week 9 Flashcards
what are the basic components of a biomembrane
lipids, sterols and proteins
why do phospholipids spontaneously form lipid bilayers in aqueous soultion
- due to amphipathicity
fatty acid
- long HC chain attached to a polar carboxyl head group
- amphippathic
- Cx:y
what is meant by a saturated fatty acid
no double bodns
unsaturated gatty acid
- one double bond
polyunsaturated fatty acid
- more than one double bond
how is melting point affected by chain legnth and saturation
- increases with longer chain lengths
- decreases with increasing unstaruation (more double bonds)
what fatty acids are used as components of membrane lipids
-phospholipids, phosopglycerides, sphingolipids, and sterols
should membranes be a fat or an oil?
neither. we need a semi-fluid membrane at 37 degrees
what do sterols do
- they fit themselves in between fatty acid chains, and can intercalate between fatty acids to alter melting temperature (an example of a sterol is cholesterol)
what are the properties of biomembranes
- they are fluid, closed compartments, semi-permeable, and assymetric
what are the characteristics of a fluid membrnae
- they are two dimensional
- fluidity is composition dependent
two dimensional fluids
- undergo rapid lateral diffusion
- rarely, there is transverse flip-flop movement between leaflets; usually movement is restricted to one leaflet
composition-dependent fluidity
- longer chais = less fluid
- more double bonds = more fluid (only cis double ones??)
- steroids (affects how much things can move around,)
- proteins (large structural things that are in teh way can alter fluidity and can either be tethered to the cytoskeletion or can influence lateral mobility as the proteins interfere with things moving around in the membrane)
- increasing temperature can result in more fluiditiy
- are cis double bonds the only ones that affect fluidity
- how do sterols actually affect melting temperatures
- how do proteins affect fluiditiy
- do all types of lip-linkages involve lateral mobility?
by what mechanisms do cells change membrane fluidity
through changes to fatty acids, and then sterols
FRAP
- used to measure lipi/protein movement
- fluorescence recovery after photo bleaching
how does FRAP work
- proteins surrounding the membrane are fluorescently labelled, and
- measure fluorescence before bleaching
- bleach that area and take another reading
- see that there is a much lower level of fluorescence
- measure the recovery of flureosence,w
what does FRAP tell us
- is indicative of the mobility of the plasma membrane
- diffusion is ten times slower in PM with proteins, as distance moved is restricted by the proteins
PM as a closed compartment
- the membrane goes all the way around the cell and has two faces
what are the two faces of the pM
- the cytosolic and exoplasmic
what is the cytosolic face for a PM
- internal face
what is the systolic face for a vesicle membrane
- external face
membrane oritentaion
- any membrane inside teh cel maintains its orientation
semi-permeable membranes
- small, uncharged or hydrophobic molecules pass freely (e.g. gases, water) while things like ions or large complex mols (large, hydrophilic or charged) are not able to pass
how are membranes considered assymetrical
- phospholipid ocmpositon differs between leaflets
- carbs only on exoplamsic fac
- proteins are either embedded in bilayer in fixed orientation or are associated with one side only (they are always in one orientation)
what carries out the biological functions of mmembranes
- membrane proteins
- phospholipids are only really used to form semi-permeable closed compartments
what ar the three types of membrane protins
- integral, lipid-linked, peripheral
integral membrane proteins
- several domains of the proteins are integrated into the membrane in the phobic region
lipid linked membrane proteins
- linked to the exoplasmic or cytosolic side of the lipid bilayerr (either side)
peripheral
- are bound to other proteins that are bound to membranes (eg linked either to lipid linked or to integral membrane proteins
integral further
- asymmetric with three distinct domains
- cytoplasmic (hydrophilic region, includes amino acids such as arg and lys- charged that are located near the cytosolic side to interact with the poler head groups
- transmembrane region is hydrophobic, with phobic secondary and territory structures that span the lipid bilayer, and are usually alpha helices or beta abrraels
- exoplasmic portion is hydrophilic, most are glycosylated
glcysoylation (e..g exopalsmic portion of integral membrane proteins)
- involves the adhesion of sugars that are charged polar groups, providing charges, and there are also sugars to prevent the protein from sliding into the inside
lipid linked proteins further
- GPI achor is used to anchor proteins to membrane by lipohilic adduct, protein is linked to the lipid bilayer on the exterior side and than that protein can diffuse laterally within the lipid bilayer on the exoplasmic side, involves sugar residues to line to the phosphatidylinositol
- acylation of fly residue of cytosolic protein
- prenylation of cys residue of protein
aacylation
- attaches through N terminal gly residue on cytosolic side
- involves N terminal lcyein, not entering the membrane
prenylation
- involves carboy terminal cysteine, prenylate the protein to cytosolic lipid
acylation and prenylation are always on…
cytoslic side
GPI anchor is always on….
exterior side
how are GPI anchor connections created
u have a phophatidylinositol linked to sugar resides that are linked to phospoethanolamine and then to the peptide C terminal?
peripheral proteins further
- are attached through non covalent interactions such as ionic interactions, h bonds , protein-protein interactions and van Der Waals
- on exterior and cytosolic sides, and cytoskeletal filaments can nteract with bilayer through peripheral membrane proteins, and so can extracellular matrix components when the peripheral proteins are located on the exterior sid
what are some examples of proteins that can link to the cytoskeleton through peripheral proteins
- cadherins and integrins
what are the true membrane proteins
- integral and lipid linked
topogenic sequences involved in insertion of proteins into membranes
- N terminal signal sequence
- stop-transfer/membrane anchor sequence (STA)
- signal-anchor- internal (uncleaned sequence)- SA
- hydrophobic C terminus
N terminal signal sequenec
- cleaved
- recognized by SRP, takes ribosome and message to ER translocon
STA sequence
where does translation always begin
- always begins in cytosol
tail anchored proteins
- occur as the hydrophobic c terminal of the protein is recognized by get3 which brings to he portion to the ER membrane, get1,2,3 are used to insert the c terminal tail into the mambren domain of the protein using atp hdyrolsysi
- no luminal doina
n terminal side of the protein stays in the cytoplasm.
