lec 6 memb structure/function

what are 3 classes of membrane proteins

1. Integral proteins: penetrate/pass thru lipid bilayer (30% of all encoded proteins, 60% of drug targets

2. Peripheral proteins: located OUTSIDE bilayer on ecm or cyto side with non-covalent bonds (easily solubilized)

3. Lipid-Anchored membrane proteins: located OUTSIDE bilayer on ecm or cyto side but COVALENTLY linked to a membrane lipid

are integral memb proteins hydrophobic, phillic, or amphipathic

amphipathic

list 4 main functions of integral proteins

1. TRANSPORTER (ions/solutes + electrons during photosynth/resp)

2. ANCHOR

3. RECEPTOR

4. ENZYME

why do integral proteins (pass thru lipid bilayer) interact closely with memb lipids?

their hydrophobic transmembrane domains form VAN DER WAALS with fatty acyl chains of bilayer, creating SEAL between protein/lipid, preserving PERMEABILITY BARRIER OF MEMBRANE

what is a common secondary structure in transmembrane protein that spans a membrane? what is it held in place by

-hydrophobic (nonpolar) a-heliz

-held by:

-van der waals between hydrophobic aa side chains w/ lipids

-ionic interactions with polar head group of lipids

what is hydropathy plot and hydropathy index

hydropathy plot: measures hydrophobicity of amino acids (determined by lipid solubility OR energy required to transfer them from nonpolar into aq medium)

uses hydropathy index: average based on hydrophobicity values for each aa (hydrophobic = positive, hydrophillic = negative)

what 2 things determine hydrophobicity of aa

-lipid solubility

-energy required to transfer them from nonpolar to aqeuous enviro

in Bacteriorhodopsin, what connects the 7 a-helices? how do a-helices keep protein anchored to membrane? how are the helices oriented and what pathway do they provide?

-helices connected by non-helical loops (at inner+outer face of memb)

-oriented in CLUSTER and provide transmembrane proton pathway

-protein anchored to memb by: hydrophobic interactions between nonpolar aa’s (of protein) and fatty acyl group (of lipid membrane)

whats another type of membrane spanning domain other than a helices? what does it come from?what amino acids line inner pore? where is it found? what can pass thru?

B-barrels (come from various B-strands)

-found on outer memb of gram negative bacteria and (like a helices: mitochondria and chloroplasts)

-pores allow passage of SMALL hydrophillic polar molec

-pores lined with hydrophillic aa

what are peripheral membrane proteins and what do they not interact with? how do they interact with membrane?

peripheral proteins: outside bilayer with non-cov bonds

-do NOT interact with hydrophobic core

-do interact with memb thru interactions with integral memb proteins OR lipids polar head groups

which membrane proteins are easily removed from membrane, and by what changes?

peripheral membrane proteins (weak non-covalent bonds)

-changes in pH and ionic strength

whats the third type of transmembrane protein

Lipid-Anchored membrane proteins (covalently anchored to outside of membrane)

what are the 2 diff ways lipid-anchored proteins can attach to membrane

1. Cytosolic side via covalently attached fatty acid chain (acylation) or prenyl group (prenylation)

2. non-Cytosolic side via oligosaccaride linker to phosphatidylinositol (PI) (called glycosylphosphatidylinositol (GPI) anchor) (in lumen of ER)

more detail, what are the 3 types of lipid-anchored proteins? are these synthesized in cytosol or ER? what groups are they attached to

1. Fatty acid-anchored memb protein (cytosol) (attached to SATURATED (single bonds) fatty acid (usually myristic acid 14C or palmitic acid 16C thats embedded in memb)

2. Isoprenylated memb proteins (cytosol) (attached to multiple isoprenyl groups 5C (usually farnesyl 15C or geranygeranyl 20C groups) then enbedded in memb)

3. GPI-anchored memb proteins (ER) (GPI post-transationally attached to C-terminus and can be release from memb via cleavage by phospholipase C)

ex. of GPI protein is a Prion protein (scrapie, mad-cow disease)

what does this mean: most plasma membrane proteins are glycosylated

glycosylated = carb added (this carb faces the exoplasmic domain)

why are integral membranes hard to isolate and what helps them

-due to hydrophobic region interact with memb hydrophobic tail = aggregate

-amphipathic detergents solubilize, purify, and reconstitute functional membrane protein systems

what model is called the “centra dogma” of membrane biology? what does it decsribe

Fluid-Mosaic Model : describes how membranes consist of a MOSAIC of proteins/lipids in a FLUIDIC state

why is membrane fluidity importat?

membrane functions rely on this: vesicle formation/fusion/secretion, cell division, muscle contraction, cell migration, signaling mechanisms

what is a transition temperature Tm and what is the phase transition? what happens below the Tm?

