Biological Membranes and Membrane Proteins (L13) Flashcards
what are a great model for studying biological membranes, lipids, integral membrane proteins, and membrane skeleton?
RBCs
8 functions of biological membranes
- compartmentalization
- barrier to diffusion
- define organelles and cell boundaries
- scaffolds for proteins
- transport molecules
- transmit information
- shape
- assemble polysaccharides
general properties of membranes
- fluidity
- membrane fusion
- selective permeability filter
- capacitance
- regulate information
- asymmetry and scaffolding
what is capacitance function of membranes?
barrier to rapid transport of ions, electrical resistance, and charge difference - maintains 20-80 mV voltage (negative inside cell)
general composition of membranes
- lipids
- proteins
- polysaccharides
what do lipids self-assemble into?
- micelles
- liposomes
- bilayers
primary lipids in biomembranes?
- phosphoglycerides
- sphingolipids
- cholesterol
importance of phosphoglycerides
important for compartment identity and precursors for signal transduction
defining part of a sphingolipid?
sphingosine - amino alcohol w/ long hydrocarbon tail
significance of plants having little cholesterol
promoted as heart healthy
- could block/compete w/ other sterols for uptake
- increase # of sterols in blood -> might signal for less cholesterol synthesis
cause of atherosclerosis
cholesterol, etc. accumulating on the inner walls of arteries and limiting blood flow and oxygen delivery to tissues
potential side effect of statins to lower cholesterol?
cholesterol is important precursor for vitamin D, bile acids, steroid hormones, cholesterol esters, modified proteins
how do proteins move around in the membrane?
mostly lateral movement - rarely flip flop (but flipases do that)
changes in membrane lipid composition that decrease freezing point
- shorter chain length
- more double bonds
- less sterol
how does a decrease in freezing point affect fluidity?
minimizes the amount of interaction b/w FA’s -> increase fluidity
three types of membrane proteins
- peripheral
- lipid-anchored
- integral or trans-membrane
integral membrane proteins
-have one or more transmembrane a-helix
what is the major membrane protein of RBCs?
glycophorin
lipid-anchored proteins
-associate via fatty acyl or prenyl groups
Ras
small G protein monomer - Tyr kinase signaling
Rabs
proteins that help specify compartment ID
peripheral proteins
-associate w/ integral membrane proteins or bind to phospholipid head groups
what type of proteins can be extracted ionically?
peripheral - extracted by salt treatments
what type of proteins can only be extracted by detergents?
integral - detergent is a small amphipathic molecule that make micelles and remove integral proteins
ionic detergents
SDS (sodium dodeylsulfate)
sodium deoxycholate
non-ionic detergents
triton X-100
octylglucoside
what are lipid rafts?
microdomains of specific lipids that cluster together and are different from surrounding lipids
components of lipid rafts
- no glycolipids, PE, PC
- enriched w/ glycosphingolipids and cholesterol
- enriched for acylated peripheral/integral proteins
6 functions of lipid rafts
- signaling platforms
- exocytosis
- endocytosis
- pathogen entry
- apoptosis
- cytoskeletal organization
functions of membrane fluidity
- response to temp change
- fusion and fission/sealing damages
- substrate exchange
- ETC
- multiprotein complex formation
evidence that proteins and lipids float in plane of membrane comes from….
- cell fusion experiments
- FRAP
- patching
- lazer tweezers
FRAP
fluorescence recovery after photobleaching
1. label membrane proteins w/ fluorescent reagent
2. bleach with laser
3. observe fluorescence recovery (characteristic kinetics)
can estimate diffusion constants and how much molecules move around
do all membrane proteins and lipids diffuse?
no
when can lipids and proteins be immobilized?
- if in lipid rafts (microdomain)
- protein-ECM interaction
- protein-membrane skeleton association
- cell-cell interaction
describe membrane skeleton
non-covalent, submembranous array of proteins
functions of membrane skeleton
- MAINTAINS CELL SHAPE
- stabilizes lipid bilayer
- creates and stabilizes domains of integral membrane proteins
- regulates exocytosis and membrane vesicle trafficking
- provides sites of attachment for cytoskeleton
parts of membrane skeleton
- a and B spectrin
- ankyrin
- band 3
- glycophorin
- band 4.1
- actin
function of Band 3
major integral membrane protein: HCO3-/Cl- antiporter
- 20-25% of total protein
- can be cross-linked to ankyrin
function of glycophorin
integral membrane protein - heavily glycosylated
- negative charge keeps surface of RBC hydrophilic
- keeps RBCs from sticking together (charge repulsion)
function of band 4.1
anchor for spectrin
what makes up junctional complex?
actin/spectrin binding with band 4.1
function of a and B spectrin
help RBC change shape
spectrin heterodimers
Bands 1 and 2 - 2 anti-parallel chains, w00 nm long
- B-subunit, N-terminus associates w/ F-actin
- modulated by CaM, phosphorylation, PIP2
- BINDS TO ANKYRIN AND BAND 4.1
function of actin in the RBC membrane skeleton
present only in small fragments - hub for spectrin molecules to associate to form a filamentous meshwork
hereditary spherocytosis mutations
can occur in several different genes:
- a and B spectrin
- protein 4.2
- band 3
MOST COMMON DEFECT = ANKYRIN
results in small, spherical RBCs with increased fragility, decreases flexibility
what molecules are permeable, slightly permeable, impermeable through membrane
permeable: gases, small uncharged polar molecules
slightly permeable: some small uncharged polar molecules (like urea)
impermeable: large uncharged polar molecules, ions, charged polar molecules
three classes of transport integral membrane proteins
- channels
- transporters
- pumps
mediate transport of ions, sugars, aa’s, and other metabolites across cellular membranes
pumps
active transport - couples movement of S against its concentration gradient to ATP hydrolysis
ex: Na+/K+ pump
Na+/K+ pump
important plasma membrane protein
-Na out, K in (antiporter)
what drugs target Na+/K+ pumps?
ouabain and digoxin
channels
facilitated diffusion - transport protein assists in movement of a specific S down its concentration gradient
ex: aquaporins, stretch-activated ion channels
transporters
secondary active transport/ co-transport - transport protein couples movement of a S against it’s concentration gradient to the movement of a second S down its concentration gradient
GLUT1
uniporter for glucose-facilitated diffusion
ABC proteins and disease
ATP-binding cassette proteins - important pumps
CFTR -> cystic fibrosis
ABCA1 -> Tangier’s disease
ABCD1 -> ADL
ABCG5/8 -> B-sitosterolemia
digoxin
- comes from digitalis/foxglove
- used for CHF
- mechanism: inhibitor of Na/K ATPase - helps heart beat more regularly and strongly
NCX
Na+/Ca2+ exchanger
- cation antiporter
- uses Na gradient to move Ca against its gradient
- pumps Ca out of muscle cells
