
- important for cell signaling
-
Proton Pump
- allow H+ to leave cell
Hydropathy Indices
- number representing the hydrophobic or hydrophilic properties of a AA sidechain
- GLU / ASP = most polar
- PHE / TRP = most hydrophobic
-
Used to PREDICT TRANSMEMRANE DOMAINS
- Bacteriorhodopsin = 7 hydrophobic domains
- Glycophorin = 1 hydrophobic domain
Membrane Permeability:
Hydrophobic Molecules
- Easily cross the membrane
- O2 / CO2 / N2 / Benzene
Membrane Permeability:
Small Uncharged POLAR Molecules
- Membranes are permeable to small noncharged molecules
- Ex. Water, Urea, Glycerol
- Must have LOW POLARITY
- Ex. Water, Urea, Glycerol
-
800 Daltons = Upper MW limit to cross cell membrane passively
- those greater than this need a transporter system
- low MW is a necessary but insufficient condition of BV
Membrane Permeability:
IONS
- CHARGED molecules can NOT cross bilayer
- especially NEGATIVE
Simple Diffusion
Non-Polar Compounds Only
Down Concentration Gradient
- <800 daltons
- Low MW is necessary but insufficient condition of BV
Fascilitated Diffusion
Down Electrochemical gradient
uses a transporter
Primary Active Transport
AGAINST electrochemical gradient
USES ENERGY
Secondary Active Transport
SYMPORTER
AGAINST electrochemical gradient
Driven by ION moving down the gradient
both molecules move in SAME direction
antiporter = two molecules in opposite directions
Ion Channel
Down Electrochemical gradient
may be gated by a ligand or ion
Ionophore-mediated ion transport
Down electrochemical gradient
Membrane Bound GLUCOSE transporter
- HydroPHILIC / phobic properties of a-helices
-
polar surface of the channel pore
- Asn / Ser / Thr
- interact w/ polar -oh groups of glucose
-
polar surface of the channel pore
Selective Serotonin Reuptake Inhibitors
SSRI
- Key in antidepressants ~ Paroxetine
- Block Serotonin transporter w/ Ki =0.34 nm
-
Toggles between outward / inward facing conformations
- Outward = open extracellularly
- Inward = open to cytosol
-
Symporter
- hydrogen and ligand go in the SAME direction
Drug Transport Predictors
- Factors that help determine Bioavailability
- Octonol / Water Partioning = logP
- Artifical Membranes
-
Cell Culture Systems - Cell Monolayer
- passive transport
- Cell assisted transport
- transcellular / paracellular (inbetween)
- Carrier mediated transport
- Transport proteins
Lipid Partitioning Vs Membrane Permeation
logD = 1-2 is MOST ideal, HIGHEST FLUX
(pH 7.4)
- flux = mass transfer via artifical membrane %
-
bimodal relationship
- lipophilicity vs mass transfer
- Very lipoPHILIC drugs can penetrate membrane
- but CAN NOT LEAVE
- Very hydroPHILIC drugs cannot get solubized in membrane
Lipid Partitioning vs Membrane Permeation
k1 & k2
-
k1 = rate constant of transport from:
-
Aqueous –> liquid phase
- INCREASES as logD Heightens
-
Aqueous –> liquid phase
-
k2 = REVERSE of K1
- DECREASES as logD Heightens
Passive Vs Carrier-Mediated Transport
-
Carrier Mediated Transport = LIMITED by transporter
- Levels off at Vmax
- high km = weak binding
- when concentration = half maximum velocity
- Overall Transport is a sum of the two components
Passive Transport
-
Linearly dependent on concentration
- Non-saturable, unlike carrier mediated transport
- NOT SUBJECT TO INHIBITION
- Less structure specific than carrier mediated
- general dependence on LIPOPHILICITY
- Less cell-type specific than carrier mediated
Carrier-Mediated Transport
-
Non-Linearly dependent on concentration
- SATURABLE
- SUBJECT TO INHIBITION
- MORE structure specific
- dependence on lipophilicity could be identified in narrow chemical series
-
Cell type specific
- requires expression of transporter
Efflux Pumps
-
can Work AGAINST the gradient (ATP)
-
pump drug out of the cell
- cytosol –> out of cell
-
pump drug out of the cell
- _Blocking efflux pump is a strateg_y:
- for maintaining higher drug conc. after administration