-Tm transition temp: temp a lipid bilayer becomes FLUID

-phase transition: this change of state from solid to liquid

-below Tm, functions that rely on memb fluidity are disrupted

what 3 things does transition temperature Tm depend on?

-lipid composition

-length of fatty acid tails (shorter=more fluid=lower Tm)

-saturation of fatty acids (unsaturated fa=double bonds=bends that prevent tight packing=more fluid=lower Tm)

note lower Tm = lower melting temperature = membrane is in liquid state at lower temp

what percent of lipid in memb of animals is sterols? whats the main sterol in animal cell membranes and whys it important

30-40% lipid is sterol, and main sterol is CHOLESTEROL (stabilize membrane)

what layer of memb is cholesterol found in? how is it oriented with respect to its hydroxyl group

-cholestrol in BOTH layers of memb

-hydroxyl group close to POLAR HEAD groups of phospholipids

cholesterol 2 roles in membranes

1. Decrease Permeability (of membranes to ions+small polar molec)

2. Fluidity Buffer (broadens temperature range of transition)

how does cholesterol decrease permeability and alter fluidity of lipid bilayers?

-DECREASED PERMEBILITY: cholesterol fills spaces between hydrocarbon chains of phospholipids and blocks routes that ions/small molecs could go thru

-FLUIDITY:

-with unsaturated fa: cholesterol DECREASE fluidity (more packed)

-with saturated fa: cholesterol INCREASE fluidity (less packed, provides space between phospholipids, and no bends, so theres room to flow thru)

how do enzymes help maintain membrane fluidity

enzymes (saturase, desaturase, transferase) ALTER FATTY ACIDS in response to need (external temp change)

other than enzymes, how else can membrane fluidity stay constant

cells can regulate their LIPID composition (fatty acid abundance) and also length/saturation of fatty acids vary

what are lipid rafts and what are their functions

LIPID RAFTS: dynamic structures (change composition as lipids/proteins move in/out of them), localized regions of membrane LIPIDS in association with specific proteins (aka lipid microdomains)

FUNCTIONS:

-detect/respond to EXTRACELLULAR SIGNALS

-lipid rafts in plasma memb of animal cells have ELEVATED LEVELS OF CHOLESTEROL + SPHINGOLIPIDS (longer/more saturated fa tails)

whats lipid raft size and fluidity compared to rest of membrane

1-2 nm thicker and less fluid (tight packing)

describ ehow phospholipids can move in monolayer? whats the enzyme that can move phospholipids from one leaf to another and what is this essential to

-rotate

-LATERAL DIFFUSION (fast movement within monolayer)

-both those types are rapid and random

-flip-flop (SLOW, hydrohillic head must pass thru interal hydrophobic sheet of memb)

-Flippases are enzymes that move phospholipifs from leaf to leaf (critical for membrane asymmetry)

what did FRAP studies show? what does FRAP stand for?

-proteins move SLOWER in living cell memb than pure lipid bilayres

-movility of many proteins was limited (cant diffuse back into bleached area)

FRAP: Fluorescence recovery after photobleaching

what is erythrocyte (rbc) membrane skeleton made of? what does this help with

Spectrin (PERIPHERAL protein) → Anykrin (PERIPHERAL protein) → Band 3 (INTEGRAL protein)

-these proteins create 2D hexagonal arrays, critical to integrity of memb (provide strength, elasticity, pliability to pass thru blood vessels)

what are 6 levels of mobility that membrane proteins can have

1. RANDOM movement

2. IMMOBILE (intracellular tethering)

3. REDUCED movement

4. FENCED

5. DIRECTED movement

6. Extracellular Entanglement

what are the names of the “membrane skeletons” that can exert significant effect on memb protein MOBILITY

CELL CORTEX (fibrillar netorks on cytoplasmic surface of memb)